What would a complete blood count (CBC) with differential show in a patient being worked up for immune checkpoint inhibitor myocarditis after being treated with nivolumab (opdivo)?

CBC Findings in Immune Checkpoint Inhibitor Myocarditis After Nivolumab Treatment

In a patient being worked up for immune checkpoint inhibitor myocarditis after nivolumab treatment, the CBC with differential would likely show lymphocytosis, potentially with eosinophilia, while other cell lines typically remain within normal limits unless there are concurrent immune-related adverse events affecting hematologic parameters.

Typical CBC Findings in ICI Myocarditis

Expected CBC Abnormalities

• Lymphocytosis: Elevated absolute lymphocyte count reflecting the T-cell mediated immune response 1
• Possible eosinophilia: May be present in some cases as part of the immune-mediated inflammatory response
• Normal red blood cell parameters: Hemoglobin and hematocrit typically remain within normal limits
• Normal platelet count: Unless there is concurrent immune thrombocytopenia

Diagnostic Context

The CBC findings should be interpreted alongside cardiac biomarkers, which are more specific for myocarditis:

• Elevated troponin levels (most sensitive marker)
• Elevated creatine kinase (CK) and CK-MB
• Elevated brain natriuretic peptide (BNP) in cases with decreased ejection fraction 1

Pathophysiology and Clinical Correlation

Immune checkpoint inhibitor myocarditis results from lymphocytic infiltration of the myocardium and myocardial conduction system 1. The CBC findings reflect this pathophysiology:

• Lymphocytosis corresponds to the T-cell mediated autoimmune response against cardiac tissue
• The lymphocytic infiltration seen on myocardial biopsies correlates with the peripheral blood lymphocytosis 2
• Peripheral blood findings may precede clinical cardiac manifestations

Concurrent Hematologic Abnormalities

While evaluating for myocarditis, be aware that nivolumab can cause other immune-related adverse events affecting the CBC:

• Autoimmune hemolytic anemia: May show decreased hemoglobin, elevated reticulocyte count, and indirect hyperbilirubinemia
• Immune thrombocytopenia: May show isolated thrombocytopenia
• Neutropenia: Rare but reported with ICIs 1

Monitoring Recommendations

For patients on nivolumab who are being evaluated for myocarditis:

• Obtain baseline CBC with differential before starting therapy
• Monitor CBC regularly during treatment (recommended by 94% of experts) 1
• Any new lymphocytosis should prompt consideration of immune-related adverse events
• Concurrent CPK/troponin monitoring is recommended by 33% of experts due to the serious consequences of myocarditis despite its low incidence 1

Clinical Pitfalls to Avoid

• Don't dismiss mild lymphocytosis: Even mild elevations can be significant in the context of cardiac symptoms
• Don't attribute all CBC abnormalities to ICI therapy: Concurrent infections or disease progression can cause similar findings
• Don't overlook other immune-related adverse events: Multiple organ systems can be affected simultaneously
• Don't delay treatment: Myocarditis can rapidly lead to death and requires immediate intervention with high-dose corticosteroids 1

Remember that myocarditis is a rare but potentially fatal complication of ICI therapy, with mortality rates as high as 50%. Early recognition of both laboratory and clinical findings is crucial for prompt intervention.