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First Trimester Screen, Hyperglycosylated hCG (h-hCG)

Test code(s) 16020, 16969 (NY)

This is an outdated version of this FAQ. It was effective 10/15/2012 to 04/23/2013.

The current version is available here.

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

A negative screen means the fetus has a reduced risk for Down syndrome or trisomy 18. But a negative screen does not guarantee the birth of a healthy baby. The first trimester screening test only screens for Down syndrome and trisomy 18; it is not a diagnostic test. Most patients with a negative screen choose not to proceed with a diagnostic test.

Recommended followup for a negative first trimester screen includes neural tube defect screening in the second trimester1; the Maternal Serum AFP test (test code 5059 [729T for NY]) can be used for this purpose.

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.

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 patient should be counseled regarding diagnostic testing options (eg, chromosome analysis of chorionic villus sampling or amniocentesis sample). Repeating the screening test is contraindicated.

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.

Question 3. My patient’s 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 patient should be counseled regarding diagnostic testing options (eg, chromosome analysis of chorionic villus sampling or amniocentesis sample). Risk recalculation and repeating the screening test are contraindicated.

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

The gestational age is derived from the crown rump length (CRL) using data from the Fetal Medicine Foundation. If the CRL is not provided, the gestational age is derived from the expected date of delivery (EDD) and the collection date provided and is an exact calculation by calendar days. Gestational wheels may be inaccurate by several days or more.

Note that the gestational age is reported in decimal weeks. For example, 11 weeks 4 days is reported as 11.6 weeks.

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

It’s appropriate to change the gestational age or EDD when the data used for screening are substantially above or below that determined by ultrasound.

The earliest average ultrasound age (AUA) calculated should be used for dating purposes. AUA is most accurate when determined in the first trimester. Accuracy decreases with advancing maternal age. For example, AUA accuracy is ±7 days in the first trimester and ±10 days in the second trimester.

If the gestational age used for this screening test is within the AUA ±7 days, the gestational age should not be changed for screening purposes. If the gestational age used for screening is outside the AUA range, it may be appropriate to change the gestational age used for screening.

If you want to change the EDD/gestational age used for a specific patient’s screening test, please call 1-800-642-4657, ext 4455. If the revised gestational age is between 10.0 to 13.9 weeks’ gestation, we can calculate and report a new NTD risk. If the revised gestational age is <10.0 weeks, we cannot calculate a new risk. Consider submitting a second specimen for screening, collected when the woman is between 10.0 to 13.9 weeks’ gestation. If the revised gestational age is >13.9 weeks’ gestation, an accurate first trimester risk assessment cannot be provided.

Question 6. What is the web calculator and how do I use it?

The web calculator is a tool that allows you to add nuchal translucency (NT) data acquired after a patient’s sample was submitted for screening and get revised Down syndrome and trisomy 18 risks. It can be used only for this test (test code 16020 [16969 for NY]).

Access the calculator at http://gocalc.QuestDiagnostics.com and perform the following steps:

  1. Enter the UNIKEY code for the patient, the sonographer’s ID number, and the patient’s last name. Note that the sonographer must be on file with the lab prior to using the calculator tool.
  2. Enter the NT value when prompted.
  3. Review the revised risks provided by the calculator.
  4. Add forwarding information when prompted so the revised final report can be sent to the primary physician as well as the client doing the NT measurement.

Question 7. 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 8. My patient had a normal maternal serum screen, but her risk for Down syndrome was higher than her age-related risk. Why was her result screen negative?

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 9. 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 10. 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 11. 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 chromosome abnormalities. Obstet Gynecol. 2007;109:217-227.
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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