Measurement of Urinary Ammonium Using a Commercially Available Plasma Ammonium Assay.

Background: Determination of urinary ammonium excretion is helpful in evaluating patients with acid-base disorders, chronic kidney disease, and nephrolithiasis. However, urinary ammonium levels are only measured by specialized laboratories in the United States, limiting widespread implementation. We evaluated the performance of a plasma ammonium assay to quantify urinary ammonium excretion and also determined ammonium stability under a variety of conditions. Methods: An enzymatic plasma ammonium assay (Randox) was modified to measure urinary ammonium concentration. Urine samples were diluted 40-fold and then assayed on an Abbott Architect ci8200 analyzer. Assay precision, limit of quantitation, and linearity were determined. The method was compared against the formalin titration method, and stability studies were conducted at different temperatures and pH. Results: After dilution, the assay had total precision of 18% at 2.54 mmol/L, 5% at 15.58 mmol/L, and 2% at 29.49 mmol/L, with a limit of quantitation of 2.92 mmol/L. Assay performance was linear in the range of 0.7-45 mmol/L. Method comparison against the formalin method showed a slope of 0.98 and intercept of -0.37 mmol/L. Urinary ammonium was determined to be stable for 48 hours at room temperature and for 9 days at 4°C and -20°C at pH 5.6-6.3. Ammonium was less stable at pH 3.8 and 8.5. When stored at -80°C, urinary ammonium was stable for at least 24 months. Conclusions: The modified enzymatic plasma ammonium assay reliably quantifies urine ammonium at physiologic concentrations. It compares well with the formalin titration method and is suitable for routine clinical use on an automated chemistry analyzer.

Reliability of Total Nucleated Cell Counts in the Setting of Hip Arthroplasty.

BACKGROUND: Total nucleated cell (TNC) count and differential are used to classify joint effusions as inflammatory or noninflammatory. Further diagnostic evaluation and management is contingent on this classification. TNC count can be measured by automated analyzers or by manual assessment using a hemocytometer. Studies have raised concerns regarding the accuracy of TNC counts measured by automated instruments, particularly in the setting of joint arthroplasty. The objective of this study was to determine whether metallosis, a complication of total hip arthroplasty in which metal debris accumulates in periprosthetic tissues and synovial fluid, is associated with inaccurate TNC counts in synovial fluid. METHODS: We compared the accuracy of cell counts measured by the Sysmex XN-1000 and Beckman Coulter Iris iQ200 with the gold standard of manual assessment using a hemocytometer in synovial fluid from patients with suspected metallosis and in fluid obtained from controls from patients with native joints and a history of arthroplasty for other indications. RESULTS: TNC counts produced by automated analyzers were associated with increased levels of discordance (relative to manual counts) in patients with metallosis. Metallosis was not associated with increased levels of discordance for RBC counts or WBC differentials. The Sysmex XN flagged all but 1 metallosis sample for manual verification of the results. CONCLUSIONS: Automated methods are generally reliable for analysis of synovial fluid. TNC counts can be inaccurate in the context of metallosis following total hip arthroplasty. Laboratories should correlate automated cell counts with a microscopic assessment of the specimen, as recommended by instrument manufacturers.

Method Verification Shows a Negative Bias between 2 Procalcitonin Methods at Medical Decision Concentrations.

BACKGROUND: Procalcitonin (PCT) concentration increases as a result of systemic inflammation owing to bacterial infection. Many PCT algorithms and medical decision concentrations (MDCs) have been clinically validated using the B·R·A·H·M·S PCT™ sensitive Kryptor™ assay. Alternative PCT assays have recently been approved by the Food and Drug Administration for clinical use in the US and require method verification before clinical implementation. METHODS: Precision, sensitivity, linearity, reportable range, and reference intervals were verified for the Architect B·R·A·H·M·S PCT assay. Accuracy of the Architect B·R·A·H·M·S PCT assay was evaluated by comparison with the B·R·A·H·M·S PCT sensitive Kryptor assay. RESULTS: The Architect B·R·A·H·M·S PCT assay was found to be precise (CV, ≤4.6%), sensitive (limit of blank, 0.001 ng/mL; limit of quantitation, ≤0.01 ng/mL), and linear according to the manufacturer's claims. The analytical measurement range (0.20-100.00 ng/mL) and the reference interval (≤0.07 ng/mL) were also verified. Patient result comparisons indicated high agreement at 0.10 ng/mL and 0.25 ng/mL and reduced positive agreement at 0.50 ng/mL and 2.00 ng/mL MDCs owing to negative bias compared with the B·R·A·H·M·S PCT sensitive Kryptor assay. CONCLUSIONS: The Architect B·R·A·H·M·S PCT assay meets most performance specifications claimed by the manufacturer; however, negative bias at 0.50 ng/mL and 2.00 ng/mL PCT concentrations is evident.