videdthe limitsofprecision are known and are acceptable. It has become common practice to verify quantitative accuracy and linearity at a threshold concentration by analyzing calibrators or controls prepared at concentrations one-half and twice the cutoff value. It should be appreciated that use of these samples provides neither special information regarding accuracy “at or near” the cutoff nor greater confidence in the results. The hypothesis that the midpoint concentration on a linear calibration graph is the most accurate on the curve when bracketed by these specific lower and higher concentrations is not scientifically supportable. Within the limits of precision, all points on a linear calibration graph are equally reliable and accurate, as long as the upper and lower limits of linearity are known. The number of calibrators necessary and their concentrations will depend on the methods used and the intended use of the analytical results. What is important in forensic applications in which a cutoff is defined administratively is the precision of the assay at that concentration. When a series of calibrators are analyzed by immunoassay, the calibration curve generated is curvilinear. The cutoff concentration should be on the linear portion of the curve, the slopeof which permits discrimination between positiveand negative samples within established statistical limits. Precision at this portion of the calibration curve is, therefore, very important. This must be established and assessed with each batch of urine specimens by the use of controls. A control with a validated concentration of the analyte at the cutoff value, another slightly below, and one slightly above the cutoff (approximately plus and minus 3 SD) should be more than sufficient todefine the precision of the assay at the cutoff concentration. Currently,many analytical technologies are used to analyze biological fluids Many for drugs clinical made daily that ical implications For employees work and even affected, positive noassays must and their metabolites. determinations are have substantial med- for patients’ health. conditions of whose liberty are likely to be test results by immu- be confirmedby a second, independent test, such as GC-MS. These technologies are absolutely dependent on the concept of calibration linearity, accuracy, and precision throughout the concentration range required Accurate by the purpose of the and precisely defined assay. quan- titative or qualitative results are critical if physicians, employers, and lawyers are to make confident decisions. To consider that only one small region or a single concentration of the calibration graph defines the method as “accurate” is misleading and ignores the tested and established concept of multi-point calibration. The finaltest of the “accuracy”of any analytical technology is to actually apply the method to a large population of specimens and successfully defend the results obtained against scientific and legal challenges. These data are now available from some of the larger contract laboratories that analyze hundreds of thousands of urine samples for drugs of abuse each year, using several different analytical instruments and procedures, but all within the framework of NIDA and AACC/CAP accreditation guidelines. The indication is that, when properly calibrated by either multi-point, full-range curves or by arithmetic means at the threshold concentration, the methods are all satisfactory for these purposes. Bryan S. Finkle Center for Human Toxicol. 417 Wakara Way, Room 290 Salt Lake City, UT 84108 David Black Aegis Analytical Labs., Inc. 624 Grassmere Park Rd., Suite 21 Nashville, TN 37211 Robert V. Blanke Consultant Toxicologist 4222 Croatan Rd. Richmond, VA 23235 Thorne J. Butler 4230 South B urn ham Ave., Suite 250 Las Vegas, NV 89119 Graham R. Jones Office of the Chief Medical Examiner P.O. Box 2257 Edmonton, Alberta, Canada T5J 2P4 R. H. Barry Sample Sports Med. Drug ID Lab Univ. Hospital N440 635 Barrhill Dr. Indianapolis, IN 46223 Furosemide as a Displacing Agent In Assay of Total Trilodothyronine To the Editor: An assay for the measurement of total triiodothyronine (T3) has been developed for the Abbott IMx#{174} Analyzer (1). Reagents consist of human serum calibrators, a T3-alkaline phosphatase conjugate, anti-T3-coated microparticles, and substrate, 4-methylumbelliferyl phosphate. In the assay, calibrators or samples are incubated with the microparticle reagent,which contains an agent to displace T3 from serum binding proteins. The reaction mixture is transferred to the glass-fiber matrix and washed, and then the T3 conjugate is added. After incubation, the matrix is washed and the substrate added. The rate of formation of 4-methylumbelliferone (4-mu) is then measured. Total T3 concentration is inversely related to 4-mu formation. In preliminary evaluations, the displacing agents furosemide (2,3), 8-anilino-1-naphthalene sulfonic acid (ANS) (4), and fenclofenac (5) were compared. ANS was rejected because of potential quenching of 4-mu; fenclofenac, because of slightly inferior displacement ofT3 in the IMx system: 62% vs 71% for furosemide. In determining the optimal amount of furosemide to use, we varied its concentrationin the microparticle reagent from 0 to 1 gIL. Furosemide at 125 mg/L providedmaximum T3 displacement without interferingwith the bindingof the T3 conjugate to the antibody-coated particles. At this optimal concentration: #{149} T3 at 8 and 100 p.g/L was displaced 85% and 98%, respectively. #{149} Mean analytical recovery of T3 added at four concentrations to four separate samples was 105% (range 95-112%). #{149} Mean analytical recovery of three separate samples diluted with zero calibrator to <0.3 ng/L (range 96-105%); original was 100% T3 content was 4.0, 2.6,and 2.0 pg/L. #{149} Results obtainedby the IMx assay (y) compared well with those by two commercially availableRLAs (x): No. of samples Slope y.Intercept, p.gIL r Amersham 166 1.20 0.10 0.97 Dalnabot 84 1.00 -0.18 0.98 Thus, furosemide appears to be an excellent displacing agent for measuring total T3 in serum. Presumably, furosemide would be equally effective in displacing thyroxin and other analytes bound to similar sites in serum or plasma. CLINICAL CHEMISTRY, Vol. 37, No. 4, 1991 587 References 1. Groskopf W, Hsu S, Sohn L. A fully automated assayfortotal T3 utilizing the AbbottIMx analyzer [Abstract]. ClinChem 1988;34:1210. 2. Stockigt JR, Lim C-F, Barlow JW, et al. High concentrations of furosemide inhibit serum binding of thyroxine. J Clin Endocrinol Metab 1984;59:62-6. 3. Stockigt JR. Lim C-F, Barlow JW, et al. Interaction of furosemide with serum thyroxine-binding sites: in vivo and in vitro studies and comparison with other inhibitors. J Clin Endocrinol Metab 1985;60: 1025-31. 4. Chopra LI, Ho RS, Lam R. An improved radioiminunoassay of triiodothyronine in serum: its application to clinical and physiological studies. J Lab Clin Med 1972; 80:729-39. 5. Rataliffe WA, Hazelton JA, Thomson JA, Ratcliffe JG. The effect of fenclofenac on thyroid function tests in vivo and in vitro. Clin Endocrinol 1980;13:569-75. W. Groskopf B. Green L. Sohn S. Hsu Abbott Laboratories Abbott Park, IL 60064 Potential Problem wIth StandardizatIon of Prealbumin (Transthyretln) Results In immunoturbldlmetry To the Editor: Recently, several centers have described immunoturbidimetric (IT) assays for measuring prealbumin (1, 2) in serum with centrifugal analyzers. We have encountered a potential problem in calibrating one such method and interpreting the results. We used Dakopatts rabbit anti-human prealbuinin antiserum (no. Q362) and Dakopatts Calibrator (no. X908) to set up an IT method (3) for the Cobas-Fara’TM (Roche Analytical Instruments Inc., Nutley, NJ 07110) centrifugal analyzer. Despite obtaining what appeared to be an excellent calibration curve, we found a lower than expected value for Standard Human Serum (no. ORDT 06/07) from Behringwerke (Hoechst UK Ltd., Middlesex TW4 6JH, U.K.). All results are shown in Table 1. Reversing the situ- ation and using dilutions bring product to calibrate of the Bethe method gave a predictably higher valueforthe Dako calibrator, demonstrating that this was neither a dilution error nor a problem with the IT methodology; we repeated the assay, using M-Partigen immunodiffusion plates (Behringwerke) for estimation of prealbumin, Table 1. BehavIor of Prealbumln CalIbrators In Immunoturbldlmetry CalIbration/control materIal !mmunoturbidimetry Behnng standard serum Dakopatts calibrator Manufacturer’s assigned value, g/L Spol SPO1 Behnng standard serum Dakopatts calibrator Calibration Assay result, g/L 0.26 Dakopatts 0.16 0.25 0.25 0.25 0.26 Behnng Dakopatts Behring SPO1 0.32 0.26 0.34 0.14 0.25 SPO1 0.26 0.25 0.25 Behring Behring 0.31 0.33 M-Partigen immunodiffusion Spol Dakoplatts calibrator calibrated with Behring Standard man Serum. The Dakopatts Hu- calibrator gave a value of 0.33 g/L, not the manufacturer’s stated value of0.25 g/L. Hamlin and Pankowsky (1) describeda similar discrepancy between calibrationmaterials from Beckman and Behring.They assayed the Beckman calibration material by iT with the Behring calibrator for standardination, and obtained a result about 16% higher than the given value. They also found that, unlike the Beckman calibrator, the Behring calibrator material apparently behaved differently in IT and nephelometric assays, giving consistently higher results with the IT method (22% positive bias). Which calibration material is correct and which reference limits should be taken? When we analyzed a third independent control (no. SPO1) obtained from the Supraregional Protein Service (Sheffield, U.K.), its assigned value was compatible with Dakopatts calibration and not with Behring Standard Human Serum. However, there is no internationally recognized prealbumin reference material for comparison. Reference ranges are another problem. Dakopatts recommends the ranges produced by Behring, namely, g/L for males and 0.1-0.4 g/L for females. Clearly, this is nonsense and the reference range for Dakopatts should be at least 25% less than these figures. Perhaps the only safe recourse is to produce a reference range at the local level, which has always been 0.2-0.5 good practicein the past. References 1. Hamlin CR, Pankowsky DA. Turbidimetric determination of transthyretin (prealbumin) with a centrifugal analyzer. Clin Chem 1987;33:144-6. 2. Konstantinides FN, Mitchell DR, Blixby E, et al. Imniunoturbidimetry of prealbumin (transthyretin) in a microcentrifugal analyzer [Tech Brief]. Clin Chem 1989;35:178-9. 588 CLINICAL CHEMISTRY, Vol. 37, No. 4, 1991 3. Ledue TB, Rifai N, Irish GR, Silverman LM. Immunoturbidimetry of transthyretin (prealbumin) in human serum [Tech Brief]. Clin Chem 1987;33:1260. J. Coore J. Ambler Dept. of Clin. Chem. Univ. Hospital, Queen’s Med. Ctr. Nottingham NG7 2UH, UJC. Two spokesmen spond: from Behring re- To the Editor: Drs. Coore and Ambler have identified a difference in standardization between two sources of prealbumin reagents and some ofits consequences. We agree that the difference exists and that such a difference can cause difficulties for users in assessing published reference ranges and in evaluating results of external proficiency surveys; this situation will continue until an international reference preparation for prealbumin becomes available. Standardization of Behringwerke prealbumin methods is based on transfer of values in a self-consistent manner from highly purified protein preparations to stable, serum-based secondary materials suitable for routine use. Such standardization is controlled-from lot to lot and across technologies-to provide a family of assay methods meeting the various demands imposed upon clinical laboratories yet able to produce equivalent results for patients’ samples (within the “state of the art”). Publications describe appropriate reference ranges for the various proteins. The reference range for prealbumin (1), established for Behringwerke’s products, is 0.250.45 g/L forboth males and females. Where international reference preparations are available, Behringwerke establishes conversion factors relating internal standards to international
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