Discrepancies in plasma alkaline phosphatase reruns on Cobas c503 Roche analysers: a retrospective analysis and impact of dilution.

OBJECTIVES: Discrepancies have been observed in the rerun results of plasma alkaline phosphatases (ALP) on Cobas c503 Roche analysers. The aims were to 1) determine the percentage of discrepancies in ALP reruns and 2) evaluate the effect of an automatic dilution on discrepancies. METHODS: 1) Raw plasma ALP results from patients were extracted from Cobas pro c503 analysers over a 2.7-year period. Reruns were triggered for any ALP above 200 U/L with no history or with a previous normal ALP, and those with a variation exceeding the total allowable error (±15 U/L if ≤150 U/L or ±10 % if >150 U/L) were classified as discrepancies. 2) During five weeks, a systematic 1:5 dilution was triggered on the analysers for all pure tests, concomitantly. RESULTS: 1) Among 9,574 reruns, 677 (7.1 %) were discordant (5.8 %, 1.2 % and 0.1 % after one, two and three reruns, respectively), and 90 % of which were decreases. 2) 9,062 pairs of concomitant pure and diluted ALP were measured, with respective medians (2.5th-97.5th percentiles) of 94 (43-559) and 90 (41-528) U/L (median of differences -4.4 %). Of these, 274 were retested, of which 16 pure reruns (5.8 %) were discordant, compared to only two diluted reruns (0.7 %). There were greater negative variations in ALP reruns as the leukocyte count increased (p<0.001). CONCLUSIONS: A substantial rate of discrepancies was observed when elevated ALP were retested on Cobas c503 analysers. This issue was almost eliminated by applying a first-line 1:5 dilution. A modification of the assay method is eagerly awaited from the manufacturer.