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Immunoassay interferences: Can we do anything to minimize their impact on patient care?
POSTER ABSTRACTS were low in group D or when the TSH was 12.1-20.0 mU/L. Similar distributions were seen for FTI. Comparing FT4 and FTI, FI'I was clearly normal in 2 cases despite marked TSH increase and FT4 decrease. Otherwise only minor discordance was seen in 11 cases. N
IMMUNOASSAY INTERFERENCES: CAN WE DO ANYTHING TO MINIMIZE THEIR IMPACT ON PATIENT CARE? Ellis. G., Wan, B. S-Y., and Makela S. K., Department of Clinical Biochemistry, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada, M5G 1X8 We have recently found three patients with incorrect TSH (Ciba-Corning ACS 180), one with incorrect TSH (BioRad) and one with incorrect ACTH (IncStar). Several of these results lead to ucertainties about diagnosis and treatment. Details of these cases will be presented. We discovered the cases in several ways: 1) as outliers during method comparisons, 2) as patients who had immunoassay results that our physicians questioned because they were difficult to explain clinically, 3) as patients randomly selected whose sera did not dilute linearly for the constituent analysed, and 4) as patients that we selected because their sera had given problems with other assays. We assume that these interferences were caused by antibodies in the patients' sera directed against some reagents in the immunoassay system, such as anti-mouse or anti-rabbit antibodies (J Clin Chem Clin Biochem 1990; 28: 881-892). In some cases, there was a history of patient contact with the animal species of the primary antibody. In one case, we think that there was transplacental passage of interference, because serum TSH from a two-week-old infant did not dilute linearly. We recommend that laboratories: 1) re-assay at several dilutions any samples where physicians doubt the validity of the results, (dilution often reduces the interferent concentration to that at which interference no longer occurs), 2) repeat the analysis with an alternate method, preferably one that uses reagents from a different species, or that incorporates an extraction procedure and reconstitution of the analyte, and 3) post the names of any patients whose sera have been found to interfere with specific assays, so that subsequent analyses can be done using appropriate steps to ensure valid results.
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EVALUATION OF THE MILES I M M U N O I ANALYSER Krahn, J., and Green, C., Department of Clinical Biochemistry, St.Boniface General Hospital, 409 Tache Avenue, Winnipeg, Manitoba, Canada, R2H 2A6 T3, T4, TSH, T3 uptake, HCG, LH, FSH, digoxin, cortisol and tobramycin assays were evaluated. The object of the
CLINICAL BIOCHEMISTRY, VOLUME 27, JUNE 1994
study was to determine 1. the precision and accuracy of the assays 2. the cross-reactivity of the appropriate assays 3. the instrument carryover 4. calibration stability 5. specimen and analyte integrity for specimens collected in a primary tube containing a serum separator gel 6. if the serum cortisol method can be used to measure urinary cortisol. The precision of all the assays is as good or better than the methods being currently used. Split-sample specimen comparisons showed two methods (LH and prolactin) where there was proportional error due to differences in standardization. This problem was solved by adjustment of results with a factor. None of the assays show cross-reactivities that would cause clinically relevant errors i.e. high HCG doesn't cross-react with the FSH, LH or TSH methods, and serum containing digoxin-like immunoreactive substances does not react with this digoxin method. The specimen carryover is only measurable for analytes like HCG where it is in the order of parts per million. Initial studies show that the Vacutainer serum separator tube does not interfere with any of the above assays. Finally, the serum cortisol method on this instrument can be used to do urinary free cortisol determinations if the urine is first extracted with dichloroethane, dried down and redissolved with the zero calibrator. Based on the above evaluation, the instrument has been successfully implemented. EVALUATION OF THE ACCESS ® IMMUNOASSAY ANALYZER DIGOXIN ASSAY Guitard. M., Service de biochimie, Hrpital du HautRichelieu, 920 boulevard du srminaire, Saint-Jean-surRichelieu, Qurbec, Canada, J3A 1B7 Rapid and accurate measurement of serum digoxin levels is usefull in the monitoring of digoxin therapy and acute intoxication. We evaluated the serum digoxin assay on the Access ® Immunoassay Analyzer (Sanofi Diagnostics Pasteur Inc., Chaska, MN) in terms of precision, sensitivity, linearity, DLIF (digoxin-like immunoreactive factor) interference and compared the Access ® results with those obtained with the Abbott TDx digoxin assay (Abbott Laboratories, Chicago, IL). The precision data over 28 days (26 runs in triplicate) was evaluated using tri-level control material (Ciba-Corning Diagnostics Corp., Irvine, CA) and is presented in the following table: Runs
Mean (nmol/L)
77 78 77
0,84 2,42 3,82
Within-run SD % CV 0,05 0.09 0,11
5,86 3,60 3,00
Between-run SD % CV 0,04 0,08 0,09
4,90 3,28 2,48
Total imprecision S.D. % C.V. 0,06 0,12 0,15
7,64 4,87 3,89
Analytical sensitivity using twenty replicates of the zero calibrator was estimated at 0,08 nmol/L (mean + 2 S.D.). The Access ® digoxin method is linear up to 7,68 nmol/L.
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IMMUNOASSAY INTERFERENCES: CAN WE DO ANYTHING TO MINIMIZE THEIR IMPACT ON PATIENT CARE? Ellis. G., Wan, B. S-Y., and Makela S. K., Department of Clinical Biochemistry, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada, M5G 1X8 We have recently found three patients with incorrect TSH (Ciba-Corning ACS 180), one with incorrect TSH (BioRad) and one with incorrect ACTH (IncStar). Several of these results lead to ucertainties about diagnosis and treatment. Details of these cases will be presented. We discovered the cases in several ways: 1) as outliers during method comparisons, 2) as patients who had immunoassay results that our physicians questioned because they were difficult to explain clinically, 3) as patients randomly selected whose sera did not dilute linearly for the constituent analysed, and 4) as patients that we selected because their sera had given problems with other assays. We assume that these interferences were caused by antibodies in the patients' sera directed against some reagents in the immunoassay system, such as anti-mouse or anti-rabbit antibodies (J Clin Chem Clin Biochem 1990; 28: 881-892). In some cases, there was a history of patient contact with the animal species of the primary antibody. In one case, we think that there was transplacental passage of interference, because serum TSH from a two-week-old infant did not dilute linearly. We recommend that laboratories: 1) re-assay at several dilutions any samples where physicians doubt the validity of the results, (dilution often reduces the interferent concentration to that at which interference no longer occurs), 2) repeat the analysis with an alternate method, preferably one that uses reagents from a different species, or that incorporates an extraction procedure and reconstitution of the analyte, and 3) post the names of any patients whose sera have been found to interfere with specific assays, so that subsequent analyses can be done using appropriate steps to ensure valid results.
[~]
EVALUATION OF THE MILES I M M U N O I ANALYSER Krahn, J., and Green, C., Department of Clinical Biochemistry, St.Boniface General Hospital, 409 Tache Avenue, Winnipeg, Manitoba, Canada, R2H 2A6 T3, T4, TSH, T3 uptake, HCG, LH, FSH, digoxin, cortisol and tobramycin assays were evaluated. The object of the
CLINICAL BIOCHEMISTRY, VOLUME 27, JUNE 1994
study was to determine 1. the precision and accuracy of the assays 2. the cross-reactivity of the appropriate assays 3. the instrument carryover 4. calibration stability 5. specimen and analyte integrity for specimens collected in a primary tube containing a serum separator gel 6. if the serum cortisol method can be used to measure urinary cortisol. The precision of all the assays is as good or better than the methods being currently used. Split-sample specimen comparisons showed two methods (LH and prolactin) where there was proportional error due to differences in standardization. This problem was solved by adjustment of results with a factor. None of the assays show cross-reactivities that would cause clinically relevant errors i.e. high HCG doesn't cross-react with the FSH, LH or TSH methods, and serum containing digoxin-like immunoreactive substances does not react with this digoxin method. The specimen carryover is only measurable for analytes like HCG where it is in the order of parts per million. Initial studies show that the Vacutainer serum separator tube does not interfere with any of the above assays. Finally, the serum cortisol method on this instrument can be used to do urinary free cortisol determinations if the urine is first extracted with dichloroethane, dried down and redissolved with the zero calibrator. Based on the above evaluation, the instrument has been successfully implemented. EVALUATION OF THE ACCESS ® IMMUNOASSAY ANALYZER DIGOXIN ASSAY Guitard. M., Service de biochimie, Hrpital du HautRichelieu, 920 boulevard du srminaire, Saint-Jean-surRichelieu, Qurbec, Canada, J3A 1B7 Rapid and accurate measurement of serum digoxin levels is usefull in the monitoring of digoxin therapy and acute intoxication. We evaluated the serum digoxin assay on the Access ® Immunoassay Analyzer (Sanofi Diagnostics Pasteur Inc., Chaska, MN) in terms of precision, sensitivity, linearity, DLIF (digoxin-like immunoreactive factor) interference and compared the Access ® results with those obtained with the Abbott TDx digoxin assay (Abbott Laboratories, Chicago, IL). The precision data over 28 days (26 runs in triplicate) was evaluated using tri-level control material (Ciba-Corning Diagnostics Corp., Irvine, CA) and is presented in the following table: Runs
Mean (nmol/L)
77 78 77
0,84 2,42 3,82
Within-run SD % CV 0,05 0.09 0,11
5,86 3,60 3,00
Between-run SD % CV 0,04 0,08 0,09
4,90 3,28 2,48
Total imprecision S.D. % C.V. 0,06 0,12 0,15
7,64 4,87 3,89
Analytical sensitivity using twenty replicates of the zero calibrator was estimated at 0,08 nmol/L (mean + 2 S.D.). The Access ® digoxin method is linear up to 7,68 nmol/L.
223