What lab tests, other than lipid panel, should a primary care physician (PCP) monitor in a patient on a JAK2 inhibitor, such as ruxolitinib (Janus kinase 2 inhibitor)?

Laboratory Monitoring for Patients on JAK2 Inhibitors

Primary care physicians should monitor complete blood count with differential, liver function tests, renal function tests, and lipid panel in patients on JAK2 inhibitors. 1

Essential Laboratory Tests

• Complete Blood Count (CBC) with differential: Monitor for thrombocytopenia, anemia, neutropenia, and lymphopenia, which are common hematologic adverse effects of JAK2 inhibitors 2
• Liver Function Tests: Check for elevated liver enzymes as JAK inhibitors can cause liver enzyme increases, particularly with upadacitinib 1
• Renal Function Tests: JAK inhibitors are partially excreted in urine and elevated levels have been reported in patients with renal impairment 1
• Lipid Panel: JAK inhibitors increase total cholesterol, HDL, LDL, and triglycerides, requiring regular monitoring 1, 2

Monitoring Schedule

• Baseline: Obtain all laboratory tests before initiating JAK2 inhibitor therapy 1
• 4-12 weeks after initiation: Repeat all laboratory tests 1
• Every 3-6 months: Continue monitoring during maintenance therapy 1

Additional Testing Based on Clinical Context

• JAK2 V617F Mutation Analysis: For patients with polycythemia vera, to monitor treatment response 1, 3
• Tuberculosis Screening: Prior to initiation and periodically during treatment due to increased infection risk 1, 2
• Hepatitis B and C Testing: Prior to initiation to prevent viral reactivation 1, 2
• HIV Testing: Consider in high-risk patients before starting therapy 1

Disease-Specific Monitoring

For Myeloproliferative Neoplasms (MPN)

• Lactate Dehydrogenase (LDH): Elevated in MPNs and can be used to monitor disease activity 1
• Erythrocyte Sedimentation Rate (ESR): May be artificially decreased due to JAK inhibition of IL-6 signaling 1
• C-Reactive Protein (CRP): May be artificially decreased due to JAK inhibition 1

For Inflammatory Conditions

• Disease-specific activity measures: Use validated measures that don't rely solely on CRP or ESR, as JAK inhibitors can reduce these markers independently of disease activity 1

Important Caveats and Pitfalls

• Misleading inflammatory markers: CRP and ESR may normalize due to JAK inhibition even when disease activity persists, potentially masking infections 1
• Hematologic toxicity management: Dose adjustments may be needed if hemoglobin decreases below 8 g/dL, absolute lymphocyte count falls below 0.5 × 10^9/L, or absolute neutrophil count is less than 1 × 10^9/L 1
• Platelet monitoring: For abrocitinib specifically, treatment is not recommended if platelet count is below 150 × 10^9/L and should be stopped if platelets fall below 50 × 10^9/L 1
• Withdrawal syndrome: Abrupt discontinuation of JAK inhibitors can cause a shock-like syndrome due to cytokine rebound; tapering is recommended 2

Clinical Response Evaluation

• Regular skin examinations: For detection of skin cancer, following national recommendations 1
• Spleen size assessment: For patients with myeloproliferative disorders 1
• Symptom burden evaluation: Using validated disease-specific instruments 1, 3

By implementing this comprehensive monitoring approach, primary care physicians can effectively manage patients on JAK2 inhibitors while minimizing adverse events and optimizing therapeutic outcomes.