Laboratory tests - full blood count (FBC) in rheumatic disease

Focus in rheumatology

In rheumatology, the FBC provides clues to disease activity, drug toxicity, and specific disease associations - cytopenias in particular can reflect either the underlying rheumatic disease itself or a complication of its immunosuppressive treatment, a distinction with major management implications
Routinely monitored both at diagnosis and serially during immunosuppressive treatment, given the myelosuppressive potential of many disease-modifying agents used in rheumatology

Anaemia of chronic disease (normochromic normocytic, from cytokine-driven iron sequestration) is common in active inflammatory disease; cytopenias as a direct disease manifestation occur in SLE specifically (haemolytic anaemia, leucopenia/lymphopenia, and thrombocytopenia are all recognised, and lymphopenia is one of the SLE classification criteria)
Drug-induced cytopenias - methotrexate, azathioprine (particularly with reduced TPMT activity), leflunomide, and cyclophosphamide can all cause myelosuppression, requiring regular FBC monitoring while on these agents

Distinguishing disease-driven cytopenia from drug-induced cytopenia requires correlating the timing (temporal relationship to a drug start/dose change vs a broader flare pattern) with other markers of disease activity (complement, anti-dsDNA in SLE) and the specific cytopenia pattern
Eosinophilia can be a clue to a specific rheumatic disease process (e.g. eosinophilic granulomatosis with polyangiitis) or a drug reaction, rather than the more common allergic/parasitic causes

Significant drug-induced cytopenia generally requires dose reduction or cessation of the causative agent, alongside standard cytopenia management (transfusion support, growth factors as indicated); disease-driven cytopenia (e.g. SLE-related) is managed by treating the underlying disease activity, generally with escalated immunosuppression rather than stopping treatment

SLE haematological manifestations (a formal SLE classification criterion), methotrexate/azathioprine myelosuppression, TPMT deficiency and thiopurine toxicity risk, eosinophilic granulomatosis with polyangiitis

The direction of the correct response (escalate vs reduce immunosuppression) is opposite depending on whether a cytopenia is disease-driven or drug-induced - correctly distinguishing the two before acting is one of the more consequential judgement calls in monitoring rheumatic disease patients on treatment

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