What baseline laboratory tests should be ordered for an adult patient being evaluated for depression before initiating pharmacotherapy?

Baseline Laboratory Testing for Depression

Before initiating antidepressant pharmacotherapy in adults with depression, order laboratory tests to identify reversible medical causes of depressive symptoms: thyroid-stimulating hormone (TSH), complete blood count (CBC), and comprehensive metabolic panel (CMP) including electrolytes, glucose, and renal/hepatic function. 1, 2

Primary Laboratory Evaluation

The essential baseline workup should include:

• Thyroid function testing (TSH): Hypothyroidism is a reversible cause of depressive symptoms and must be excluded before attributing symptoms to primary psychiatric illness 1, 2

• Complete blood count (CBC): To screen for anemia and other hematologic abnormalities that can present with fatigue and depressive symptoms 1

• Comprehensive metabolic panel: Including electrolytes, glucose, blood urea nitrogen, creatinine, and liver function tests to identify metabolic derangements, renal dysfunction, hepatic disease, or electrolyte imbalances that can cause or exacerbate depressive symptoms 1, 2

• Vitamin B12 level: B12 deficiency is a reversible cause of depressive symptoms, particularly in older adults 1

Clinical Rationale

First, treat medical causes of depressive symptoms (such as unrelieved pain, fatigue, infection, or electrolyte imbalance) before initiating psychiatric treatment. 1 The American Academy of Neurology and American Geriatrics Society recommend screening for reversible conditions including thyroid disorders, vitamin B12 deficiency, and metabolic abnormalities in patients presenting with depressive symptoms 1, 2

Key Medical Mimics to Exclude

Medical conditions that must be ruled out include:

• Endocrine disorders: Hypothyroidism, hyperthyroidism, Cushing's syndrome, and diabetes can all present with depressive symptoms 1, 2

• Metabolic disturbances: Electrolyte imbalances (particularly sodium, calcium), hypoglycemia, uremia, and hepatic encephalopathy 1

• Nutritional deficiencies: Vitamin B12 and folate deficiency 1

• Systemic illness: Chronic infections, anemia, and inflammatory conditions 1, 2

Special Population Considerations

Older Adults (≥65 years)

For elderly patients, the laboratory evaluation is particularly critical because:

• Cognitive impairment must be distinguished from depression: Screen for delirium and reversible causes of cognitive decline including thyroid dysfunction, B12 deficiency, and metabolic abnormalities 1

• Higher prevalence of medical comorbidities: Older adults are more likely to have medical conditions causing or contributing to depressive symptoms 1, 3

• Medication effects: Review current medications as many can cause depressive symptoms; baseline labs help establish safety for antidepressant initiation 3

Patients with Chronic Medical Conditions

For patients with diabetes, cardiovascular disease, or other chronic conditions:

• Baseline metabolic panel is essential: To establish safe prescribing parameters and identify metabolic contributors to depression 1

• Depression worsens medical outcomes: Unrecognized depression leads to poorer outcomes in chronic disease management, making accurate diagnosis critical 1

Common Pitfalls to Avoid

Do not skip the medical workup and assume symptoms are purely psychiatric. 1, 2 Many clinicians fail to adequately screen for medical causes before initiating antidepressants, leading to:

• Missed diagnoses of treatable medical conditions 1, 2
• Ineffective treatment when depression is secondary to an unaddressed medical problem 1
• Potential medication adverse effects in patients with unrecognized metabolic or hepatic dysfunction 1

Do not rely solely on screening questionnaires without clinical evaluation. 1, 4 While tools like the PHQ-9 are valuable for screening, all positive screens must trigger a full diagnostic interview using DSM criteria and medical evaluation 1, 4, 5

Additional Testing Based on Clinical Presentation

Consider additional laboratory tests when clinically indicated:

• Substance screening: If substance abuse is suspected, as this affects treatment approach 1

• Inflammatory markers: If systemic inflammatory condition is suspected 2

• Medication levels: For patients on medications that can cause depression (e.g., corticosteroids, beta-blockers) 3

Documentation Requirements

Before initiating pharmacotherapy, document:

• Results of laboratory testing and exclusion of medical causes 1, 2
• Baseline symptom severity using validated instruments (PHQ-9 score) 1, 4, 5
• Assessment of suicide risk and safety plan if indicated 1, 4
• Target symptoms for monitoring treatment response 1