What laboratory tests are typically ordered during the first obstetric (OB) visit?

Laboratory Tests Ordered at First Obstetric Visit

The first obstetric visit should include a comprehensive panel of laboratory tests including complete blood count, blood type and Rh status, urinalysis, hepatitis B surface antigen, HIV, syphilis, gonorrhea, chlamydia, and diabetes screening. 1

Core Laboratory Tests

Blood Tests

• Complete blood count (CBC) - To assess for anemia 1, 2
• Blood type and Rh status - Essential for all pregnant women 1, 2
• Hepatitis B surface antigen (HBsAg) - Universal screening recommended regardless of previous vaccination or testing history 1
• HIV antibody testing - Recommended for all pregnant women 1, 2
• Syphilis screening - Universal screening recommended 1, 2
• Rubella antibody status - To identify women at risk of congenital rubella syndrome 1

Urine Tests

• Urinalysis - To screen for asymptomatic bacteriuria, proteinuria, and glucosuria 1, 2
• Gonorrhea and chlamydia screening - Recommended for all pregnant women 1, 2

Metabolic Screening

• Diabetes screening - Either random glucose, fasting glucose, or glucose challenge test depending on risk factors 1, 2

Timing Considerations

Laboratory testing should be performed at the first prenatal visit, ideally in the first trimester. This timing is critical because:

1. Early detection of infections allows for timely intervention to prevent maternal-fetal transmission 1
2. Neural tube defects and other birth defects can only be prevented if risk factors are identified early 1
3. Placental development begins at implantation, and early interventions may prevent complications 1

Special Considerations

Hepatitis B Screening

HBsAg testing should be performed in each pregnancy, regardless of previous hepatitis B vaccination or previous negative HBsAg tests 1. This is crucial as:

• An estimated 24,000 infants are born annually to HBV-infected women in the US
• 30-40% of chronic HBV infections result from perinatal transmission
• Early identification allows for appropriate prophylaxis of the newborn 1

HIV Testing

HIV screening is particularly important as:

• Early antiretroviral therapy can significantly reduce mother-to-child transmission
• Results should be available quickly enough to allow for intervention during labor if needed
• For women with unknown HIV status during labor, rapid testing should be performed 1

Genetic Screening

Assessment for genetic disorders should be based on:

• Family history
• Ethnic background
• Maternal age
• Carrier screening for conditions like cystic fibrosis should be offered as indicated 1, 2

Common Pitfalls to Avoid

1. Inadequate syphilis screening - Studies show that despite recommendations, screening rates can be suboptimal, particularly in high-risk populations 3

2. Missing HIV screening opportunities - Despite recommendations for universal HIV screening, studies show that actual screening rates may be as low as 82-85% 4

3. Failure to repeat testing when indicated - In high-prevalence areas, seroconversion rates during pregnancy for both syphilis (2.7%) and HIV (2.2%) justify re-screening later in pregnancy 5

4. Overlooking screening based on practice setting - Research indicates that women attending private clinics or having private insurance may be less likely to receive adequate screening for certain infections 3

5. Neglecting to order hepatitis B screening - This is one of the most important tests as timely intervention can prevent chronic infection in the newborn 1

By ensuring all recommended laboratory tests are performed at the first obstetric visit, healthcare providers can significantly reduce maternal and infant morbidity and mortality through early detection and intervention for various conditions.