Discrepancies between two total IgE assays and difference in reference intervals in healthy adults.

OBJECTIVE: To compare total immunoglobulin (Ig) E assay performance characteristics between Abbott Architect and Siemens Immulite test systems. Reference intervals were also determined for both platforms in an American population of healthy adults. METHODS: Agreement of the two total IgE assays was evaluated in a cohort of 331 subjects with normal complete blood count (CBC) and comprehensive metabolic panel (CMP) results. Reference intervals were established in 302 subjects after exclusion of atopic individuals on the Abbott Architect and Siemens Immulite test systems. RESULTS: We demonstrated a 32% positive bias for total IgE quantitation on the Siemens Immulite platform compared to the Abbott Architect, despite both methods calibrated against the same WHO international reference material (75/502), Furthermore, the upper limit of the reference interval (95th percentile) was determined to be higher for the Siemens Immulite assay compared to the Abbott Architect (132 and 102 IU/mL, respectively). CONCLUSION: Despite the use of a common WHO reference material for total IgE assay calibration, significant differences in quantitation was observed between two FDA-cleared test systems. Given that, it is warranted for clinical laboratories to verify vendor established reference intervals and adjust accordingly based on internal assessment of the normal range.

Falsely decreased vancomycin caused by rheumatoid factor: A case report.

BACKGROUND: Vancomycin is associated with potential nephrotoxicity and trough concentrations need to be monitored in certain patients. Falsely decreased vancomycin measurement may result in overtreatment and need to be identified promptly by clinicians and pharmacists to avoid toxicities. METHODS AND RESULTS: We report a case of rheumatoid factor-mediated falsely low vancomycin measurement with Abbott particle-enhanced turbidimetric inhibition immunoassay (PETINIA) method. Reanalyzing the sample using an alternative method, removing the interferences using heterophile blocking reagent as well as rheumatoid factor clean-up solution all helped to solve the false results. Results from alternative method and interference studies showed vancomycin concentrations reached toxic concentrations in the patient and administration of the drug was immediately terminated. The patient experienced a transient increase in serum creatinine. CONCLUSIONS: Even though most modern immunoassays use blocking agents to neutralize interfering antibodies such as rheumatoid factor, it is important for health care professionals to understand that occasional interference still occurs due to the heterogeneous nature of rheumatoid factor.

A suspected case of falsely low digoxin and vancomycin concentrations caused by free kappa light chains with PETINIA method.

Vancomycin and digoxin are associated with potential toxicity and serum concentrations need to be monitored in certain patients. Previous reports suggested IgM paraproteins could interfere with vancomycin assays, and no paraprotein interference has been reported with digoxin assays. Here we present a suspected case of free-kappa light chains-mediated falsely low digoxin and vancomycin concentrations with Abbott particle-enhanced turbidimetric inhibition immunoassay (PETINIA) method. A 53-year-old patient received multiple doses of vancomycin and digoxin intravenously, but trough vancomycin and random digoxin concentrations repeatedly measured as <1.1 µg/mL and <0.2 ng/mL respectively with Abbott PETINIA method. Results from alternative methods showed concentrations reaching toxic levels and administration of the drugs was immediately terminated. A significantly elevated level of free-kappa light chains, possibly in polymeric form as suggested by protein electrophoresis result, was suspected to be the cause of falsely low results. During the laboratory investigation, absorbance curves revealed increased agglutination in the patient's samples in the latter part of the reaction, suggesting interfering substances led to production of turbidity after reagents were added. Protein-free filtration partially recovered the drugs with Abbott PETINIA. When drug concentrations do not correlate with clinical judgment, clinicians and pharmacists should consult clinical laboratories for investigation of potential interfering substances.