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First Trimester Screen, hCG

Test code(s) 16145, 16968 (NY)

This is an outdated version of this FAQ. It was effective 04/20/2012 to 04/23/2013.

The current version is available here.

Question 1. My patient had a negative First Trimester Screen, hCG test. What should I do next?

A negative screen means it is unlikely the fetus has either Down syndrome or trisomy 18. But a negative screen does not guarantee the birth of a healthy baby. The demographic information provided at the time of testing is used in calculating the patient’s Down syndrome and trisomy 18 risk. Please check the demographic information to ensure accuracy of calculated results.

The first trimester screening test screens only for Down syndrome and trisomy 18. Guidelines recommend neural tube defect screening be performed in the second trimester.1 The Maternal Serum AFP test is available for this purpose.

Most patients with negative screening tests choose not to proceed with a diagnostic test.

Question 2. My patient’s result was “screen positive” for Down syndrome. What should I do next?

A positive Down syndrome screen result means there is an increased risk for the fetus to be affected with Down syndrome or other chromosome abnormalities. The demographic information provided at the time of testing is used in calculating the patient’s Down syndrome and trisomy 18 risk. Please check the demographic information to ensure accuracy of calculated results.

Guidelines recommend counseling women with a positive screening test. Such counseling may include a discussion of the significance of the screening results and diagnostic testing options (eg, careful ultrasound examination of the fetus, chorionic villus or amniocentesis testing). Guidelines do not recommend repeating Down syndrome positive screening tests.

Question 3. My patient result was screen positive for trisomy 18. What should I do next?

A positive trisomy 18 result means there is an increased risk for the fetus to be affected with trisomy 18 or other chromosome abnormalities. The demographic information provided at the time of testing is used in calculating the patients Down syndrome and trisomy 18 risk. Please check the demographic information to ensure accuracy of calculated results.

Guidelines recommend counseling women with a positive screening test. Such counseling may include a discussion of the significance of the screening results and diagnostic testing options (eg, careful ultrasound examination of the fetus, chorionic villus or amniocentesis testing). Guidelines do not recommend recalculating or repeating trisomy 18 positive screening tests.

Question 4. The report indicates a different gestational age than what I determined. How is the gestational age calculated?

We report the gestational age, based on the crown-rump length (CRL), in decimal weeks; for example, 11 weeks 4 days is reported as 11.6 weeks. If the CRL is not provided, we derive the gestational age from the expected date of delivery (EDD) and the sample collection date provided; we use an exact calculation of calendar days. Gestational wheels may be inaccurate by several days or more.

Question 5. When is it appropriate to change the gestational age or estimated date of delivery (EDD) on a maternal serum screen result report?

The criteria for changing the EDD of a pregnancy consists of multiple factors including the average ultrasound age (AUA). The earliest ultrasound at which the AUA was calculated should be used for dating purposes. It is generally accepted that the accuracy of AUA is +/- 7 days in the first trimester and +/- 10 days in the second trimester. If the estimated gestational age indicated on a maternal serum screening test report is within the time interval estimated by ultrasound, then the dates should not be changed.

If you want to change the EDD for a patient who had a maternal serum screening test performed, please contact Quest Diagnostics Nichols Institute (phone number on the test report) or call Quest Diagnostics Genetic Testing Center at 1-866 GENE-INFO. If the revised EDD results in an acceptable gestational age for the maternal serum screening test (10 to 13.9 weeks’ gestation for first trimester screening), a revised risk assessment can be calculated using measurements from the original specimen. If the revised gestational age is <10 weeks, a second specimen, collected between 10 and 13.9 weeks’ gestation, should be submitted to obtain an accurate risk assessment. If the revised gestational age exceeds 13.9 weeks, an accurate first trimester risk assessment cannot be provided.

Question 6. My patient has a family history of Down syndrome or trisomy 18. What impact does this have on these results?

Please call 1-866-GENE-INFO to discuss this case with a genetic counselor. Documentation of the abnormality in the family may enable a more specific risk assessment or indicate whether additional studies should be performed.

Question 7. My patient had a normal maternal serum screen, but her risk for Down syndrome was higher than her age-related risk. How do I explain this?

A cutoff of 1 in 270, the risk of a 35-year old, is used to determine if a pregnancy is "screen negative" or "screen positive" for Down syndrome. This cutoff is used regardless of the woman’s age, since it’s the historical cutoff for offering diagnostic testing (amniocentesis).

When counseling a pregnant woman, it may be helpful to compare her age-related risk (ie, pre-test risk) with her screen-derived risk (post-test risk) and the general population risk (1 in 600 live births). This allows the woman and her partner to better understand her risk of carrying a Down-syndrome affected fetus and to weigh it against the risks and consequences of amniocentesis.

Question 8. What does a low PAPP-A result mean and is there a follow-up test?

There is no consensus as to exactly what constitutes a low PAPP-A value in first trimester screening. However, low PAPP-A levels have been associated with low birth weight, reduced fetal growth, hypertension, pre-eclampsia, pre-term labor, and fetal demise. When a physician considers a woman’s PAPP-A to be low, he/she usually treats her as if she has a high risk pregnancy and follows her more closely as the pregnancy progresses.

Question 9. What is a Down syndrome pseudo-risk in a twin gestation? Why don't we give twin-specific risks?

Prenatal screening in twin pregnancies is complex. The serum markers can be measured in a woman with a twin gestation and then divided by corresponding medians for unaffected twins in order to provide a “pseudo-risk” for Down syndrome. This calculation accounts for the presence of two fetuses, but does not take into account the chorionicity of the pregnancy or the nuchal translucency measurement of each specific fetus. The result is a pregnancy-specific pseudo-risk, rather than a fetus-specific risk.

Question 10. In a twin gestation, why is there no risk assessment reported for trisomy 18?

Prenatal screening in twin pregnancies is complex. A trisomy 18 risk assessment is not calculated for twin gestations due to insufficient screening marker data from affected twin pregnancies.

 

Reference

  1. ACOG Committee on Practice Bulletins. ACOG Practice Bulletin No. 77: screening for fetal chromosomal abnormalities. Obstet Gynecol. 2001;109:217-222.
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

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