Intra-individual Biological Variation Research Articles

Cerebrospinal fluid beta-amyloid(1-42) in Alzheimer disease: differences between early- and late-onset Alzheimer disease and stability during the course of disease.

To study the diagnostic potential of the 42 amino acid form of beta-amyloid (beta-amyloid(1-42)) in cerebrospinal fluid (CSF) as a biochemical marker for Alzheimer disease (AD), the intra-individual biological variation of CSF-beta-amyloid(1-42) level in patients with AD, and the possible effects of differential binding between beta-amyloid and apolipoprotein E isoforms on CSF-beta-amyloid(1-42) levels. A 20-month prospective follow-up study. Community population-based sample of consecutive patients with AD referred to the Piteå River Valley Hospital, Piteå, Sweden. Fifty-three patients with AD (mean +/- SD age, 71.4 +/- 7.4 years) diagnosed according to the National Institute of Neurological and Communicative Disorders and Stroke and Alzheimer's Disease and Related Disorders Association criteria and 21 healthy, age-matched (mean +/- SD age, 68.8 +/- 8.0 years) control subjects. Cerebrospinal fluid beta-amyloid(1-42) level--analyzed using enzyme-linked immunosorbent assay--and severity of dementia--analyzed using the Mini-Mental State Examination. Mean +/- SD levels of CSF-beta-amyloid(1-42) were decreased (P<.001) in patients with AD (709 +/- 304 pg/mL) compared with controls (1678 +/- 436 pg/mL). Most patients with AD (49 [92%] of 53 patients) had reduced levels (<1130 pg/mL). A highly significant correlation (r = 0.90; P<.001) between baseline and 1-year follow-up CSF-beta-amyloid(1-42) levels was found. There were no significant correlations between CSF-beta-amyloid(1-42) level and duration (r = -0.16) or severity (r = -0.02) of dementia. Low levels were also found in patients with mild dementia (Mini-Mental State Examination score, >25). The sensitivity of CSF-beta-amyloid(1-42) level as a diagnostic marker for AD is high. The intra-individual biological variation in CSF-beta-amyloid(1-42) level is low. Low CSF-beta-amyloid(1-42) levels are also found in the earlier stages of dementia in patients with AD. These findings suggest that CSF-beta-amyloid(1-42) analyses may be of value in the clinical diagnosis of AD, especially in the early course of the disease, when drug therapy may have the greatest potential of being effective but clinical diagnosis is particularly difficult.

Biochemical monitoring of treatment for galactosaemia: biological variability in metabolite concentrations.

Red cell galactose 1-phosphate (Gal-1-P) concentrations and urinary galactitol excretion have been suggested as biochemical indices of dietary compliance in classical transferase-deficient galactosaemia. We report our experience of measuring both in 32 patients over 0-10.9 years (median 3.45). A total of 438 blood specimens for Gal-1-P and 383 urine specimens for galacitol assay were received; 317 pairs of specimens were collected at the same time. Concentrations of both analytes fell rapidly over the first 2-3 months following dietary intervention, to mean (geometric SD) levels of 225 (1.60) mumol/L red cells for Gal-1-P and 388 (1.19) mumol/mmol creatinine for galactitol. Concentrations then fell exponentially over the next 7-8 years, with times to half-disappearance of 6.3 years for Gal-1-P and 6.4 years for galactitol, to levels of 104 (1.58) and 193 (1.36) respectively in patients aged over 10 years. Concentrations of both analytes were independent of the presence of the common Q188R mutation. Mean intra- and inter-individual coefficients of variation (CV) across the range of values studied were 36% and 61% for Gal-1-P, and 37% and 42% for galactitol. Analytical CVs were 3.6% for Gal-1-P and 5.5% for galactitol, indicating that the major source of variability is biological. The correlation coefficient between Gal-1-P and galactitol in paired samples overall was 0.33; the regression equation being [Galactitol] = 0.84[Gal-1-P] + 176. Serial measurements of both Gal-1-P and galactitol may be valuable in monitoring galactosaemia, but high intra-individual biological variability limits their usefulness. Standardization of sample collection times may improve this. Further work is needed to assess the predictive values of both analytes for long-term outcome.

Intra-individual variation of plasma lipids and lipoproteins in prepubescent children.

Estimates of the average intra-individual biological variability for plasma lipids and lipoproteins differs substantially among published studies. Moreover, this topic does not appear to have received consideration in exercise and health literature with normal, healthy children as subjects. The purpose of this study was, therefore, to determine the short-term, day-to-day variability of the lipid-lipoprotein profile from 19 children [mean (SD), 11.5 (0.8) years] from 3 separate venous blood samples. Intra-individual standard deviations, variances and coefficients of variance were determined for total cholesterol (TC), triacylglycerol (TG), high-density lipoprotein-cholesterol (HDL-C), HDL-C sub-fractions HDL2 and HDL3, and low-density lipoprotein-cholesterol (LDL-C). The intra-individual variation for TC and LDL-C was used to calculate the 95% confidence intervals (CIs) around the National Cholesterol Education Programme (NCEP 1991) cut-off points. The main finding was that all of the measured blood analytes including TC, TG, HDL-C, HDL2, HDL3, and LDL-C varied considerably from day-to-day. Coefficients of total variation ranged from 3.5% for HDL3 to 25.4% for TG. Classification of individuals using NCEP guidelines was difficult based on only one or two blood samples. The magnitude of variation for LDL-C meant that a 95% CI could not be constructed around the NCEP borderline-high classification from either one or two samples. However, averaging three TC and LDL-C measurements increased the likelihood of classification within the 95% CI. The results indicate that when using the NCEP guidelines for children and adolescents, true concentrations for TC and LDL-C should be based on the mean of multiple samples.

Consensus and accuracy in haematology laboratories of developing countries: the Jordanian experience.

A study lasting for 18 months using interlaboratory surveys was carried out to assess the analytical quality of Jordanian haematology laboratories that represent one of the developing countries. The study surveyed 50 laboratories constituting the majority of clinical laboratories in the central region of Jordan using 15 control specimens of whole fresh blood and eight freshly prepared blood smears. The study covered the routine haematological parameters of PCV, Hb, RBC, WBC and differential white blood cell count. More than 97% of Jordanian laboratories using cell counters achieved the medically useful criteria for analytical performance, this figure was reduced to 84% in laboratories using manual methods. Jordanian laboratories, however, were far from achieving the analytical goals that have been proposed based on intraindividual biological variation. This study stressed the need for a national EQA scheme in haematology, to reach a common level of standardization.

Intra individual variability in markers of proteinuria for normal subjects and those with cadmium induced renal dysfunction: interpretation of results from untimed, random urine samples

The project aimed to help interpretation of urinary protein measurements, namely -2-microglobulin, retinol-binding protein, albumin and total protein in untimed, random urine samples as indicating significant changes in renal tubular reabsorption and glomerular permeability in an individual. A standard methodology used in clinical laboratory medicine was applied to calculate the intra-individual biological variation for these analytes. This parameter in conjunction with a laboratory's analytical variation allows definition of uncertainty about a single urine protein measurement, significant changes above normal variation in serial measurements within an individual and a defined level of maximum acceptable analytical imprecision. Repeat urine samples were obtained over a period of one week from a group of cadmium-exposed workers, 90% of whom had long-term tubular proteinuria, and a group of five unexposed volunteers with normal renal function. Dilute samples defined as having creatinines less than 3 mmol l-1 were excluded, as were urines with pH less than 5.5 for -2-microglobulin. Samples were analysed twice after randomisation in large batches. There was no evidence of any diurnal variation in the four protein measurements from samples collected between early morning and 16:00 hours. Creatinine or specific gravity correction of urine results for all four proteins only marginally reduced the uncertainty associated with an individual measurement asreflecting the true excretion value. For those subjects with defined tubular proteinuria, variability in retinol-binding protein excretion was less than that for -2- microglobulin. About 30% of the samples had urine pHs of 5.5 or less where -2- microglobulin degradation occurs. Using our laboratory analytical precision the minimum changes between serial creatinine-corrected measurements that are needed to be considered statistically significant (p < 0.05) is 110% for retinol-binding protein, 177% for -2-microglobulin, 70% for total protein, and 81% for albumin. Unlike published data for cadmium and mercury, the use of creatinine or specific gravity correction of random untimed urine samples for the urinary proteins does not make a large improvement to the interpretation of data by reducing the uncertainty associated with a measurement. There are significant advantages to the use of retinol-binding protein in contrast to -2- microglobulin as an indicator of renal tubular damage. Levels for defined acceptable analytical precision are calculated for laboratories undertaking protein estimations. The data in this report will help in the interpretation of urinary protein measurements without monitoring cadmium workers.

Biological Variability and Reference Intervals for Total Plasma Homocysteine

We determined the intra-individual biological variability of plasma homocysteine in 20 healthy subjects. The intra-individual coefficient of variation was relatively low (8.3%), indicating that a single measurement can be used to characterize the average homocysteine concentration. A population study measuring plasma homocysteine and serum folate levels was conducted on serum samples collected from 1109 randomly selected, fasting adults with a wide age range. We determined age- and gender-specific central 0.95 intervals and found that subjects in the highest quartile of serum folate had significantly lower (P = 0.0001) mean plasma homocysteine concentrations than did those in the lowest quartile of folate values. An 'ideal' homocysteine reference range, based on targeting those subjects who are likely to be folate replete, is preferable to the population-based range using the central 0.95 interval.

Monitoring of proteinuria in phase I studies in healthy male subjects.

The quantitative measurement of urinary marker proteins may improve the sensitivity of monitoring renal function in healthy male subjects in phase I studies. Little is known about the variability of physiological proteinuria in young, healthy male subjects. Thus, the biological and analytical variability of three marker proteins, i.e. albumin, alpha(1)-microglobulin and N-acetyl-beta-D-glucosaminidase (NAG), were investigated in this population. Seven young, healthy male subjects participated in a prospective two-way cross-over study, and 139 in a retrospective study. Albumin and alpha(1)-microglobulin were determined by immunological methods (radial immunodiffusion and/or kinetic nephelometry), and NAG by enzyme activity in a colorimetric assay. The inter-assay precision of NAG, albumin and alpha(1)-microglobulin is good (< 15%) if automated kinetic nephelometry is applied for albumin and alpha(1)-microglobulin determination, but less impressive (< 25%) with radial immunodiffusion. The highest frequency of detectable proteinuria and highest creatinine-adjusted protein levels are found in the second morning urine voided after a night's rest. The intra-individual biological variability of NAG excretion from day to day is low (CV: 15-25%), irrespective of outpatient or inpatient settings. By contrast, albumin and alpha(1)-microglobulin excretion can differ by a factor of 2-3 from day to day, and higher levels are predominantly found in outpatient settings. The reference ranges for young, healthy male subjects are generally lower than published in cross-sectional studies in the total healthy population. These findings and established reference ranges for young, healthy male subjects may assist in the evaluation of proteinuria in clinical pharmacological phase I trials.

Effect of serum-clot contact time on clinical chemistry laboratory results

The effect of serum-clot contact time on laboratory results was studied by dividing each blood specimen into four blood collection tubes. The control sera were separated from the clot within 30 min of the collection. The other tubes were incubated at 32 degrees C, and the sera were separated at 3, 6, and 24 h. The sera were stored at 4 degrees C and analyzed at the same time. The stability of the tests was determined by comparing the results of the 3-, 6-, and 24-h samples with the values from the 30-min samples. The acceptable limits around the 30-min values were derived from the analytical and intraindividual biological variation of the tests. A total of 63 analytes were studied. Potassium, phosphorous, and glucose were the least stable, and the serum should be separated from the clot within 3 h for these analytes. Albumin, bicarbonate, chloride, C-peptide, HDL-cholesterol, iron, LDL-cholesterol, and total protein should be separated within 6 h. The other analytes were stable for 24 h of serum-clot contact.

Influence of biological and analytical variation on urine measurements for monitoring exposure to cadmium.

OBJECTIVE: To define the mean intraindividual biological variation in urinary concentrations of cadmium and retinol binding protein (RBP) in untimed, random urine samples and the influence of creatinine or specific...

The evolution and limitations of accuracy and precision standards

Limits of maximum allowable analytical imprecision have been defined on the basis of normal range (Tonks), composite biological variation (Cotlove), intraindividual biological variation (Aspen Conference, College of American Pathologists), medical significance (Barnett, Skendzel), and a combination of medical significance and biological variation (Fritsche, Klee). Because error limits based on medical significance are less stringent than those based on biological variation, performance goals based on medical significance are more likely to be attained by laboratories than those defined by biological variation. Most clinical scientists realize that the goal to limit the analytical C.V. to one-half or less of the biological C.V. is arbitrary, and for the large majority of laboratory tests of no benefit to the patient, for the major component of total variation is not the analytical imprecision, but the intraindividual variation itself. Furthermore, the purpose of laboratory testing is to help physicians confirm or exclude potential diagnoses and monitoring therapy rather than detecting small deviations from normal values. Small changes in test values are quite often uninterpretable or lead to costly albeit fruitless investigations. In view of diminishing resources for health care we must establish the accuracy and precision required for each test. While improving the accuracy of some tests would be desirable, for most of them further improvement would be irrelevant to patient care because few tests are pathognomonic by themselves and quite often diagnosis is not made on the basis of a single laboratory result. If more accuracy is desirable, it must also be affordable and benefits should outweigh costs.

Biological variability in concentrations of serum lipids: sources of variation among results from published studies and composite predicted values

To obtain the best estimates of the average intraindividual biological variability (CVb) in the concentrations of total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride serum lipids in a person's blood, we evaluated results from 30 studies published from 1970 to 1992. The usually more applicable random-effects model estimated an average CVb of 6.1% for TC, 7.4% for HDLC, 9.5% for LDLC, and 22.6% for triglyceride. Composite estimates of the average CVb from all evaluated published studies by different models of estimation ranged from 6.0% to 6.4% for TC, from 6.2% to 7.5% for HDLC, from 7.0% to 9.6% for LDLC, and from 22.4% to 22.9% for triglyceride. Two important factors influenced the reported biological variation of the study subjects: (a) the magnitude of the variability of the analytical method used and (b) the design characteristics of the study--primarily the number of subjects, the sampling interval, and the number of measurements per subject. For TC, we found a statistically significant positive correlation between the reported mean CVb and both the number of study subjects and the analytical variation. For TC and LDLC we estimate CVb as a function of the study design features. The number of patient specimens required to obtain reliable estimates for serum lipid concentrations are determined from the CVb and the current analytical variation.

Effect of biological and analytical variation in cholesterol measurement on the cost-effectiveness of cholesterol-lowering therapy.

A number of recently published studies on the cost-effectiveness of cholesterol-lowering therapy use data from the Framingham Study to model the effect of cholesterol lowering on coronary heart disease risk. However, the risk estimates from the Framingham Study underestimate the association between coronary heart disease risk and serum cholesterol level because they do not account for intraindividual biological variation and analytical variation in cholesterol measurement. Cost-effectiveness studies that use these risk estimates are therefore likely to overestimate the cost per year of life saved of cholesterol-lowering interventions. We have developed an algorithm that can be used to improve current estimates of the cost-effectiveness of cholesterol-lowering therapy. Our results show that adjusting for intraindividual biological variation and analytical variation lowers the cost per year of life saved by 17 to 29%, depending on sex, pretreatment cholesterol level, and age at initiation of therapy.

Blood lipid measurements: Variations and practical utility

Objective. —To describe the magnitude and impact of the major biological and analytical sources of variation in serum lipid and lipoprotein levels on risk of coronary heart disease; to present a way to qualitatively estimate the total intraindividual variation; and to demonstrate how to determine the number of specimens required to estimate, with 95% confidence, the true underlying total cholesterol value in the serum of a patient. Data Sources. —Representative references on each source of variation were selected from more than 300 reviewed publications, most published within the past 5 years, to document current findings and concepts. Most articles reviewed were in English. Study Selections. —Studies on biological sources of variation were selected using the following criteria: representative of published findings, clear statement of either significant or insignificant results, and acquisition of clinical and laboratory data under standardized conditions. Representative results for special populations such as women and children are reported when results differ from those of adult men. Data Extraction. —References were selected based on acceptable experimental design and use of standardized laboratory lipid measurements. Data Synthesis. —The lipid levels considered representative for a selected source of variation arose from quantitative measurements by a suitably standardized laboratory. Statistical analysis of data was examined to assure reliability. The proposed method of estimating the biological coefficient of variation must be considered to give qualitative results, because only two or three serial specimens are collected in most cases for the estimation. Conclusions. —Concern has arisen about the magnitude, impact, and interpretation of preanalytical as well as analytical sources of variation on reported results of lipid measurements of an individual. Preanalytical sources of variation from behavioral, clinical, and sampling sources constitute about 60% of the total variation in a reported lipid measurement of an individual. A technique is presented to allow physicians to qualitatively estimate the intraindividual biological variation of a patient from the results of two or more specimens reported from a standardized laboratory and to determine whether additional specimens are needed to meet the National Cholesterol Education Program recommendation that the intraindividual serum total cholesterol coefficient of variation not exceed 5.0. A National Reference Method Network has been established to help solve analytical problems. ( JAMA . 1992;267:1652-1660)

Blood Lipid Measurements

To describe the magnitude and impact of the major biological and analytical sources of variation in serum lipid and lipoprotein levels on risk of coronary heart disease; to present a way to qualitatively estimate the total intraindividual variation; and to demonstrate how to determine the number of specimens required to estimate, with 95% confidence, the "true" underlying total cholesterol value in the serum of a patient. Representative references on each source of variation were selected from more than 300 reviewed publications, most published within the past 5 years, to document current findings and concepts. Most articles reviewed were in English. Studies on biological sources of variation were selected using the following criteria: representative of published findings, clear statement of either significant or insignificant results, and acquisition of clinical and laboratory data under standardized conditions. Representative results for special populations such as women and children are reported when results differ from those of adult men. References were selected based on acceptable experimental design and use of standardized laboratory lipid measurements. The lipid levels considered representative for a selected source of variation arose from quantitative measurements by a suitably standardized laboratory. Statistical analysis of data was examined to assure reliability. The proposed method of estimating the biological coefficient of variation must be considered to give qualitative results, because only two or three serial specimens are collected in most cases for the estimation. Concern has arisen about the magnitude, impact, and interpretation of preanalytical as well as analytical sources of variation on reported results of lipid measurements of an individual. Preanalytical sources of variation from behavioral, clinical, and sampling sources constitute about 60% of the total variation in a reported lipid measurement of an individual. A technique is presented to allow physicians to qualitatively estimate the intraindividual biological variation of a patient from the results of two or more specimens reported from a standardized laboratory and to determine whether additional specimens are needed to meet the National Cholesterol Education Program recommendation that the intraindividual serum total cholesterol coefficient of variation not exceed 5.0. A National Reference Method Network has been established to help solve analytical problems.

Quality Goals for Hormone Testing

Estimates of intra-individual biological variation in normal subjects have been made for 17 hormones commonly measured for diagnostic purposes and the results have been compared with state-of-the-art analytical imprecision data. The implications of using these results for setting goals for analytical performance are discussed.

Evaluation of medically significant error limits and quality control procedures on a multi-channel analyser

Quality control (QC) programs are designed to monitor the precision and accuracy of the analytical process. Goals Tor analytical accuracy and precision have mainly been derived from “state of the art” performance, opinions of clinicians and laboratory professionals and more recently from intra-individual variation data. This study derived medically significant error limits based on both clinicians’ opinions and intra-individual variation data. The performance of both a standard QC rule (mean ± 3 sd, 1/3s rule) and an improved QC system (mean and range rules, 4 controls per run) in the detection of medically significant errors when using a SMAC multi-channel analyser was investigated. Probability of critical size error detection was calculated using power function curves. It was found that when error limits based on clinicians’ opinions were used, error detection was satisfactory when the mean and range QC system was applied but unsatisfactory for some analytes when the 1/3s rule was used. Error detection was unsatisfactory when error limits based on intra-individual biological variation were used and this could not be improved by using better QC systems. The limiting factor is analytical precision. The use of error limits based on biological variation as proposed by major clinical chemistry professional bodies cannot be achieved using most current types of analytical technology.

Intra-individual biological variation in the N-acetyl-beta-glucosaminidase/creatininium ratio for first morning urine.

Intra-individual biological variation in the N-acetyl-beta-glucosaminidase/creatininium ratio for first morning urine.

Intra-individual variation in plasma and erythrocyte cholinesterase activities and the monitoring of uptake of organo-phosphate pesticides.

The measurement of plasma and erythrocyte cholinesterase activities is used to monitor absorption of anti-cholinesterase organo-phosphorus compounds. A fall in an individual's enzyme activity signifies excessive exposure if it is clearly greater than the normal intra-individual variation found in unexposed subjects. The extent of normal variation in enzyme activity as measured is dependent on both the true intra-individual biological variation and the precision of the method used. This report defines normal variation in plasma and erythrocyte cholinesterase activity using the assay developed and used routinely in our laboratory. We have also defined the relation between the precision of the assay used by a laboratory and the sensitivity with which a significant depression in successive enzyme measurements can be detected. This allows occupational physicians, who use cholinesterase measurements to monitor organo-phosphate exposure, to establish percentage depressions from their method precision data that may possibly indicate organo-phosphate uptake between successive enzyme measurements. We have calculated that, with our analytical precision, percentage drops between two successive measurements that are greater than 15 and 7.5 per cent for the plasma and erythrocyte enzymes respectively suggest significant organo-phosphorus absorption in pesticide workers.

Response to the Proposal of Setting Goals for Imprecision Based on Average Intraindividual Biologic Variation

Response to the Proposal of Setting Goals for Imprecision Based on Average Intraindividual Biologic Variation

Desirable standards for hematology tests: a proposal.

Few analytic goals have been proposed for hematology tests, and these are based upon subjective clinical opinions. It is proposed that the current consensus strategies for delineating analytic goals in clinical chemistry can be validly used in the other disciplines of clinical pathology. Goals for imprecision are best based upon average intraindividual biologic variation; such goals are more stringent than those previously published. There are many cogent reasons that justify the goal for inaccuracy of no bias. In consequence, goals for imprecision should be viewed as goals for total laboratory error. Further experimental work on biologic variation is required to develop a wider range of goals for hematology and coagulation tests, if these strategies are considered worthy of adoption.