Unexpected overestimation of methotrexate plasma concentrations: importance of the assay implementation and its validation

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Impact of bicarbonate interference on routine ion-selective electrode chloride measurements

BACKGROUND: Chloride measurement is crucial for calculating the anion gap. Bicarbonate can interfere with chloride measurements; however, there is no information on the specific types of electrodes affected or the changes in bicarbonate non-selectivity over time. We evaluated the interference of bicarbonate on chloride measurements using different electrodes and the stability of this interference over time. METHODS: The effect of bicarbonate on chloride measured with electrodes of various manufacturers was assessed. When non-selectivity toward bicarbonate was observed, the stability of this interference during the electrode’s lifetime was explored. The impact of the bicarbonate concentration on the calibrator was also evaluated. RESULTS: Non-selectivity was observed for electrodes using quaternary ammonium salts (Beckman Coulter, Siemens, and Roche), with overestimated or underestimated chloride values observed at high or low bicarbonate concentrations, respectively. The degree of selectivity varied among electrodes. With the Roche electrode, interference became more pronounced over time, whereas the Siemens electrode appeared to gain selectivity during its lifetime. For the Roche system, adjusting the calibrator’s bicarbonate concentration from 30 mmol/L to 20–24 mmol/L reduced the number of samples with unacceptable bias (>3%) from 77.3% to 12.6%. Lot-to-lot variations in the calibrator bicarbonate concentration increased the uncertainty of chloride measurements. CONCLUSIONS: The extent of bicarbonate-induced error varied according to the type, manufacturer, and wear of the electrode; the bicarbonate concentration in the calibrators and the tested sample; and the chloride content. Laboratories should be aware of the impact of bicarbonate on routine chloride measurements to establish appropriate QC procedures

Multicenter method evaluation of the ARK™ Methotrexate Immunoassay

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Nationwide Harmonization Effort for Semi-Quantitative Reporting of SARS-CoV-2 PCR Test Results in Belgium.

From early 2020, a high demand for SARS-CoV-2 tests was driven by several testing indications, including asymptomatic cases, resulting in the massive roll-out of PCR assays to combat the pandemic. Considering the dynamic of viral shedding during the course of infection, the demand to report cycle threshold (Ct) values rapidly emerged. As Ct values can be affected by a number of factors, we considered that harmonization of semi-quantitative PCR results across laboratories would avoid potential divergent interpretations, particularly in the absence of clinical or serological information. A proposal to harmonize reporting of test results was drafted by the National Reference Centre (NRC) UZ/KU Leuven, distinguishing four categories of positivity based on RNA copies/mL. Pre-quantified control material was shipped to 124 laboratories with instructions to setup a standard curve to define thresholds per assay. For each assay, the mean Ct value and corresponding standard deviation was calculated per target gene, for the three concentrations (10, 10 and 10 copies/mL) that determine the classification. The results of 17 assays are summarized. This harmonization effort allowed to ensure that all Belgian laboratories would report positive PCR results in the same semi-quantitative manner to clinicians and to the national database which feeds contact tracing interventions