Type, Rh, and Antibody Screening
Direct Coombs test
The patient’s blood type and Rh is determined with the direct Coombs test. If the patient is Rh negative, she is at risk for anti-D isoimmunization.
Indirect Coombs test or atypical antibody test (AAT)
The presence of atypical RBC antibodies is determined with the indirect Coombs test. Isoimmunization is identified if atypical antibodies are present. Follow-up testing is necessary to identify whether the fetus is at risk.
STD Screening
Cervical cultures
Screening cultures for chlamydia and gonorrhea will identify whether the fetus is at risk from delivery through an infected birth canal.
Syphilis
Nonspecific screening tests (veneral disease research laboratory [VDRL] or rapid plasma reagin [RPR]) are performed on all pregnant women. Positive screening tests must be followedup with treponema-specific tests (microhemagglutination assay for antibodies to T. pallidum
[MHA-TP] or fluorescent treponema antibody absorption [FTA]). Treatment of syphilis in pregnancy requires penicillin to ensure adequate fetal treatment.
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HIV Screening
Screening
HIV screening is recommended for all pregnant women as part of the initial lab testing. The CDC recommends Informed Refusal (or “Opt Out,” where a patient is tested unless she refuses), rather than Informed Consent (or “Opt In,” where a patient must specifically consent). Retesting should take place in the third trimester in areas of high HIV prevalence or an at-risk patient. Rapid HIV testing in labor is recommended if the patient’s HIV status is not known.
ELISA test
This screening test assesses presence of detectable HIV antibodies. A 3-month lag exists between HIV infection and a positive ELISA test. All babies born to HIV-positive women will be HIV antibody positive from passive maternal antibodies.
Western blot test
This definitive test identifies the presence of HIV core and envelope antigens. Triple antiviral therapy is recommended for all HIV-positive women starting at 14 weeks and continuing through delivery. With cesarean delivery and triple antiviral therapy, transmission rates are as low as 1%.
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Cervical Pap Smear
Cervical cytologic screening can identify if the mother has cervical dysplasia or malignancy.
SECOND TRIMESTER LABORATORY TESTS
A 23-year-old woman (G3 P1 Ab1) is seen at 16 weeks’ gestation. Her previous pregnancy resulted in an anencephalic fetus that did not survive. She took 4 mg of folate preconception before this pregnancy but wants to know whether this fetus is affected.
Maternal Serum a-Fetoprotein (MS-AFP)
AFP
This is the major serum glycoprotein of the embryo. The concentration peaks at 12 weeks in the fetus and amniotic fluid (AF), then rises until 30 weeks in the maternal serum. Fetal struc-tural defects (open neural tube defect [NTD] and ventral wall defects) result in increased spill-age into the amniotic fluid and maternal serum. Other causes include twin pregnancy, placental bleeding, fetal renal disease, and sacrococcygeal teratoma.
Table 5-2. Alpha-Fetoprotein
Major Serum Glycoprotein of the Embryo
| Normal AFP changes | Fetal serum | Peaks at 12 weeks |
| Amniotic fluid | Peaks at 12 weeks | |
| Maternal serum | Peaks at 30 weeks | |
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Chapter 5 l Prenatal Laboratory Testing
MS-AFP
MS-AFP is reported in multiples of the median (MoM) and is always performed as part of mul-tiple marker screenings. Maternal serum testing is performed within a gestational window of 15–20 weeks. Because reference ranges are specific to gestational age, accurate pregnancy datingis imperative.
1.0
.75 3.8 7.7
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| 0.5 | 1.0 | 2.0 | 3.0 | 4.0 | 7.0 | 8.0 |
| Trisomy 21 | Normals | Open NTD | Anencephaly | |||
| Figure I-5-1. Midpoints of MSAFP | ||||||
Elevated MS-AFP
A positive high value is >2.5 MoM. The next step in management is to obtain an obstetric ultrasound to confirm gestational dating. The most common cause of an elevated MS-AFP is dating error.
• If the true gestational age is more advanced than the assumed gestational age, it would explain the positive high value. In cases of dating error, repeat the MS-AFP if the pregnancy is still within the 15- to 20-week window. A normal MS-AFP will be reassuring.
• If the dates are correct and no explanation is seen on sonogram, perform amniocentesis for AF-AFP determination and acetylcholinesterase activity. Elevated levels of AF ace-tylcholinesterase activity are specific to open NTD.
• With unexplained elevated MS-AFP but normal AF-AFP, the pregnancy is statistically at risk for intrauterine growth restriction (IUGR), stillbirth, and preeclampsia.
Low MS-AFP
A positive low value is <0.85 MoM. The sensitivity of MS-AFP alone for trisomy 21 is only 20%.
The next step in management is to obtain an obstetric ultrasound to confirm gestational dating.
The most common cause of a low MS-AFP is dating error.
• If the true gestational age is less than the assumed gestational age, it would explain the positive low value. In cases of dating error, repeat the MS-AFP if the pregnancy is still within the window. A normal MS-AFP will be reassuring.
• If the dates are correct and no explanation is seen on sonogram, perform amniocentesis for karyotype.
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| This is an elective, | MS−AFP | |||||
| not routine, | ||||||
| prenatal test. | ||||||
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| Sonogram | ||||||
| for dating and anomalies | ||||||
Dating error
Recalculate MS−AFP
Dating is correct
MS−AFP is true (+)
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| Sonogram explains | Sonogram normal | ||||||||||||
| NTD, VWD, twins, renal | Unexplained | ||||||||||||
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| No further workup if | No further | Amniocentesis | |||||||||||
| MS−AFP result is WNL | workup | for AF-AFP/AChE | |||||||||||
Figure I-5-2. Midtrimester Labs
Quadruple Marker Screen
Trisomy screening
The sensitivity for trisomy 21 detection can be increased to 80% by performing maternal serum screen for not only MS-AFP, but also hCG, estriol, and inhibin-A. The window for testing is also 15–20 weeks. Because reference values are gestational age specific, accurate dating is important.
Trisomy 21
With Down syndrome, levels for MS-AFP and estriol are decreased, but hCG and inhibin-A are increased. Perform an amniocentesis for karyotype.
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Trisomy 18
With Edward syndrome, levels for all 4 markers (MS-AFP, estriol, inhibin-A, and hCG) are decreased. Perform an amniocentesis for karyotype.
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