Basic Laboratory Testing Before Starting an SSRI
No routine baseline laboratory tests are required before initiating SSRI therapy in otherwise healthy adults. However, targeted testing should be performed based on specific clinical risk factors.
Risk-Stratified Laboratory Approach
Standard Practice (Low-Risk Patients)
For healthy adults without comorbidities or risk factors, you can initiate SSRI therapy without any baseline laboratory testing 1. The evidence shows that SSRIs do not cause clinically significant changes in routine blood chemistry values in most patients.
Targeted Testing Based on Risk Factors
Check baseline sodium (Na+) if the patient has:
- Age ≥65 years
- Concurrent diuretic use
- History of hyponatremia
- Volume depletion or dehydration
Hyponatremia is a well-documented SSRI adverse effect, particularly in elderly patients 2. While one study of sertraline found no significant sodium changes 1, this remains a recognized class effect requiring vigilance in high-risk populations.
Check baseline metabolic panel (including creatinine) if the patient has:
- Pre-existing renal disease
- Diabetes
- Cardiovascular disease
- Multiple medications
The evidence shows minimal impact on creatinine (only 0.5% variance related to sertraline dose) 1, but baseline values are useful for monitoring in patients with existing metabolic conditions 2.
Consider baseline hormonal testing (LH, FSH, testosterone, prolactin) only if:
- Patient specifically reports pre-existing sexual dysfunction
- You need to differentiate baseline from treatment-emergent sexual side effects
This is not routinely recommended but may be useful for medicolegal documentation, as 79-84% of patients on SSRIs develop hormonal abnormalities including elevated prolactin and decreased testosterone 3. However, this testing does not change initial prescribing decisions.
What NOT to Order
Do NOT order CYP450 genetic testing before starting SSRIs. The EGAPP Working Group found insufficient evidence to support routine CYP450 polymorphism testing (CYP2D6, CYP2C19) for adults beginning SSRI treatment 4. Despite FDA approval of commercial tests like the AmpliChip, there is no demonstrated clinical utility or improvement in patient outcomes.
Do NOT order routine complete blood counts, liver function tests, or thyroid panels unless clinically indicated by symptoms or comorbidities unrelated to SSRI initiation 1.
Common Pitfalls to Avoid
- Over-testing: Ordering comprehensive metabolic panels "just to be safe" in healthy young adults wastes resources without improving outcomes
- Delaying treatment: Waiting for unnecessary test results postpones effective therapy for a disabling condition
- Ignoring medication interactions: Focus clinical attention on reviewing the patient's current medication list for serotonin syndrome risk (particularly tramadol, fentanyl, MAOIs, linezolid, SNRIs, other antidepressants) rather than ordering labs 5
Monitoring After Initiation
While not baseline testing, recheck sodium at 2-4 weeks in elderly patients or those on diuretics, as this is when hyponatremia typically manifests 2. The key safety concerns with SSRIs—sexual dysfunction, GI bleeding risk, serotonin syndrome, and discontinuation symptoms—are not prevented by baseline laboratory testing but rather by careful medication review and clinical monitoring.