Initial Investigation: Urine Drug Test
The most appropriate initial investigation for this patient is a urine drug test (Option B). This provides objective confirmation of cannabis use, establishes a baseline for monitoring treatment progress, and can identify other substances that may be contributing to the clinical presentation 1, 2.
Rationale for Urine Drug Testing
Confirming substance use is the critical first step in this clinical scenario where the patient openly reports marijuana use for stress relief and demonstrates medication non-compliance 1. The urine drug test serves multiple essential functions:
- Objective documentation of cannabis use patterns, which is crucial since self-reported substance use may be unreliable 2
- Detection of polysubstance use that could complicate the psychiatric presentation and treatment plan 1, 2
- Baseline establishment before initiating or modifying psychiatric medications, particularly given the patient's non-adherence to current depression treatment 1
Why Cannabis Assessment Takes Priority
Cannabis withdrawal syndrome can significantly exacerbate depression and anxiety symptoms, presenting with irritability, anxiety, sleep disturbances, decreased appetite, restlessness, and altered mood 3, 1. These withdrawal symptoms overlap substantially with the patient's presenting complaints of depression, loss of appetite, and social isolation 1.
The temporal relationship between cannabis use and psychiatric symptoms must be clarified before attributing all symptoms to primary depression 4, 5. Non-medical marijuana use in depression is associated with:
- Higher suicidal ideation and worse mental health functioning 5
- Reduced improvement in depression symptoms over time compared to non-users 5
- Decreased likelihood of attending psychiatry visits, which aligns with this patient's medication non-compliance 5
Why Other Options Are Less Appropriate
Complete blood count (CBC) would be a reasonable secondary investigation to rule out medical causes of depression (such as anemia), but it does not address the immediate clinical priority of substance use assessment 1.
Thyroid function testing should be considered in the broader workup for depression, but given the clear history of active substance use and medication non-compliance, establishing the extent of cannabis use takes precedence 1.
Pulmonary function testing has no role in the initial psychiatric evaluation unless specific respiratory complaints are present, which are not mentioned in this case 1.
Clinical Implications
The urine drug test results will guide critical management decisions:
- If heavy cannabis use is confirmed (equivalent to >1.5 g/day inhaled), the patient is at higher risk for cannabis withdrawal syndrome when attempting cessation 3, 1
- Cannabis use disorder assessment should follow positive testing using validated screening tools 1
- Medication non-compliance may be directly related to cannabis use patterns, as non-medical marijuana use creates barriers to psychiatric care 5
Starting or adjusting antidepressants without understanding the full substance use picture would be premature and could mask underlying cannabis-related psychiatric symptoms 1, 4. The bidirectional relationship between cannabis use and depression requires clarification—depression may be driving cannabis use as self-medication, or cannabis use may be worsening depressive symptoms 6, 7.