First trimester screening

 CVS/ Amniocentesis for definitive diagnostic (not screening tets but confirmatory tets) :-All women  with positive results of prenatal screening, and then  karyotyping of a fetus (from CVS / Liquor amni) have to be done  with amniocentesis. Using a sensitivity analysis method, it has been determined with high probability that a pathological value of the marker implies the presence of risk. Using a specificity analysis method, it has been determined with high probability that a normal value of the marker implies the absence of significant risk. Using a positive expected value method, it has been determined with high probability that risk is present only if the marker implies so. Using a negative expected value method, it has been determined with high probability that significant risk is absent if the marker implies so.

If the findings of genetic screening evidence of risk of Down’s syndrome in the PRISCA software greater than 1: 250, which is indicated based on prenatal karyotyping.

In a study it was observed that in sample of 340 high-risk findings of the screening, there were 18 (6.1%) results with the pathological values of NT markers.

Pathological PAPP-A values were found in 174 (59.1%) cases.

Free beta-hCG showed extreme values in 168 (57.1%) pregnant women.

 Values of the markers  are being usually  reported in deviation from the median – MoM (multiple of median).

The risk of Down’s syndrome is shown in PRISCA software at two levels, the risk of biochemical correlations of biochemical markers PAPP-A and free beta HCG and finite risk adding the ultrasound marker NT. The results show the effect of each marker in the formation of risk of Down’s syndrome, influence of biochemical markers on biochemical and finite risk and impact of NT marker on the final risk.

Table I

Influence of PAPP-A marker on biochemical risk.

	PAPP-A MOM normal value	PAPP-A MOM pathological value
Risk-free	98	86
Risk	68	88

Sensitivity 0.5563 (probability 55.63%)

Specificity of PAPP-A 0.4815 (probability 48.15%)

Positive expected value of PAPP-A 0.4615 (probability 46.15%)

Negative expected value of PAPP-A 0.5759 (probability 57.59%)

Table II

Influence of free beta HCG marker on biochemical risk.

	Normal value free beta HCG	Free beta HCG pathological value
Risk-free	109	77
Risk	63	91

Sensitivity of free beta HCG 0.5909 (probability 59.09%)

Specificity of free beta HCG 0.5860 (probability 58.60%)

Positive expected value of free beta HCG 0.5416 (probability 54.16%)

Negative expected value of free beta HCG 0.6337 (probability 63.37%)

Table III

Influence of PAPP-A marker on the final risk (biochemical + NT).

	PAPP-A MOM normal value	PAPP-A MOM pathological value
Risk-free	145	139
Risk	21	35

Sensitivity of PAPP-A 0.6250 (probability 62.50%)

Specificity of PAPP-A 0.5106 (probability 51.06%)

Positive expected value of PAPP-A 0.2011 (probability 20.11%)

Negative expected value of PAPP-A 0.8735 (probability 87.35%)

In the last two decades, there have been numerous reports about the detection rate for different methods of screening for trisomy 21. Detection rate of the risk of maternal age and fetal NT is 75–80%, while the risk for age and biochemical screening of PAPP-A and free beta HCG is 70%. The combination of age-related risk markers NT, PAPP-A and free beta HCG increases the detection of trisomy 21 to 85–95%

The ability to achieve a reliable measurement of NT is dependent on the appropriate training of sonographers

 

Biochemical analyzers provide auto mated, precise and reproducible measurements. Presenting selectively the biochemical and ultrasound screening in the first trimester and representing a separate risk of sonographic and biochemical markers give a much better insight than the pure presentation of their combination. This type of screening is achieved by a policy in which the first-stage screening is based on maternal age, fetal NT and either tricuspid or ductus venosus flow, and biochemical testing is then performed only in those with an intermediate risk. An alternative first trimester contingent screening policy consists of maternal serum biochemistry in all pregnancies followed by fetal NT only in those with an intermediate risk after biochemical testing