Boehringer Mannheim Diagnostics Research Articles

Chemical exposure in garage workers and related health risks on the biochemical levels: A comparative study in Harar town, eastern Ethiopia.

Objectives:Occupational exposure to chemicals causes a wide range of biological effects depending on the level and duration of exposure. The current study is intended to determine the differences in biochemical levels among garage workers compared with occupationally nonexposed participants in Harar town, eastern Ethiopia.Methods:A comparative cross-sectional study was conducted in Harar town, eastern Ethiopia. Thirty (30) garage workers were selected and compared with 30 age- and sex-matched control group of teachers and students. Demographic and occupational data were collected using a structured questionnaire by trained data collector. Biochemical levels were measured by automated clinical chemistry analyzer (Autolab 18, Boehringer-Mannheim Diagnostics, the United States). Data were analyzed using STATA Version 13.Results:All of the included garage workers were male. A statistically significant increase were found in alanine aminotransferase (35.60 ± 7.93 vs 19.17 ± 0.91 U/L; P value = 0.0440), aspartate aminotransferase (47.23 ± 4.89 vs 27.03 ± 1.13 U/L; P value = 0.0002), total protein (85.83 ± 1.16 vs 76.40 ± 0.86 g/l; P value < 0.0001), uric acid (7.34 ± 0.29 vs 5.19 ± 0.21 mg/dl; P value < 0.0001), glucose (85.13 ± 3.92 vs 75.60 ± 2.40 mg/dl; P value = 0.0425); total cholesterol (199.40 ± 13.11 vs 140.37 ± 3.81 mg/dl; P value = 0.0001) and triglyceride (143.40 ± 5.79 vs 110.60 ± 8.98 mg/dl; P value = 0.0033) in garage workers compared with control group. On the contrary, a statistically significant decrease were found in albumin (39.37 ± 1.78 vs 46.37 ± 0.56 g/l; P value = 0.0004) and urea (21.63 ± 1.04 vs 27.60 ± 1.69 mg/dl; P value = 0.0039) among garage workers compared with the control group.Conclusion:Our finding indicates that working in the garage changes most of the biochemical levels. Therefore, appropriate and effective safety measures need to be implemented to prevent possible chemical exposure during routine work.

Localization of genes for V+LDL plasma cholesterol levels on two diets in the opossum Monodelphis domestica

Plasma cholesterol levels among individuals vary considerably in response to diet. However, the genes that influence this response are largely unknown. Non-HDL (V+LDL) cholesterol levels vary dramatically among gray, short-tailed opossums fed an atherogenic diet, and we previously reported that two quantitative trait loci (QTLs) influenced V+LDL cholesterol on two diets. We used hypothesis-free, genome-wide linkage analyses on data from 325 pedigreed opossums and located one QTL for V+LDL cholesterol on the basal diet on opossum chromosome 1q [logarithm of the odds (LOD) = 3.11, genomic P = 0.019] and another QTL for V+LDL on the atherogenic diet (i.e., high levels of cholesterol and fat) on chromosome 8 (LOD = 9.88, genomic P = 5 x 10(-9)). We then employed a novel strategy involving combined analyses of genomic resources, expression analysis, sequencing, and genotyping to identify candidate genes for the chromosome 8 QTL. A polymorphism in ABCB4 was strongly associated (P = 9 x 10(-14)) with the plasma V+LDL cholesterol concentrations on the high-cholesterol, high-fat diet. The results of this study indicate that genetic variation in ABCB4, or closely linked genes, is responsible for the dramatic differences among opossums in their V+LDL cholesterol response to an atherogenic diet.

Genotype-by-diet effects on co-variation in Lp-PLA2 activity and LDL-cholesterol concentration in baboons fed an atherogenic diet

Both lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) activity, a biomarker of inflammation, and concentration of its primary associated lipoprotein, LDL, are correlated with adverse coronary outcomes. We previously reported a quantitative trait locus (QTL) corresponding to HSA2p24.3-p23.2 with pleiotropic effects on Lp-PLA(2) activity and LDL-cholesterol (LDL-C) concentration in baboons fed a basal diet. Here, our goal was to locate pleiotropic QTLs influencing both traits in the same baboons fed a high-cholesterol, high-fat (HCHF) diet, and to assess whether shared genetic effects on these traits differ between diets. We assayed Lp-PLA(2) activity and LDL-C concentration in 683 baboons fed the HCHF diet. We used a bivariate maximum likelihood-based variance components approach in whole-genome linkage screens to locate a QTL [logarithm of odds (LOD) = 3.13, genome-wide P = 0.019] corresponding to HSA19q12-q13.2 with pleiotropic effects on Lp-PLA(2) activity and LDL-C levels in the HCHF diet. We additionally found significant evidence of genetic variance in response to diet for Lp-PLA(2) activity (P = 0.0017) and for LDL-C concentration (P = 0.00001), revealing a contribution of genotype-by-diet interaction to covariation in these two traits. We conclude that the pleiotropic QTLs detected at 2p24.3-p23.2 and 19q12-q13.2 on the basal and HCHF diets, respectively, exert diet-specific effects on covariation in Lp-PLA(2) activity and LDL-C concentration.

The Nonlinearity Seen for LDL-Cholesterol with Lyophilized Material Is a Matrix Effect

Recently, Genzyme Corporation (Cambridge, MA) developed and patented a direct method for LDL-cholesterol (LDL-C) determination. The method uses an antibody to separate LDL from VLDL and HDL. Because the method is analogous to that for HDL-cholesterol (HDL-C) in that it requires a separation step and that matrix effects have been documented to interfere with the assessment for linearity for that analyte, we investigated the effect matrix has on the evaluation of linearity for the direct measurement of LDL-C (1). We determined LDL-C on the Cobas FARA (Roche Diagnostic Systems), using cholesterol reagents: cholesterol esterase and oxidase (Boehringer Mannheim Diagnostics) and an LDL-C kit (Sigma Diagnostics). We tested linearity, using the following materials: CAP Linearity Survey material LN2–21 (College of American Pathologists, Northfield, IL); reconstituted according to instructions, mixed high- and low-concentration vials to make five, equally-spaced in concentration solutions; HDL-C and LDL-C control material (Sigma Diagnostics); and Enzyme diluent (DuPont Clinical Diagnostic …

The percentage of free prostate specific antigen is an age-independent tumour marker for prostate cancer: establishment of reference ranges in a large population of healthy men.

To define the reference range for the ratio of free to total prostate-specific antigen (fPSA%) in a population of healthy men with no clinically evident prostate cancer and to assess the influence of age on this tumour marker, thus determining the utility of fPSA% in enhancing the discriminatory power of PSA to differentiate healthy men and patients with benign prostatic hyperplasia from those with prostate cancer. In a prospective cohort study between May and August 1996, 1160 white men aged 20-89 years (957 were 40-69 years old, 82% of all subjects) from nine European and eight non-European countries were assessed. None of the participants who had a history of prostate cancer had undergone prostatectomy. A standard clinical examination including a digital rectal examination was performed to exclude the presence of prostate cancer. Transrectal ultrasonography was not an inclusion criterion, as it was not available in every case. Total PSA (tPSA) and free PSA (fPSA) were determined in 61 laboratories using the appropriate Enzymun-Test for tPSA and fPSA (Boehringer Mannheim Diagnostics, Mannheim, Germany). Serum tPSA, fPSA and fPSA% were then assessed as a function of the subjects' age. The serum tPSA and fPSA were significantly different among age decades 2-8 (P < 0.001), with increasing median values, indicating that both variables depend on age. The recommended upper reference limit (95th percentile) for tPSA is 1.78 ng/mL for men aged 30-39 years, 1.75 ng/mL for 40-49 years, 2.27 ng/mL for 50-59 years, 3.48 ng/mL for 60-69 years and 4.26 ng/mL for 70-79 years. The fPSA% was not significantly different between decades 3-8 (P = 0.06). Those aged 20-29 years had a slightly higher median value (P = 0.03) than the other age groups. The recommended lower reference limit (fifth percentile) for fPSA% is 12.6%. The fPSA% for clinically relevant age groups in healthy men was independent of age, which simplifies the use and interpretation of this relatively new tumour marker.

New Immunoelectrochemiluminometric Assay to Measure Serum Thyrotropin,

The log/linear relation between serum thyrotropin (TSH) and free thyroxine (FT4) makes TSH, when measured by a highly sensitive method, a more sensitive indicator of thyroid disfunction than FT4 (1). Nonisotopic immunometric assays are the methods of choice to measure serum TSH, as they are at least equally sensitive to IRMAs, can be automated, are generally faster and more convenient than IRMAs, and do not use radioisotopes. Within the former group of methods, the immunochemiluminometric assay is increasingly popular because of its inherent sensitivity, but instrumental and triggering modes may jeopardize it. A new immunoelectrochemiluminometric assay (IECMA) has recently come on the market (2). It is based on electrogenerated chemiluminescence that, aiming to avoid the instability of luminophores, does not proceed in free solution but on to the surface of a platinum electrode (3)(4). We describe the evaluation of such a method using the random access analyzer Elecsys® 2010 (Boehringer Mannheim Diagnostics). The method consists of two immunological “sandwich” reactions linking, at one side, sample TSH to a solid phase of paramagnetic streptavidin-coated microparticles by means of a biotinylated monoclonal TSH-specific antibody and, at the other side, a Ru-labeled TSH-specific antibody. Separation of bound and unbound TSH is made by a magnet, and luminescence from a luminophore is triggered electrochemically. The immunological reaction consists of a first incubation where 50 μL of sample, the biotinylated antibody, and the Ru-labeled antibody react for 9 min, when antibodies capture the TSH present in the sample to form a sandwich complex. In a second step, streptavidin-coated paramagnetic microparticles are added, and during a second 9-min incubation time the biotinylated antibody attaches to the streptavidin-coated surface of the microparticles. The reaction mixture …

Effect of Hemopure® on the Performance of Ektachem and Hitachi Clinical Analyzers

Hemopure®, an ultrapurified polymerized hemoglobin-based oxygen-carrying solution, is currently undergoing evaluation in elective surgery patients (1). This experimental drug may obscure routine clinical chemistry assays because it is naturally pigmented and circulates in the plasma (2). Because chemistry analyzers use colorimetric methods for many of their tests, the presence of Hemopure in a sample may cause interference in the assay, depending upon the instrument used, the methodology, and the reagents. To obtain accurate chemistry results for both the clinical study and for patient management, it was necessary to determine the extent of these interferences. At each clinical trial site, the chemistry analyzers were challenged with samples containing Hemopure up to 50 g/L to determine the maximum amount of Hemopure that may be present in a sample to obtain accurate results for each test. Hitachi (Boehringer Mannheim Diagnostics) and Ektachem/Vitros …

Beyond viruses A to E: possible new causes of hepatitis identified

Despite the discovery of several hepatitis-related viruses, the cause of hepatitis infection in some 15% of cases continues to baffle physicians. Extensive international research efforts have been channelled into identifying and characterising causative agents other than the now-familiar hepatitis A-E viruses. The results of this research have led to more than one candidate. Details of the ‘new’ pathogens were presented at satellite symposia sponsored by Boehringer Mannheim and Abbott Diagnostics, at the 3rd International Meeting on Hepatitis C Virus and Related Viruses [Gold Coast, Australia; August 1995].

Nonlinearity of high-density lipoprotein cholesterol determinations is matrix dependent

The majority of methods for determining high-density lipoprotein (HDL) cholesterol failed the 1991 College of American Pathologists (CAP) linearity survey (sets LN2-A, B, C). We hypothesized that they failed because of the survey material matrix. We evaluated linearity with dextran sulfate or phosphotungstate as the precipitating reagents for several methods: Ektachem; TDx; Sigma; Dimension; Cobas Fara, with Roche reagents; and the Hitachi 736, with precipitating reagents from Boehringer Mannheim Diagnostics. We tested CAP survey material, Sigma HDL-cholesterol control material, and a fresh serum pool for linearity, using the polynomial method. All of the methods were nonlinear for the CAP material and for the controls precipitated after dilution. Five of eight methods were linear for the control materials precipitated before dilution. All methods but one were linear for the serum pool. These results demonstrate that the source of the nonlinearity is located in the precipitation step and depends on the sample matrix.

Creatinine assay by the fuller's earth procedure or by enzymatic determination is adequate for urine but not plasma of mice

1. 1. Mouse plasma creatinine measured by fuller's earth (15.1 ± 0.5(41) μM, mean ± SE) exceeded the same pools read by HPLC [7.5 ± 0.2(41); P < 0.001]. 2. 2. For mouse urine, the fuller's earth method (3.26 ± 0.21 mM) slightly exceeded the HPLC method (2.65 ± 0.18; P < 0.001) with good correlation ( r = 0.915, P < 0.001). 3. 3. An enzymatic method (creatinine PAP, Boehringer-Mannheim Diagnostics) for plasma creatinine (5.2 ± 0.3 μM) was equivalent to HPLC (6.1 ± 0.1, NS) but with poor correlation ( r = 0.23, NS). 4. 4. Creatinine in the urine agreed closely between HPLC (2.84 ± 0.12 mM) and the enzymatic procedure (2.95 ± 0.17, NS) with good correlation ( r = 0.845, P < 0.001).

The Influence of Hematocrit Uremia, and Hemodialysis on Whole Blood Glucose Analysis

Blood glucose concentrations were simultaneously measured on venous whole blood and plasma specimens from 207 hospitalized patients, 32 of whom were undergoing hemodialysis due to end-stage renal disease. Whole blood glucose values were obtained by a glucose oxidase-peroxidase electrode system (Stat Profile 5, Nova Biomedical Corp, Waltham, Mass) and by two glucose oxidase-peroxidase–based dry reagent reflectometry systems (Accu Chek II, Boehringer Mannheim Diagnostics, Indianapolis; One Touch, Lifescan Inc, Milpitas, Calif). Glucose, hematocrit, urea nitrogen, creatinine, and osmolality levels were determined on predialysis and postdialysis specimens. The three whole blood systems tested demonstrated linearity of response over the glucose range of 50 to 550 mg/dL, and had good correlation with plasma glucose values ( r >0.988). The meter systems, however, had a statistically significant negative proportional bias, which was eliminated after correcting for red cell volume displacement. Hematocrit values of 20% to 60%, 22.5% to 55%, and 20% to 55%, did not significantly affect the precision or accuracy of the Stat Profile 5, Accu Chek II, or One Touch systems. Elevated hematocrit values negatively affected the dry reagent strip systems, contrary to what one might predict as the effect of volume displacement. The uremia of end-stage renal disease depressed the strip glucose values by about 9%, without affecting the electrode system. Hemodialysis reversed the depression of glucose values with the Accu Chek II system, but not with the One Touch system. Despite this decrease in reactivity, these systems appear suitable for use with patients undergoing hemodialysis.

Evaluation of the Boehringer Mannheim ES 300 immunoassay analyzer and comparison with enzyme immunoassay, fluorescence polarization immunoassay, and radioimmunoassay methods

The ES 300 (Boehringer Mannheim Diagnostics, Indianapolis, IN) is a new automated immunoassay analyzer intended for the quantitative determination of a wide range of analytes. We compared its performance to enzyme immunoassay (EIA), fluorescence polarization immunoassay (FPIA), and radioimmunoassay (RIA) methods for cortisol, digoxin, ferritin, prolactin, T4-uptake, total-T3, and TSH. The ES 300 methods showed excellent precision and the manufacturers' linearity claims were met in all cases. Cortisol, prolactin, total-T3, and TSH showed no bias and acceptable correlation with other methods. Digoxin, ferritin and total T4 showed positive bias but acceptable correlation. The ES 300 T4-uptake correlated poorly with the TDx method and showed positive bias; however, these assays appear comparable (although deficient) in diagnostic sensitivity when compared to TSH and T4 data for the same patient population. In all, we found the ES 300 to be an acceptable instrumental alternative for the high volume immunoassay laboratory.

Total Blood Cholesterol and Contributory Risk Factors in an Adolescent Population

Total blood cholesterol (TBC) levels and contributory risk factors in an adolescent population were investigated. Existing TBC screening records were reviewed on 452 10th grade students in two schools. The sample consisted of 52% males and 48% females whose mean age was 15.47 years. Blood samples were analyzed by the Reflotron. Risk factors investigated included age, gender, ethnicity, individual and family history of high cholesterol, history of high blood pressure, smoking tobacco products, and oral contraceptive use. The sample mean for TBC was 150.61. The only significant factors identified by ANOVA were gender and use of oral contraceptives. Females had higher TBC levels than males, and females who used oral contraceptives had higher TBC levels than nonusers.

Clinical evaluation of an automated chemical inhibition assay for lactate dehydrogenase isoenzyme 1

We evaluated an automated assay for lactate dehydrogenase (LD; EC 1.1.1.27) isoenzymes, supplied by Boehringer Mannheim Diagnostics (BMD) and based on selective chemical inhibition of non-LD-1 isoenzymes by guanidine thiocyanate. Results were compared with the Roche Isomune LD-1 method. The Hitachi 717 analyzer was used to measure enzyme activity for both procedures in 229 serum samples. One hundred specimens were also analyzed by the Helena rapid electrophoresis (REP) method. We determined the limit of linearity of the BMD method to be about 1200 U of LD-1 per liter. The analytical correlation of BMD (y) with Isomune (x) yielded y = 1.0x + 0.5 U/L, r = 0.997, Sy/x = 16.9 (range 20-1397 U/L). The regression equation for BMD vs REP was y = 1.1x + 7.2% (r = 0.800, Sy/x = 7.4, range 14-83%). Average values for within-run precision for low (38 U/L), medium (180 U/L), and high (865 U/L) controls were 4.1%, 1.0%, and 0.5%, respectively (16 trials of six each). The average values for run-to-run precision were 4.1%, 1.7%, and 1.1%, respectively, for these controls (n = 16). We used receiver-operating characteristic curves to determine optimum decision limits. Using an LD-1 cutoff of 40% of total LD, we obtained a clinical sensitivity of 97-100% and a specificity of 95% when blood was collected during the optimum interval, 24-48 h after the onset of chest pain. We conclude that the BMD LD-1 assay is equivalent to the immunochemical and electrophoretic assays for measuring the LD-1 isoenzyme.

Cholesterol screening: comparative evaluation of on-site and laboratory-based measurements

We measured cholesterol in capillary blood samples from 9683 volunteers over a four-day on-site community screening program, using the "Reflotron" desk-top analyzer (Boehringer-Mannheim Diagnostics, Indianapolis, IN). We also measured cholesterol in venous blood samples from 3% of those screened (a) with the Reflotron at the screening sites, (b) in a qualified hospital clinical laboratory, and (c) in a Centers for Disease Control standardized lipoprotein research laboratory. The sensitivity (and specificity) of the Reflotron measurements, with use of the lipoprotein laboratory measurements as the point of reference, was 0.95 (0.73) in capillary blood samples and 0.88 (0.93) in venous blood samples, compared with 0.99 (0.87) in the hospital clinical laboratory. The Reflotron measurements correlated less well with the lipoprotein laboratory values in both venous blood (r = 0.91) and capillary blood (r = 0.89) samples than did the clinical laboratory values (r greater than 0.99). Furthermore, the capillary blood measurements averaged 7% higher than venous measurements when both kinds of samples were analyzed in the Reflotron.

Assessment of renal function by serum creatinine and creatinine clearance: glomerular filtration rate estimated by four procedures.

We compared creatinine concentrations in serum and urine and creatinine clearances determined by two Jaffé (Beckman's "Astra," Boehringer Mannheim Diagnostics) and two enzymatic (Kodak, Boehringer Mannheim Diagnostics) methods. Serum creatinine and creatinine clearances determined by each method were also compared with the glomerular filtration rate as measured with use of sodium [125I]iothalamate in patients with a wide range of renal function. Results between methods correlated excellently, but we saw clear method-dependent biases of up to 2.9 mg/L for serum. The highest serum creatinine values and the lowest creatinine clearances were obtained with Boehringer Mannheim Diagnostics' Jaffé method. The reciprocal of the serum creatinine and the creatinine clearance also correlated well with the glomerular filtration rate, but all methods over-estimated the glomerular filtration rates to varying degrees. Appropriate standardization of methods appears to be as important as method principle for establishing an accurate relationship between creatinine determinations and glomerular filtration rate.

Lipase activity measured in serum by a continuous-monitoring pH-Stat technique--an update.

Using recent knowledge regarding the roles of colipase, bile acids, Ca2+, and emulsifiers, we optimized a previously published pH-Stat method for lipase (EC 3.1.1.3) activity measurements. The recommended assay conditions are: olive oil/triolein, 100 mL/L; sodium glycocholate, 35 mmol/L; Ca2+, 8.5 mmol/L; and colipase, 6.0 mg/L. The sample volume is 0.10 mL, the reaction pH 9.0, the temperature 30 degrees C, and the concentration of titrant 15 mmol/L. Hydroxypropyl methylcellulose, 20 g/L, replaces acacia as emulsifier to avoid inhibition by excess Ca2+. The standard curve is linear to greater than 4566 U/L. The reference interval with olive oil as substrate is 30-235 U/L. Lipase activities with triolein substrate are 9.9% greater than with olive oil. Interference by pancreatic carboxylesterase (EC 3.1.1.1) activity is inhibited by incubating the sample with diisopropylfluorophosphate. Results correlate well with those by the optimized SingleVial method of Boehringer Mannheim Diagnostics (r = 0.997) and the immunochemical assay of Beckman Instruments, Inc. (r = 0.995). Correlation with the aca method (E.I. DuPont de Nemours & Company) is less satisfactory (r = 0.892), probably owing to lack of colipase in the latter method.

Immediate Alanine Aminotransferase Testing for Donor Screening: Use of the Reflotron on Blood Drives

Screening blood donors for elevated levels of alanine aminotransferase (ALT) is required to help reduce the incidence of transfusion-transmitted non-A, non-B hepatitis. A study of the cost-effectiveness and usability of the Reflotron (Boehringer-Mannheim Diagnostics, Indianapolis, Ind), an on-site instrument for measuring ALT, is reported. On four blood drives, 118 donors were tested, with a rejection rate of 4.2%. The Reflotron was found to be cost-saving in terms of the College of American Pathologists workload units at a rejection rate of 0.85% or greater. Advantages regarding utility include portability, easy oper ability, and savings on donor time. The disadvantages (single-specimen analysis and lack of sample identification) are relatively minor and can be overcome. We conclude that the Reflotron is a convenient and cost-effective instrument for use in donor screening.

Standardization of a commercial (Boehringer Mannheim diagnostics) enzymic method for cholesterol.

The Laboratory Standardization Panel of the National Cholesterol Education Program recommends that cholesterol method accuracy ideally be within 3% of the true value determined by the Abell-Kendall Reference Method, a component of the National Reference System for Cholesterol. As one of the Abell-Kendall network laboratories established to facilitate cholesterol standardization, the approach we recommend for determining accuracy involves a comparison analysis on patients' specimens by the method in question and by the Abell-Kendall method. Use of fresh specimens precludes matrix interactions that may influence enzymic measurement. Using this approach, we assessed an enzymic method for cholesterol with two instruments (Boehringer Mannheim/Hitachi 717 and 737), with BMD reagent, controls, and calibrator. Fresh and frozen sera were analyzed with both instruments over three days. The Abell-Kendall method was used at the Northwest Lipid Research Center on frozen aliquots of the same sera. Both instruments demonstrated good agreement with the Reference Method, as determined by linear regression; overall bias averaged less than -2% for the Hitachi 717 and -1% for the Hitachi 737 at 2000 mg/L--i.e., within the accuracy recommendation. We observed a difference in bias for fresh and frozen specimens; with the Hitachi 717, fresh specimens exhibited -3% bias at 2000 mg/L, but there was virtually no bias of determinations of frozen specimens.

Reflotron cholesterol measurement evaluated as a screening technique.

We evaluated the analytical performance of Boehringer Mannheim Diagnostics' "Reflotron" analyzer for the measurement of cholesterol. Coefficients of variation (CVs) for whole-blood cholesterol were: within-day 2.0% and 2.2% at 1680 and 2670 mg/L, respectively; between-day 1.8% and 2.4% (n = 9 and 8). Results were similar for serum and heparinized or EDTA-treated single-donor plasma (CV 1.4% to 2.6%). CVs of results for two reconstituted commercial quality-control materials were 3.4% and 4.6%. Heparin and hematocrit were evaluated as interferents, and critical limits for interference were identified for bilirubin, hemoglobin, and triglyceride in blood and plasma or serum. When sample collection and analysis were controlled by trained personnel, results with the Reflotron (y) compared well with those by the Ektachem procedure (x) for both blood and serum samples: r = 0.950, y = 0.944x + 130 mg/L; and r = 0.955, y = 0.93x + 43.5 mg/L, respectively. The same comparability was observed when the analysis was performed by briefly trained high-school students: r = 0.980, y = 0.949x + 23 mg/L. Performance decreased when both collection and analysis were performed by laymen: r = 0.880, y = 0.870x + 186 mg/L.