Reporting Measurement Uncertainty Research Articles

PAWG pilot study on quantification of total haemoglobin in blood

Main textUnder the auspices of the Protein Analysis Working Group (PAWG) of the Comité Consultatif pour la Quantité de Matière (CCQM) a pilot study, CCQM-P201, was organised. Five National Metrology Institutes (NMIs) participated in this pilot study and provided a total of eleven measurement results using six different methods based on three different measurement principles. The purpose of this pilot study was to develop measurement capabilities for the accurate quantification of intact haemoglobin (Hb) whereby measurement results could be made directly traceable to the SI. The choice of protein resulted from its ability to be quantified via different measurement principles providing different routes of traceability. Hb also provided a model of an intact protein with a complex tetrameric structure which is often measured via quantification of a specific subunit or the incorporated haem group. To achieve appropriate results for the chosen measurand, it was necessary to distinguish between the whole protein and fragments of the tetrameric structure, such as dimers or monomers. The participants were encouraged to investigate several of the measurement methods available. This was considered particularly advantageous in this instance as various methods applied to the same analyte may assist in identifying possible previously unknown biases or interferences of the various methods. Hb is being used as a model system to build capacity in methods for the quantification of proteins with a molar mass ≤ 100 kDa in biological fluids in a concentration range > 1∙106 pmol/g.This was the first pilot study for the quantification of an inherent protein in blood with all the natural variations that protein shows in a clinical sample. The different methods applied by the participants included isotopic dilution (ID) organic mass spectrometry (MS) of specific peptides after tryptic digestion, species-specific ID inductively coupled plasma mass spectrometry (ICP-MS), post-column ID ICP-MS, quantification via total iron after chromatographic isolation of Hb and acid digestion as well as two optical methods. One of the latter was the reference method proposed by the International Committee for Standardisation in Haematology (ICSH) based on the conversion of Hb to hemiglobincyanide (HiCN), the other used the conversion of Hb with alkaline haematin detergent both followed by the measurement of spectroscopic absorbance.Considering the use of very different measurement techniques for the quantification of total Hb, the results are in good agreement with a reference value with corresponding expanded uncertainty of (120.6 ± 1.2) mg/g calculated as DerSimonian-Laird mean. However, the observed differences between the results are not completely reflected in the reported measurement uncertainties. The calculated interlaboratory standard deviation or Tau value (dark uncertainty) was approximately 3 % whilst the average reported measurement uncertainty was approximately half this (1.4 %). A possible reason for this large difference may lie in the underestimation of any necessary conversion factors and their uncertainty contributions, between the target analyte measured and the requested measurand.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Reduction of Signal Drift in a Wavelength Modulation Spectroscopy-Based Methane Flux Sensor.

Accurately quantifying unsteady methane venting from key oil and gas sector sources such as storage tanks and well casing vents is a critical challenge. Recently, we presented an optical sensor to meet this need that combines volume fraction and Doppler shift measurements using wavelength modulation spectroscopy with 2f harmonic detection to quantify mass flux of methane through a vent line. This paper extends the previous effort through a methodical component-by-component investigation of potential sources of thermally-induced measurement drift to guide the design of an updated sensor. Test data were analyzed using an innovative signal processing technique that permitted quantification of background wavelength modulation spectroscopy signal drift linked to specific components, and the results were successfully used to design a drift-resistant sensor. In the updated sensor, background signal strength was reduced, and stability improved, such that the empirical methane-fraction dependent velocity correction necessary in the original sensor was no longer required. The revised sensor improves previously reported measurement uncertainties on flow velocity from 0.15 to 0.10 m/s, while markedly reducing thermally-induced velocity drift from 0.44 m/s/K to 0.015 m/s/K. In the most general and challenging application, where both flow velocity and methane fraction are independently varying, the updated design reduces the methane mass flow rate uncertainty by more than a factor of six, from ±2.55 kg/h to ±0.40 kg/h. This new design also maintains the intrinsic safety of the original sensor and is ideally suited for unsteady methane vent measurements within hazardous locations typical of oil and gas facilities.

Common sorption isotherm models are not physically valid for water in wood

Sorption isotherm models are frequently used to understand the interactions of water with cellulosic materials. Many commonly used models, such as the Guggenheim-Anderson-de Boer, Hailwood-Horrobin, and Dent isotherm models, are based on idealized physical systems. In the scientific literature, these models are often used to predict properties of wood such as the monolayer capacity, which is a representation of the number of available sorption sites in the material. Reporting such properties assumes that an idealized system adequately represents actual wood-water interactions. These models can be reduced to the same form, where the ratio of water activity, aw, to equilibrium moisture content (EMC), is a second-order polynomial function of aw. Here we review twelve models that have a parabolic form, fit these models to high quality water vapor sorption isotherm data for wood at multiple temperatures, and calculate physical properties from the model parameters. Moreover, variability in model predictions arising from measurement uncertainties in aw and EMC values is propagated using a Monte Carlo method. Predicted physical properties are tested against independently measured values of hydroxyl accessibility, relative amounts of primary and secondary water, and enthalpy of sorption. None of the models accurately predicts any of the physical properties, even when the reported measurement uncertainties are increased by a factor of three. Therefore, it must be concluded that parabolic sorption isotherm models are not valid for water in wood cell walls.

Effect of Open-Ended Coaxial Probe-to-Tissue Contact Pressure on Dielectric Measurements.

Open-ended coaxial probes are widely used to gather dielectric properties of biological tissues. Due to the lack of an agreed data acquisition protocol, several environmental conditions can cause inaccuracies when comparing dielectric data. In this work, the effect of a different measurement probe-to-tissue contact pressure was monitored in the frequency range from 0.5 to 20 GHz. Therefore, we constructed a controlled lifting platform with an integrated pressure sensor to exert a constant pressure on the tissue sample during the dielectric measurement. In the pressure range from 7.74 kPa to 77.4 kPa, we observed a linear correlation of % and % per kPa for, respectively, the relative real and imaginary complex permittivity. These values are statistically significant compared with the reported measurement uncertainty. Following the literature in different biology-related disciplines regarding pressure-induced variability in measurements, we hypothesize that these changes originate from squeezing out the interstitial and extracellular fluid. This process locally increases the concentration of membranes, cellular organelles, and proteins in the sensed volume. Finally, we suggest moving towards a standardized probe-to-tissue contact pressure, since the literature has already demonstrated that reprobing at the same pressure can produce repeatable data within a 1% uncertainty interval.

Measurement uncertainty of β-lactam antibiotics results: estimation and clinical impact on therapeutic drug monitoring

Background Despite that measurement uncertainty data should facilitate an appropriate interpretation of measured values, there are actually few reported by clinical laboratories. We aimed to estimate the measurement uncertainty of some β-lactam antibiotics (β-LA), and to evaluate the impact of reporting the measurement uncertainty on clinicians' decisions while guiding antibiotic therapy. Methods Measurement uncertainty of β-LA (aztreonam [ATM], cefepime [FEP], ceftazidime [CAZ], and piperacillin [PIP]) values, obtained by an UHPLC-MS/MS based-method, was estimated using the top-down approach called the single laboratory validation approach (EUROLAB guidelines). Main uncertainty sources considered were related to calibrators' assigned values, the intermediate precision, and the bias. As part of an institutional program, patients with osteoarticular infections are treated with β-LA in continuous infusion and monitored to assure values at least 4 times over the minimal inhibitory concentration (4×MIC). We retrospectively evaluated the impact of two scenarios of laboratory reports on clinicians' expected decisions while monitoring the treatment: reports containing only the β-LA values, or including the β-LA coverage intervals (β-LA values and their expanded measurement uncertainties). Results The relative expanded uncertainties for ATM, FEP, CAZ and PIP were lower than 26.7%, 26.4%, 28.8%, and 25.5%, respectively. Reporting the measurement uncertainty, we identified that clinicians may modify their decision especially in cases where 4×MIC values were within the β-LA coverage intervals. Conclusions This study provides a simple method to estimate the measurement uncertainty of β-LA values that can be easily applied in clinical laboratories. Further studies should confirm the potential impact of reporting measurement uncertainty on clinicians' decision-making while guiding antibiotic therapy.

Measurement uncertainty in laboratory medicine: the bridge between medical and industrial metrology

Abstract Uncertainty is an inseparable part of all types of measurements, that is, in metrology a measurement without uncertainty is not possible. Calculation of uncertainty increases the awareness of the certainty of measurement results. Reporting measurement uncertainty is mandatory in almost all industrial sectors but not in laboratory medicine. Test results without analytical uncertainty increases the diagnostic uncertainty, causing errors that could seriously affect patients health. To improve diagnostic certainty, we should calculate and upon request report measurement uncertainty to laboratory users (ISO 15189). In this mini-review, we summarized the theoretical perspective of uncertainty concept, its relation to Six Sigma and finally compared it with Total Error Method.

A comparative analysis of AC/DC transfer standards for comparison of national standards

The development of standards based on quantum effects, in particular, alternating voltage synthesizers, has not yet allowed defining the metrological characteristics of measuring instruments for alternating voltage up to 1,000 V at a frequency up to 1 MHz. Therefore, a comparative analysis of international comparisons of national standards has summarized the possibilities of metrological support with the use of AC/DC voltage transfer standards.The conducted analytical and experimental studies give grounds to state the decisive contribution of national metrological institutes in the formation of the modern equivalence level of AC/DC voltage transfer standards. A comparative analysis of the uncertainty of measurements achieved by the leading national metrology institutes has made it possible to distinguish the most accurate type of thermal voltage converters based on the thermocouples connected in series. Such a measuring instrument allows measuring AC/DC transfer difference with an uncertainty of less than 1 μV/V at certain points of the measuring range.Consideration of the capabilities of the travelling standards to ensure a stable storage of a value of AC/DC voltage transfer difference has indicated the advantage of the thermal converter of an indicated above type, relative to the other types used in the comparisons of AC/DC transfer standards. The calculation of stability coefficients for different types of standards has shown an approximately twofold advantage of thermal converters based on the thermocouple comparing with multi-range thermal comparators on the basis of the root-mean-square voltage sensor. The results of the considered comparisons have shown that there were no advantages of any of the measurement schemes used by the laboratories since no relation has been found between the reported measurement uncertainty and the scheme.The results of the estimation of the frequency influence of the input voltage on the transformation coefficient of the AC/DC voltage transfer standards of the two types have given the grounds to neglect correcting the contribution of this source of uncertainty. The proposed approach to measuring the AC/DC transfer difference with providing the connection with a direct definition allows us to estimate more appropriately this metrological characteristic in two ways

RELAP5 simulation of CANDU Station Blackout accidents with/without water make-up to the steam generators

In the event of a complete Station Blackout (SBO) of a CANDU reactor, the current safety practice is to initiate depressurization early in the transient once a lack of power has been declared. This requires operator actions to manually open the main steam safety valves (MSSVs) on the secondary side initiating a crash-cool procedure. The depressurization of the secondary side allows make-up water to be supplied to the steam generator (SG) secondary-side thereby extending natural-circulation driven heat removal from the fuel in the primary heat transport system. Once depressurized there are several additional water sources available to replenish the secondary-side inventory, and in the event that emergency power cannot be restored emergency mitigating equipment (EME) are available to provide alternative water make-up. The objective of this paper is to examine the processes and phenomena involved during and after crash-cooling and compare these results to cases where operator actions are not credited. Simulations are performed until such time as the secondary-side inventory is stabilized from alternative water sources or until it is depleted.A detailed RELAP5 model of a 900MW CANDU plant has been created including the primary heat transport system (PHTS), the feed and bleed system, the steam generator secondary side, the moderator system, and the shield water cooling system. The 480 fuel channels were grouped into 20 channels by elevation and channel power. The models were benchmarked against the 1993 loss-of-flow event at Darlington NGS and agreed with the station data within the reported measurement uncertainty. Then the models were used to simulate the Station Blackout accidents with loss of class IV, class III, and emergency power supplies. Five different scenarios with/without crash-cool and with different water make-up options are modeled and key sensitivities determined. The results show that the depressurization of the secondary side may create a situation where continuous natural circulation breaks down and intermittent buoyancy induced flows (IBIF) takes place. The RELAP5 predicted IBIF phenomena are discussed, as well as the limitations of the current RELAP5 code. The main focus of this paper is on the early stage of the accidents, i.e. when adequate steam generator secondary side inventory exists and where damage to the main heat transport system can be precluded. The results demonstrate that EME actions to maintain SG inventory are effective and ensure fuel and fuel channel integrity.

National Interlaboratory Comparison in Scope of Relative Humidity in the Range from 11 %rh to 95 %rh at 23 $${}^{\circ }\hbox {C}$$ ∘ C

The goal of interlaboratory comparisons is to verify the competence of accredited or non-accredited laboratories, including verification of the reported measurement uncertainties, whenever possible. An interlaboratory comparison consists of measurement results of a laboratory and a reference laboratory which is competent to run the comparison and provides traceability at the required uncertainty level. In Turkey, the demand for an interlaboratory comparison in the humidity field increased drastically in parallel to an increasing number of accredited laboratories in this field. Therefore, a national interlaboratory comparison in the scope of relative humidity in the range between 11 %rh and 95 %rh at \(23\, {}^{\circ }\hbox {C}\) was initiated by TUBITAK UME in 2011. Two hygrometers were calibrated at TUBITAK UME in the above-mentioned range and characterized in terms of long-term stability, temperature dependence, and hysteresis. One of the hygrometers is used throughout this comparison. The second one was kept aside as a backup device in case of a malfunction of the first device. Organization and evaluation of the interlaboratory comparison in the scope of relative humidity were performed according to the EN ISO/IEC 17043:2010 “Conformity assessment—general requirements for proficiency testing” among thirteen national laboratories. Evaluation of comparison results are performed by normalized error analysis. It was found that among the 95 realized points of comparison, 85 (89 %) had \(E_{n} \le 1\) and only 10 (11 %) had \(E >1\).

Validation of ORIGEN for LWR used fuel decay heat analysis with SCALE

The energy release rate from the decay of radionuclides can be a critical design parameter for used nuclear fuel storage, transportation, and repository engineered systems. Validation of the SCALE nuclear analysis code system capabilities in predicting decay heat for commercial used fuel applications has been performed using decay heat measurements for fuel assemblies irradiated in pressurized and boiling water reactors. The experimental data used for validation include a large number of full-length-assembly decay heat measurements that were performed between 2003 and 2010 at the Swedish Central Interim Storage Facility for Spent Nuclear Fuel, Clab, operated by the Swedish Nuclear Fuel and Waste Management Company, SKB. The measured fuel assemblies cover the burnup range 14–51GWd/MTU and cooling times between 12 and 27 years, which are times of interest to used fuel transportation and storage applications. The validation results indicate good agreement between calculated and measured decay heat values, generally within the reported measurement uncertainty. The effects of key modeling assumptions and data used in the calculations are presented and discussed.

Estimating the Measurement Uncertainty in Forensic Blood Alcohol Analysis

For many reasons, forensic toxicologists are being asked to determine and report their measurement uncertainty in blood alcohol analysis. While understood conceptually, the elements and computations involved in determining measurement uncertainty are generally foreign to most forensic toxicologists. Several established and well-documented methods are available to determine and report the uncertainty in blood alcohol measurement. A straightforward bottom-up approach is presented that includes: (1) specifying the measurand, (2) identifying the major components of uncertainty, (3) quantifying the components, (4) statistically combining the components and (5) reporting the results. A hypothetical example is presented that employs reasonable estimates for forensic blood alcohol analysis assuming headspace gas chromatography. These computations are easily employed in spreadsheet programs as well. Determining and reporting measurement uncertainty is an important element in establishing fitness-for-purpose. Indeed, the demand for such computations and information from the forensic toxicologist will continue to increase.

Relative measurements of ozone absorption cross‐sections at three wavelengths in the Hartley band using a well‐defined UV laser beam

New measurements of absorption cross sections of ozone are reported using a photometer with a well‐defined UV laser beam at three different wavelengths in the Hartley band, by comparison with the conventional cross‐section value at the mercury line wavelength of 253.65 nm. Reported measurement results and uncertainties are dependent on the value of the cross section used at 253.65 nm. Using the value determined by Hearn (1961), ozone absorption cross‐section values of 9.64 × 10−18, 10.61 × 10−18, and 11.26 × 10−18 cm2 molecule−1 are reported at wavelengths of 244.06, 248.32, and 257.34 nm, respectively; whereas, use of the Malicet (1995) value leads to ozone absorption cross‐section values of 9.50 × 10−18, 10.45 × 10−18, and 11.12 × 10−18 cm2 molecule−1, respectively. In all cases, the uncertainty of the results is dominated by the contribution from the cross‐section value at 253.65 nm. The experimental setup described in this article not only provides a minor contribution to the entire reported measurement uncertainties but also confirms the internal consistency of previously published ozone absorption cross‐section data sets and demonstrates the biases that can be introduced by changing data sets. The results are important for the accurate measurement of ozone concentrations in the troposphere by UV absorption methods and also for all measurements of ozone concentration in the atmosphere by spectroscopic methods for which the values of the absorption cross sections have been scaled to measured values in the Hartley band.

The estimation of traceability and uncertainty of measurement on the result of HBV DNA with different detecting system

Objective To study the uncertainty and traceability of HBV DNA assays and discuss the comparability of among different detection systems. Methods Different detecting systems were used to detect HBV DNA using the national standard substance as control The uncertainty of the was evaluated referring Guidelines for estimating and reporting measurement uncerTAinty of chemical test results of NATA The were traced back to the national standard substance. According to the CLSI document EP9-A2, the were analyzed and subjected to bias estimation with the t(0.05sv) √u2b1+ u2b2 as the criterion clinically accepted to investigate the comparability of different detecting systems. Results The means (-y) measured by 3 HBV DNA assay systems were 6.15,5.88,and 6.31 lg(kIU/L) respectively. Except system A,both the biases of system B and C had statistical significance (all P ＜ 0. 05) and expanded uncertainty of three detection systems was varied, but the difference was within the maximum acceptable range (± 0. 5) of the external quality assessment by National Center for Clinical Laboratory. Being traceable to national standard substance, the of HBV DNA of the three detecting systems were (5.45 ± 1.23), (5.55 ± 1.32) and (5.42 ± 1.25) lg(kIU/L), respectively.There was significant difference among three systems (F = 5.63, P ＜ 0. 05). Comparing system A and B,there was significant difference in statistic (q = 5. 12, P ＜ 0. 05) and the difference between system B and C also had statistically significant (q = 6. 85, P ＜ 0. 05), but the between system A and C had no statistical difference (q = 1.85,P ＞ 0. 05). Among these three systems, the difference of any two detection systems had no statistical significance (all P ＞ 0. 05). It showed that system bias was acceptable in clinical application and the between different systems were comparable. Conclusions It is necessary to estimate the uncertainty and traceability when comparing the HBV DNA assay among the different labs. It also needs to estimate the bias of different systems and evaluate the clinical acceptability to ensure the accuracy and comparability of the results. Key words: Uncertainty; Hepatitis B virus; DNA,viral; Polymerase chain reaction

On the uncertainty estimation of electromagnetic field measurements using field sensors: a general approach

One of the most common and popular practices on measuring the non-ionising electric and/or magnetic field strength employs field meters and the appropriate electric and/or magnetic field strength sensors. These measurements have to meet several requirements proposed by specific guidelines or standards. On the other hand, performing non-ionising exposure assessment using real measurement data can be a very difficult task due to instrumentation limits and uncertainties. In addition, each measuring technique, practice and recommendation has its own drawbacks. In this paper, a methodology for estimating the overall uncertainty for such measurements, including uncertainty estimation of spatial average values of electric or magnetic field strengths, is proposed. Estimating and reporting measurement uncertainty are of great importance, especially when the measured values are very close to the established limits of human exposure to non-ionising electromagnetic fields.

The true cyclotron frequency for particles and ions in a Penning trap

The true cyclotron frequency of a particle or ion, needed for mass spectrometry and other accurate measurements in a Penning trap, cannot be measured directly. It is not one of the oscillation frequencies of the trapped particle, and the three oscillation frequencies that can be measured vary with the misalignment and the harmonic distortion of the trap potential. Two methods to determine the cyclotron frequency are discussed. First, when all three eigenfrequencies of a trapped particle can be measured, the true cyclotron frequency is given by the prescription of the Brown–Gabrielse invariance theorem. This prescription makes possible a surprising number of the most accurate measurements in particle, nuclear and atomic physics because it accounts exactly for the lowest order electrostatic imperfections and magnetic misalignments. Second, when less accuracy is required, as when the masses of unstable nuclei are measured, a single sideband frequency is often measured instead—the frequency of a driving force that optimally couples two of the motions of the ion in the trap. A missing theoretical justification for this alternate method is provided using an expansion of the same invariance theorem. A remarkable suppression of systematic measurement errors is predicted, showing why these are not larger than reported measurement uncertainties, despite the contrary indication of simple estimates.

Draft radioanalytical water and soil performance testing: Uncertainty acceptance criteria derived from historical mixed-analyte performance evaluation program results

Radioanalytical performance testing programs provide radioassay laboratories, regulators and the public performance-based evidence that measurement capabilities are in control. Performance acceptance criteria that combine aspects of measurements difference from a known value and the associated combined uncertainty establishes a quantitative statement of statistical confidence. However, there is need for a reasonable upper limit of the reported measurement uncertainty. Evaluation of thousands of historical U.S. DOE Mixed-Analyte Performance Evaluation Program measurement results for 17 radionuclides in soil and water samples provides predictive expectations for future measurement results and a statistical basis for establishing reasonable upper limits for reported measurement uncertainties for performance evaluation programs.

Recommendations on quantities, symbols and measurement units for publications in ACQUAL: reporting measurement uncertainty in chemical analysis

Measurement uncertainty is an integral part of a measurement result. Without a statement of uncertainty a measurement result is not complete. Concluding about compatibility with other measurement results obtained for the same measurand or with compliance limits is not possible and the measurement result does therefore, not serve its purpose. The measurement uncertainty is a quality criterion, which is important for the use of the result, in particular if decisions are to be based on that result. The Guide to the Expression of Uncertainty in Measurement (GUM), ISO 1995, and the CITAC/EURACHEM document Quantifying Uncertainty in Analytical Measurement (QUAM), 2001, an introduction to the GUM for chemists, are useful guides for evaluating measurement uncertainty in chemical analysis. These guides and their recommendations on how to report measurement results together with the associated uncertainty serve as the basis for the following. The final measurement result obtained after corrections for all known systematic effects is regarded as the best estimate y of the measurand Y within the range of values which could reasonably be attributed to the measurand under the given conditions of measurement. It is reported in the form: Y 1⁄4 y U with U 1⁄4 kuðyÞ

Reporting measurement uncertainty and coverage intervals near natural limits

Different methods of treating data which lie close to a natural limit in a feasible range, such as zero or 100% mass or mole fraction, are discussed and recommendations made concerning the most appropriate. The methods considered include discarding observations beyond the limit, shifting observations to the limit, truncation of a classical confidence interval based on Student's t (coupled with shifting the result to the limit if outside the feasible range), truncation and renormalisation of an assumed normal distribution, and the maximum density interval of a Bayesian estimate based on a normal measurement distribution and a uniform prior within the feasible range. Based on consideration of bias and simulation to assess coverage, it is recommended that for most purposes, a confidence interval near a natural limit should be constructed by first calculating the usual confidence interval based on Student's t, then truncating the out-of-range portion to leave an asymmetric interval and adjusting the reported value to within the resulting interval if required. It is suggested that the original standard uncertainty is retained for uncertainty propagation purposes.

On Conformity Testing and the Use of two Stage Procedures

SummaryConformity testing is a systematic examination of the extent to which an entity conforms to specified requirements. Such testing is performed in industry as well as in regulatory agencies in a variety of fields. In this paper we discuss conformity testing under measurement or sampling uncertainty. Although the situation has many analogies to statistical testing of a hypothesis concerning the unknown value of the measurand there are no generally accepted rules for handling measurement uncertainty when testing for conformity.Usually the objective of a test for conformity is to provide assurance of conformity. We therefore suggest that an appropriate statistical test for conformity should be devised such that there is only a small probability of declaring conformity when in fact the entity does not conform. An operational way of formulating this principle is to require that whenever an entity has been declared to be conforming, it should not be possible to alter that declaration, even if the entity was investigated with better (more precise) measuring instruments, or measurement procedures.Some industries and agencies designate specification limits under consideration of the measurement uncertainty. This practice is not invariant under changes of measurement procedure. We therefore suggest that conformity testing should be based upon a comparison of a confidence interval for the value of the measurand with some limiting values that have been designated without regard to the measurement uncertainty. Such a procedure is in line with the recently established practice of reporting measurement uncertainty as “an interval of values that could reasonably be attributed to the measurand”.The price to be paid for a reliable assurance of conformity is a relatively large risk that the procedure will fail to establish conformity for entities that only marginally conform. We suggest a two‐stage procedure that may improve this situation and provide a better discriminatory ability. In an example we illustrate the determination of the power function of such a two‐stage procedure.

Measurement Variability Associated with KXRF Bone Lead Measurement in Young Adults

In vivo bone lead measurement using K X-ray fluorescence (KXRF) has been used to estimate long-term lead exposure, especially in adults. Relatively few studies have been conducted on young subjects with this technique. To explore the measurement variability of KXRF bone lead measurements in young subjects, the tibiae of two male cadavers from Boston, Massachusetts, 17 and 20 years of age, were obtained for repeated bone lead measurements. Bone lead concentrations were measured using a grid of nine locations, 1 cm apart, centered at the midpoint of the tibia. Each location was sampled using five 60-min measurements. Measured concentrations ranged from < 0 to 11.8 microg Pb/g bone mineral across a tibia with mean concentrations for the midpoint locations of 0.8 microg Pb/g bone mineral SD = 2.5 and 2.0 microg Pb/g bone mineral (SD = 1.9) for the left and right legs of the younger subject and 3.6 microg Pb/g bone mineral (SD = 2.6) and 6.0 microg Pb/g bone mineral (SD = 3.3) for the left and right legs of the older subject. Although bone lead concentrations did not vary significantly by measurement location in an individual leg, reported measurement uncertainty increased significantly at locations that were 1 cm from the center of the tibia horizontally (p < 0.0001). Symmetry in bone lead concentration between legs was observed for the 17-year-old subject. Potential asymmetry between the left and right legs was suggested for the 20-year-old subject (p = 0.06). These data describe the degree of variability that may be associated with bone lead measurements of young subjects with low bone lead concentrations using a standard spot-source KXRF instrument. Because of the importance of conducting additional research on adolescent lead toxicity, further improvements to the precision of KXRF measurement are needed.