Comparison of two immunoassay systems for hCGβ and PAPP-A in prenatal screening for trisomy 21, 18, and 13 in the first trimester.

OBJECTIVES: The biochemical serum markers free β-human chorionic gonadotropin (hCGβ) and pregnancy associated plasma protein A (PAPP-A), used in screening for trisomy 21 (T21), trisomy 18 (T18), and trisomy 13 (T13) during the first trimester, can be measured on different laboratory instruments e.g. Kryptor (Brahms) and Cobas (Roche). We compared the performance of these two analytical instruments when used for first trimester combined testing.

DESIGN AND METHODS: Serum samples from 944 singleton pregnant women attending for first trimester combined testing were routinely assayed for hCGβ and PAPP-A on Kryptor, and re-analyzed on Cobas. In addition, serum samples from 70 pregnant women carrying a fetus affected by T21, T18 or T13, were re-assayed on Cobas.

RESULTS: For the screening population, the hCGβ and PAPP-A results in multiples of the median (MoM) from Kryptor and Cobas were significantly lower on Cobas when compared to Kryptor. The number of pregnant women with a risk above 1:300 for T21 was 48 for both Cobas and Kryptor, although a few patients only had a high risk with one of the methods. Overall, the screen positive rate was 5.1% for both instruments. In the trisomy groups the calculated risks for T21, T18, and T13 agreed well between Cobas and Kryptor.

CONCLUSIONS: The screen positive rate for T21 (5.1%) did not differ between the two analytical platforms in our screening population, although PAPP-A measurements form Cobas were significantly lower than those from Kryptor. The calculated risks for the pregnancies affected by trisomies using hCGβ MoM and PAPP-A MoM from Kryptor agreed well with those from Cobas.