Serum biochemistry

The measured concentration of free ß-hCG and PAPP-A is influenced by the machine and reagents used, gestational age, maternal weight, ethnicity, smoking status and method of conception.

In the calculation of patient-specific risks it is necessary to make adjustments in the measured free ß-hCG and PAPP-A. Each measured level is first converted to a multiple of the expected normal median (MoM) specific to a pregnancy of the same gestation, maternal weight, smoking status, ethnicity and method of conception.

• In Black women the PAPP-A level is about 60% higher than in White women. Failure to take into account ethnic origin would result in substantial underestimate of the true risk of trisomy 21 in Black women.
• In women who smoke and those conceiving by IVF serum PAPP-A is decreased and this could be misinterpreted for increased risk for trisomy 21 and a substantial increase in false positive rates.