First-trimester screening for aneuploidy

Prenatal Screening Markers for Down Syndrome: Sensitivity, Specificity, Positive and Negative Expected Value Method

Background

Genetic screening for chromosomopathy is performed in the first trimester of pregnancy by determining fetal nuchal translucency (NT), and the pregnancy associated plasma protein-A (PAPP-A) and free human chorionic gonadotropin (free-beta HCG) biomarkers in maternal serum.

Methods

We tested the sensitivity, specificity, positive and negative expected values of each marker with the aim of setting a model for prenatal screening readings. Statistical data treatment has been performed on a sample of 340 pregnant women with positive results of prenatal screening.

Results

Sensitivity of PAPP-A was 0.6250 (probability 62.50%), free beta HCG 0.5893 (58.93%), NT 0.1785 (17.85%). Specificity of PAPP-A was 0.5106 (probability 51.06%), free beta HCG 0.5246 (52.46%), NT 0.9718 (97.18%). Positive expected value of PAPP-A was 0.2011 (probability 20.11%), free beta HCG 0.1964 (19.64%), NT 0.556 (55.56%). Negative expected value of PAPP-A was 0.8735 (probability 87.35%), free beta HCG 0.8662 (86.62%), NT 0.8571 (85.71%). The NT marker has a significantly higher specificity, which means that its normal value will significantly reduce the final risk of trisomy 21. The sensitivity of NT is much lower than that of biochemical markers, which means that a pathological value of NT does not have a significant influence on the final risk, i.e. the significantly higher sensitivity of biochemical markers will reduce the final risk of trisomy 21.

Conclusions

The analyses stress the importance of using a software which has the possibility to separate the level of a biochemical risk by correlating PAPP-A and free beta HCG and, by adding the NT marker, calculate the level of a final risk of Down syndrome.

False Positive Values of Biomarkers of Prenatal Screening on Chromosomopathy as Indicators of a Risky Pregnancy

Genetic screening on chromosomopathy has been performed on 2000 pregnant women in their first trimester of pregnancy by determining Pregnancy associated plasma protein-A and free-beta HCG biomarkers in maternal serum. After obtaining a normal fetal karyotype, the pathological values of the biomarkers have been correlated with other pregnancy disorders, and the possible causes of the positive genetic screening have been tested. 340 false positive biomarkers (17%) have been detected. The increased free-beta HCG (48.24%) had a significant influence. A significant correlation (p > 0.01) between the increased free-beta HCG and bleeding during pregnancy has been established. Complications occurred in 78.52% pregnancies with pathological biomarkers, MISSed in 13.82%, miscarriages in 10.88%, induced pregnancy terminations caused by fetal anomalies in 8.82% and births with disturbed fetal vitality in 45%. The research results have shown a significant correlation (p > 0.01) between the increased value of the free-beta HCG biomarkers and fetal hypoxia. The false positive genetic screening, caused by the increased free-beta HCG, can indicate placental dysfunction and fetal vitality disruption.

Non-invasive screening: The probability of events

Congenital anomalies are the cause of 20.0-25.0% of cases of perinatal death, while 3.0% of children are born with malformations of varying size. We examined the predictive values and defined the credibility ratio of the combined test results. Sensitivity of the test is 94.0%, and specificity is 99.0%. The positive likelihood ratio [likelihood ratio test (LR+)] is 94.00; a negative likelihood ratio [likelihood ratio test (LR-)] is 0.06. The pretest probability that pregnant women carry a fetus with chromosomal abnormality is 1:250. Posttest odds after the combined test to discover this abnormality are 0.3760, and probability of the same case is 0.2732 if it happens that the test result is positive. The result of our study confirms the justification of combined test usage in routine clinical practice, since the posttest odds rate in the case of a positive screening increases several times over (almost 90 times); the probability of detecting a chromosomal abnormality was about 70 times.

Value of the Combined Test in Prenatal Diagnostics

Congenital anomalies are the cause of 20.0-25.0% of cases of perinatal death, while 3.0% of children are born with malformations of varying size. We examined the predictive values and defined the credibility ratio of the combined test results. Of 317 examined pregnant women, 16 (5.05%) gave a pathological karyotype after amniocentesis: of these, nine (2.84%) had chromosomal number aberrations and seven (2.21%) had chromosomal structure aberrations. We determined the ultrasonographic parameters using the standards of the Fetal Medicine Foundation (location please). We measured free β-subunit of choriogronadotropin (β-HCG) and pregnancy associated plasma protein A (PAPP-A) in venous blood from pregnant women using a combined commercial assay. Sensitivity of the test is 94.0%, and specificity is 99.0%. The positive likelihood ratio [likelihood ratio test (LR+)] is 94.00, a negative likelihood ratio is [likelihood ratio test (LR-)] 0.06. The pretest probability that pregnant women carry a fetus with chromosomal abnormality is 1:250. Posttest odds after the combined test to discover this abnormality is 0.3760, and probability of the same case is 0.2732 if it happens that the test result is positive. The result of our study confirms the justification of combined test usage in routine clinical practice, since the posttest odds rate in the case of a positive screening increases several times over (almost 90 times); the probability of detecting a chromosomal abnormality was about 70 times. The combined screening test, if used methodologically correctly, has a high predictive value in detecting fetal congenital anomalies.

Population screening for fetal trisomy 21: easy access to screening should be balanced against a uniform ultrasound protocol

OBJECTIVES

To evaluate the performance of a first-trimester fetal aneuploidy screening program, with a documented underestimation of nuchal translucency thickness measurements (NT) compared to the Fetal Medicine Foundation (FMF) reference range.

METHODS

We analysed the data of Algemeen Medisch Laboratorium (AML) in Antwerp, Belgium, on combined screening with pregnancy-associated plasma protein-A (PAPP-A), free beta-human chorionic gonadotropin (FB-hCG) and NT. NT-multiples of the median (MoM), relative to the FMF reference range, were used for risk calculations.

RESULTS

The proportion of first-trimester screening tests in the total of serum screening tests increased from 1.3% (125/9424) in 2000 to 53.1% (6577/12 377) in 2003. Only 11.4% (1514/13 267) of NT measurements were performed according to FMF criteria. The 80.8% (21/26) trisomy 21 (T21) detection rate (DR) at cut off 1:300 resulted from maternal serum screening. NT measurements did not add to this DR, but reduced the false-positive rate from 16.8% (2212/13181) to 8.6% (1130/13181). Only 23.8% (5/21) of T21 detections were by FMF trainees.

CONCLUSION

Easy access to screening and maternal serum parameters accounted for the majority of T21 detections in our first-trimester combined screening program.