Some Points about Serum biochemistry in prenatal screening

Some Points about Serum biochemistry in prenatal screening 

Screening for trisomy 21

- In trisomy 21 free ß-hCG is higher than in euploid pregnancies and the difference between the two is higher at 13 than at 11 weeks.

- In trisomy 21 PAPP-A is lower than in euploid pregnancies and the difference between the two is higher at 11 than at 13 weeks.

- -The difference from euploid pregnancies in PAPP-A at 11 weeks is greater than the difference in ß-hCG at 13 weeks and therefore the overall performance of biochemical screening is better at 11 than at 13 weeks.

- In euploid pregnancies the average free ß-hCG is 1.0 MoM and PAPP-A is 1.0 MoM.

- In aneuploid pregnancies the average MoM values of free ß-hCG and PAPP-A are:

 
                          free ß-hCG          PAPP-A
Trisomy 21            2.0                    0.5

Trisomy 18            0.2                    0.2

Trisomy 13            0.3                    0.3

Turner                   1.2                    0.5

Triploidy:

» Digynic               0.2                    0.1

» Diandric            9.0                     0.7

- 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.

Dr. Sarang Younesi, [13.04.20 20:29]
In trisomy 21 compared to euploid pregnancies:

- The difference in biochemical markers is greater at 11 than at 13 weeks.

- The difference in fetal NT is greater at 11 than at 13 weeks.
 
- The performance of combined screening is better at 11 than at 13 weeks.

Effect of gestational age:

- The overall performance of combined screening is better at 11 than at 13 weeks and may be best at 10 weeks.

- Ultrasound scanning for fetal abnormalities is better at 12 than at 11 weeks and much better than at 10 weeks.

- A good way of achieving a high performance of screening for trisomy 21 and diagnosing major fetal defects by ultrasound is to carry out the blood test at 10 or 11 weeks and the ultrasound scan at 12 weeks (about 94% versus 84% for the time that both of them are performed in 12 weeks).

Trisomies 18 and 13

- Trisomies 18 and 13 are the second and third most common chromosomal abnormalities after trisomy 21.

- At 11-13 weeks the relative prevalence of trisomies 18 and 13 to trisomy 21 are about 1 to 2.5 and 1 to 7, respectively

 (for example in a 36 yrs old pregnant women in 12 weeks has 1:200 prior risk for Trisomy 21, 1:450 for Trisomy 18, and 1:1400 for Trisomy 13).

- All three trisomies are associated with increased maternal age, increased fetal NT and decreased maternal serum PAPP-A.

- A beneficial consequence of first-trimester combined screening for trisomy 21 is the early diagnosis of trisomies 18 and 13. 

- At a false positive rate of 3% the detection rate of trisomy 21 is 90% and of trisomies 18 and 13 is about 75%

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