Sample to sample carryover: a source of analytical laboratory error and its relevance to integrated clinical chemistry/immunoassay systems

BACKGROUND

Integrated systems that combine clinical chemistry and immunoassay analyzers are used routinely. Sample to sample carryover is an inherent risk and can cause erroneously high patient test results for immunoassays. IVD manufacturers and laboratories must be aware of this phenomenon and guard against it.

METHODS

We used a sample carryover protocol that directs the clinical chemistry module to process samples with very high immunoassay analyte concentrations followed by samples with very low concentrations for the same analyte. Low concentration samples were then tested by the immunoassay module to determine if the clinical chemistry module caused primary sample tube to primary sample tube carryover of the immunoassay analyte.

RESULTS

Sample carryover was assessed on the Abbott ci8200 for HBsAg, AFP, beta-hCG, and PSA. Observed HBsAg carryover met the design specification of <0.1 ppm. Carryover for the other analytes was <0.1 ppm or below the assay limit of detection.

CONCLUSIONS

IVD manufacturers must design integrated systems to minimize primary specimen tube carryover and avoid analytical laboratory error that can impact patient safety. Carryover testing is difficult for clinical laboratories to perform in order to verify system performance. Laboratories must consider the potential for specimen carryover and its impact on results whether moving primary sample tubes between separate analyzers or using an integrated system.

Point-of-Care Time-resolved Immunofluorometric Assay for Human Pregnancy-associated Plasma Protein A: Use in First-Trimester Screening for Down Syndrome

Background: Screening for Down syndrome in the first trimester by a combination of fetal nuchal translucency thickness and maternal serum pregnancy-associated plasma protein A (PAPP-A) and free β-human chorionic gonadotropin has been shown to be effective and efficient. We aimed to develop a fast point-of-care assay that could be placed in one-stop clinics for the measurement of PAPP-A.

Methods: We developed a two-site, one-step assay that uses two monoclonal antibodies (mAbs) to PAPP-A, based on a dry-reagent, all-in-one immunoassay concept with a stable fluorescent lanthanide chelate and time-resolved fluorometry. One antibody (mAb 10E1) was biotinylated, and the other (mAb 234-5) was europium-labeled, both via the ε-amino groups of surface lysine residues. The assay was performed on an AIO immunoanalyzer at 36 °C in single, streptavidin-coated microtitration wells that contained the dry reagents. PAPP-A, either in free or complexed form, was detected by the antibodies used.

Results: The assay procedure required 20 min and used 10 μL of sample. The calibration curve was linear from 5 to 10 000 mIU/L. The detection limit was 0.5 mIU/L. Intra- and interassay imprecision (CV) was ≤4.3% and 8.3%, respectively, for whole blood, plasma, or serum samples. Recovery was 93–96% for serum, 95–108% for heparin-derived whole blood, and 98–103% for heparin-derived plasma. Parallelism was observed in all three matrices. Results correlated [slope = 0.85 (confidence interval, 0.82–0.87); intercept = −33 (confidence interval, −58 to −9); Sy|x = 85 mIU/L; r = 0.991; n = 100] with those obtained by a Delfia assay. Heparin did not affect the assay, but EDTA markedly reduced PAPP-A values. PAPP-A was stable at 4 °C for at least 18 days in serum and for 8 days in heparin-derived whole blood or plasma.

Conclusions: The present assay appears suited for use in one-stop clinics for screening for Down syndrome in the first trimester, with results available within 1 h.

Analytical and clinical evaluation of TSH and thyroid hormones by electrochemiluminescent immunoassays

OBJECTIVES

To perform an analytical evaluation of the new electrochemiluminescent immunoassays (ECLIA) for TSH, FT4, and T3 in the Elecsys 2010 immunoassay system. To assess the clinical classification of patients under suspicion of thyroid disease based on these laboratory assays.

MATERIALS AND METHODS

The analytical evaluation included the performance of minimum detectable concentrations, within-assay and inter-assay precision for the three analytes, functional sensitivity and linearity studies for TSH, and method comparison with the previous methods of RIA for FT4 and T3, and IRMA for TSH in current protocols of our institution. 102 patients with clinical suspicion of thyroid disease were assayed by ECLIA and radioactive techniques. Their differential clinical classification based on laboratory tests was studied as well.

RESULTS

The minimum detectable concentrations coincided with the manufacturer's: <0.005 mU/L for TSH, <0.30 pmol/L for FT4, and <0.30 nmol/L for T3. Functional sensitivity for TSH was 0.044 mU/L. Over the analytical range tested, within-assay imprecision was below 3.2% for TSH, 2.2% for FT4 and 9.6% for T3, and interassay CVs were below 4.0% for TSH, 5.9% for FT4 and 12.9% for T3. Measurement of diluted sera showed the TSH assay to overestimate recoveries by 18.6%. We have compared sera results of the Elecsys ECLIA assays with those obtained from the IRMA (Spectria-Orion Diagnostica) for TSH: TSH (ECLIA) = 0.074+0.953 TSH (IRMA), (r = 0.974; Sy/x = 2.638), and RIA (Coat a Count-DPC) for FT4:FT4 (ECLIA) = 5.043+0.682 FT4 (RIA), (r = 0.770; Sy/x = 4.774) and RIA (Spectria-Orion Diagnostica) for T3: T3(ECLIA) = -0.461+1.084 T3 (RIA), (r = 0.970; Sy/x = 0.412). When sera from 102 patients were processed by both methods, minimal disagreement in the area of diagnostic classification was observed in 8/102 (7.8%) of the cases.

CONCLUSION

The Elecsys 2010 is specially attractive as a routine assay because it is fully automated, obtaining results in only 18 minutes. The analytical assay performance for TSH, FT4 and T3 was shown to be acceptable. Using two different sets of diagnostic tests minimal discrepancies were found in the laboratory assessment for the classification of patients with clinical suspicion of thyroid disease.