How do you interpret lab results and determine treatment in a pregnant patient?

Interpretation of Laboratory Tests in Pregnancy

Laboratory test interpretation during pregnancy requires understanding of normal physiological changes that occur during this period, as these changes significantly alter reference ranges compared to non-pregnant individuals.

Normal Physiological Changes in Pregnancy

Pregnancy causes substantial physiological alterations that affect laboratory values:

• Blood Volume Changes:

  • Plasma volume increases by approximately 50% (about 1250 ml) by 34-36 weeks 1
  • Red cell mass increases by only 18-25% (250-450 ml depending on iron supplementation) 1
  • Results in physiologic anemia of pregnancy with decreased hematocrit (from ~40% to ~33%) 1

• Coagulation System:

  • Shifts toward a hypercoagulable state 2
  • Increased levels of most clotting factors
  • Decreased endogenous anticoagulants
  • Inhibition of fibrinolysis 2

• Renal Function:

  • Glomerular filtration rate (GFR) increases up to 50% above pre-pregnancy levels 3
  • Affects interpretation of renal function tests

Appropriate Laboratory Testing by Trimester

First Antenatal Screen (First Trimester)

• Complete blood count
• Blood group and antibody screen
• Rubella antibody status
• Syphilis serology
• Hepatitis B serology
• HIV antibody testing 3
• Additional tests may include:
  • Varicella immunity
  • Chlamydia screening
  • Vitamin D levels 3

Second Trimester

• Gestational diabetes screening (most important)
• Protein detection in urine to rule out preeclampsia 3
• Screening for neural tube defects (AFP testing) between 15-20 weeks 4

Third Trimester

• Group B streptococcal (GBS) infection screening at 37 weeks 3
• Repeat complete blood count
• Monitoring for preeclampsia markers if indicated

Interpretation Principles for Pregnant Patients

1. Use pregnancy-specific reference ranges when available, as normal pregnancy values often differ significantly from non-pregnant ranges 5, 2

2. Consider gestational age when interpreting results, as many parameters change progressively throughout pregnancy 6

3. Monitor trends rather than isolated values, especially for parameters known to change dynamically during pregnancy 6

4. Recognize that many parameters take 3-12 months to return to baseline postpartum 6

Special Considerations for Specific Tests

Coagulation Parameters

• D-dimer, PT ratio, APTT ratio, fibrinogen, and platelet count should be monitored using pregnancy-specific reference ranges 7
• Particularly important when assessing thrombotic risk or bleeding disorders

Renal Function Tests

• Creatinine levels are typically lower in pregnancy due to increased GFR
• Urinary protein excretion increases during normal pregnancy

Liver Function Tests

• Slight increases in alkaline phosphatase are normal due to placental production
• Other liver enzymes should remain within normal non-pregnant ranges

Glucose Metabolism

• Lower fasting glucose levels are normal in pregnancy
• Higher postprandial values may be observed

Monitoring in High-Risk Conditions

COVID-19 in Pregnancy

• Monitor coagulation parameters closely due to combined hypercoagulable state of pregnancy and COVID-19-associated coagulopathy 7
• Consider thromboprophylaxis with weight-adjusted low molecular weight heparin for hospitalized pregnant women 7

Preeclampsia Monitoring

• Regular monitoring of blood pressure, urinary protein, liver enzymes, and platelet counts
• Monitor for signs of HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets)

Common Pitfalls to Avoid

1. Misinterpreting physiologic anemia as pathologic and unnecessarily treating with iron
2. Failing to use pregnancy-specific reference ranges leading to inappropriate interventions
3. Not considering gestational age when interpreting results
4. Overlooking the hypercoagulable state of pregnancy when assessing thrombotic risk
5. Misinterpreting elevated alkaline phosphatase as liver dysfunction when it's often placental in origin

Conclusion

Laboratory test interpretation in pregnancy requires specialized knowledge of the physiological changes that occur during this period. Using pregnancy-specific reference ranges, considering gestational age, and monitoring trends over time are essential for accurate interpretation and appropriate clinical decision-making.