What is the most appropriate initial investigation for a patient presenting with depression, loss of appetite, social isolation, and non-compliance with anxiety and depression medications, who also exhibits unkempt appearance and slurred speech, and expresses a desire to use marijuana for stress relief?

Initial Investigation: Urine Drug Test

For a patient presenting with depression, social isolation, medication non-compliance, unkempt appearance, and slurred speech who expresses desire to use marijuana for stress relief, a urine drug test (Option B) is the most appropriate initial investigation. 1

Primary Rationale

The clinical presentation strongly suggests active substance use that must be confirmed before proceeding with any psychiatric treatment plan:

• Slurred speech is a cardinal sign of acute intoxication or withdrawal, not typically seen in primary depression or thyroid dysfunction alone 2
• The patient's stated desire to use marijuana combined with behavioral changes suggests current substance use that requires objective confirmation 1
• Medication non-compliance in the context of substance use may reflect active addiction rather than primary psychiatric pathology 3, 1

Why Urine Drug Test Takes Priority

Substance use must be identified first because it can fully mimic or exacerbate psychiatric symptoms, making accurate diagnosis impossible without establishing a baseline of current drug use 1:

• Cannabis and other substances can cause depression, anxiety, social withdrawal, and appetite changes that are indistinguishable from primary psychiatric illness 2, 1
• Urinalysis provides objective evidence of recent cannabis use and can detect other substances contributing to the clinical picture 1
• Starting psychiatric treatment without confirming substance use status would be premature and could mask underlying addiction 1

Why Not the Other Options

Thyroid Function Test (Option D) - Second Priority

While thyroid dysfunction can present with depression, anxiety, and social withdrawal 3, several factors make it less urgent than urine drug testing in this specific case:

• The presence of slurred speech and stated marijuana use intention points more directly to substance-related pathology 2, 1
• Thyroid dysfunction does not typically cause slurred speech 3
• Thyroid testing should be performed after substance use is confirmed and addressed, as part of the comprehensive medical workup 3, 1

Complete Blood Count (Option A)

CBC would be appropriate for evaluating fatigue and ruling out anemia as a contributor to depressive symptoms 3, but:

• It does not explain the acute presentation with slurred speech 2
• It should be part of the comprehensive workup after substance use is confirmed 1

Pulmonary Function Test (Option C)

This would only be indicated if specific respiratory complaints were present, not as an initial investigation for psychiatric symptoms 1

Clinical Approach After Urine Drug Testing

Once substance use is confirmed:

1. Assess for withdrawal syndromes: Cannabis withdrawal includes irritability, anxiety, sleep disturbances, decreased appetite, and restlessness 1
2. Evaluate for other substances: The urine test may reveal polysubstance use requiring different management 1
3. Perform thyroid function testing: After establishing substance use status, screen for medical causes of psychiatric symptoms 3
4. Obtain CBC and comprehensive metabolic panel: Complete the medical workup to exclude other contributing factors 3, 1

Critical Pitfall to Avoid

Do not prematurely diagnose primary psychiatric illness or initiate antidepressant therapy without first confirming and addressing substance use 3, 1. Substance-induced mood and anxiety symptoms may resolve with abstinence, and treating with antidepressants before establishing sobriety can complicate diagnosis and mask the true underlying pathology 1, 4.