What labs and side effects should be monitored in a patient newly started on mirtazapine?

Monitoring for Patients Newly Started on Mirtazapine

Patients newly started on mirtazapine should be monitored for weight gain, sedation, and complete blood counts, with particular attention to signs of agranulocytosis. 1 These monitoring parameters are essential as they directly impact patient morbidity, mortality, and quality of life.

Laboratory Monitoring

Baseline Labs (Before Starting Treatment)

• Complete blood count (CBC) with differential 1
• Liver function tests 2
• Consider baseline weight and body mass index (BMI) 2

Follow-up Labs

• CBC monitoring, particularly if signs of infection develop 1
• If a patient develops sore throat, fever, stomatitis, or other signs of infection along with a low WBC count, mirtazapine should be discontinued immediately 1

Side Effect Monitoring

Common Side Effects (>5% incidence)

• Somnolence/sedation (54%) - most prominent side effect 1
• Increased appetite (17%) 1
• Weight gain (12%) 1
• Dry mouth (25%) 1
• Dizziness (7%) 1
• Constipation (13%) 1

Serious Side Effects (Requiring Immediate Attention)

• Agranulocytosis - rare but potentially life-threatening (approximately 1.1 per thousand patients) 1
  • Monitor for: fever, sore throat, stomatitis, signs of infection with low WBC
• Serotonin syndrome (when combined with other serotonergic medications) 1
  • Monitor for: mental status changes, autonomic instability, neuromuscular symptoms, seizures, GI symptoms
• Suicidality - especially in the first few weeks of treatment 1
  • Monitor for: emergence of agitation, irritability, unusual changes in behavior

Monitoring Schedule

1. First 4-8 weeks:

   • Weekly clinical assessment for sedation, appetite changes, weight gain
   • Monitor for emergence of suicidality, especially in younger patients
   • CBC if symptoms of infection develop

2. Subsequent monitoring:

   • Regular weight checks (every 1-3 months)
   • CBC as clinically indicated (with signs of infection)
   • Assess ongoing efficacy and side effects at each visit

Special Considerations

• Elderly patients: Lower clearance of mirtazapine; monitor more closely for side effects 1
• Hepatic/renal impairment: Reduced clearance; may require dose adjustments 1
• Drug interactions: Monitor when combined with other serotonergic medications 1
• Sleep and appetite effects: Mirtazapine promotes sleep, appetite, and weight gain - beneficial in some patients but problematic in others 2

Clinical Pearls

• Sedation tends to be more prominent at lower doses (7.5-15mg) and may actually improve at higher doses 2, 3
• Mirtazapine has a therapeutic plasma concentration range of 40-80 ng/mL, though routine monitoring of levels is not generally recommended 2
• Unlike many other antidepressants, mirtazapine has minimal sexual side effects 4
• The medication is typically administered as a single dose at bedtime to take advantage of its sedative properties 2

Common Pitfalls to Avoid

1. Failing to warn patients about sedation, which can impair driving and other activities requiring alertness
2. Not monitoring weight gain, which can lead to metabolic complications
3. Missing early signs of rare but serious blood dyscrasias
4. Overlooking the potential for drug interactions, especially with other CNS depressants

By implementing this monitoring protocol, clinicians can optimize the benefits of mirtazapine therapy while minimizing risks to patient health and quality of life.