What is the most important lab test before starting a woman on oral contraception (OC) therapy?

Blood Pressure Measurement is the Only Essential Lab Test Before Starting Oral Contraception

Blood pressure measurement is the only essential and mandatory test required before initiating oral contraceptive therapy in women. 1

Evidence-Based Classification of Pre-Contraception Testing

The CDC's Selected Practice Recommendations for Contraceptive Use clearly categorizes examinations and tests needed before starting combined hormonal contraceptives:

• Class A (essential and mandatory): Blood pressure measurement only 1, 2
• Class C (does not contribute substantially): All laboratory tests including glucose, lipids, liver enzymes, hemoglobin, thrombogenic mutations, cervical cytology, STD screening, and HIV screening 1

Blood pressure measurement is critical because:

• Women with severe hypertension (≥160/100 mmHg) or vascular disease should not use combined hormonal contraceptives (U.S. MEC 4) 1
• Women with less severe hypertension (140-159/90-99 mmHg) generally should not use combined hormonal contraceptives (U.S. MEC 3) 1

Practical Implementation

If access to healthcare is limited, blood pressure can be obtained in non-clinical settings (e.g., pharmacy, fire station) and self-reported to the provider 1.

Why Other Tests Are Not Required

The CDC and WHO have determined that other tests do not contribute substantially to safe and effective use of oral contraceptives:

• Weight/BMI: Not needed to determine medical eligibility as all contraceptive methods can be used by obese women (though baseline measurement may be useful for monitoring) 1
• Laboratory tests: Not required because:
  • The prevalence of undiagnosed conditions that would contraindicate OC use is low
  • Women with conditions that contraindicate OC use likely already have these conditions diagnosed
  • The benefits of providing contraception outweigh the risks of not performing these tests 1, 2

Special Considerations

For combined hormonal contraceptives containing drospirenone:

• Consider checking serum potassium during the first treatment cycle in women on long-term medications that may increase potassium levels due to drospirenone's anti-mineralocorticoid activity 3

Clinical Pitfalls to Avoid

1. Unnecessary testing: Requiring additional tests can create barriers to contraceptive access without improving safety 1
2. Delayed initiation: The "Quick Start" approach (starting contraception immediately) is safe when pregnancy can be reasonably ruled out 1
3. Overlooking hypertension: Failing to measure blood pressure could put women with undiagnosed hypertension at risk for cardiovascular complications 4, 5
4. Overestimating risks: Even with modern low-dose estrogen formulations, blood pressure should still be monitored as studies show small but significant increases in blood pressure among OC users 6, 4

By following these evidence-based guidelines, providers can ensure safe initiation of oral contraceptives while removing unnecessary barriers to contraceptive access.