Objective: Combine test (between the 11 th and 14th weeks of gestation) and triple test (between the 16 th and 18th weeks of gestation) are being used for trisomy 21 and trisomy 18 screening. We have investigated the precision of fetal nuchal translucency (NT), free β HCG and pregnancy associated protein of Pregnancy - A (PAPP- A) measurements in maternal serum and the effects of their precision to the risk evaluation results of the patients.

Materials and Methods: To measure analytic repeatibility, 3 different serum pools have been prepared, and 5 consecutive measurements were done from these serum pools. For NT measurement, 5 separate experienced obstetricians have measured NTÔs of 9 patients. The effects of the results from these separate measurements on the first trimester test results have been investigated. While studying the serum pools, IMMULITE®Free BetaHCG and IMMULITE®PAPP-A kit (Siemens Medical Solutions Diagnostics Limited, UK) for free β HCG and for PAPP-A have been used. In risk evaluation, the prenatal screening program (PRISCA typolog Software Gmbh, Hamburg) has been utilized.

Results: The same day repeatibility results (min - max %CV values) 3.9-8.6%, 4.2-7.0% and respectively 4.03-17.6% for HCG, PAPP-A and NT have been found as. When the end effect of thesebfree variations to the first trimester risk has been calculated, (using the three separate pools of serum samples), influences of 0-16.3%, 0-0.8%, 0-16.4% for free HCG results, 0-9.5%, 0-34.6%, 8-17.1% for PAPP- A results and 27.4-37%, 24.3-35.1%, 50.7-57.2% for NT measurements were found βConclusion:While the analytic performances are within the acceptable ranges, the variations have been found to influence the end risk calculation significantly. The end results are not influenced in the low risk group, but in high risk population and in advanced maternal age groups, it causes the final results that could effect the clinical decision making These results once again showed us that, the analytic performance must be kept in optimum for the first trimester tests. Very strict internal and external quality control programs are a must. NT certification is critical therefore a solid training and definitive certification is crucial.