Continued provision of WHO International Standards for total and free PSA: Content and commutability of replacement preparations

Objectives: Replacements are required for the WHO International Standards (IS) for free PSA, coded 96/668 and total PSA (90:10), coded 96/670, which were established in 1999 to support efforts to harmonise PSA assays and address non-equimolarity. An important consideration is that the introduction of the replacements should have minimal impact on PSA measurements. Design and methods: We report the development of a replacement strategy, informed by field assessment of preparations through an external quality assessment scheme and the subsequent evaluation of the candidate ISs in worldwide collaborative studies. Results: By immunoassay, data from participants confirmed the value assigned to the current standards. Robust geometric mean estimates of the free PSA content of the candidate replacement for 96/668 coded 17/102 was 0.533 mu g/ampoule (n = 21). The ratio of the content estimates of 17/102:96/668 was 0.516 (GCV 12.5%, n = 21). Robust geometric mean estimates of the total PSA content of the candidate replacement for 96/670, coded 17/100, was 0.505 mu g/ampoule (n = 22). The ratio of the content estimates of 17/100:96/670 was 0.490 (GCV 5.3%, n = 22). Through concomitant measurement of a panel of 15 representative patient samples, the candidate ISs were shown to exhibit commutability with patient samples that was comparable with that of the current ISs. Conclusion: On the basis of these results, the preparations coded 17/102 and 17/100 were established by the WHO Expert Committee on Biological Standardization as the 2nd ISs for free and total PSA (PSA-ACT+free PSA) respectively, with assigned contents of 0.53 mu g/ampoule and 0.50 mu g/ampoule.