A patient takes methotrexate together with adalimumab (Humira) for rheumatoid arthritis. Which laboratory investigation is most important before surgical dental treatment?

Correct answer: C. Complete blood count with differential

• A. Bleeding time is obsolete, poorly reproducible and no longer recommended for preoperative assessment.
• B. INR monitors warfarin. Neither of these drugs affects the vitamin-K-dependent clotting factors.
• C. CORRECT. A COMPLETE BLOOD COUNT is the key investigation. Methotrexate is a folate antagonist causing bone-marrow suppression — leukopenia, thrombocytopenia and megaloblastic anaemia — and adalimumab, a TNF-alpha inhibitor, adds further immunosuppression. The CBC with differential quantifies both the infection risk (neutrophil count) and the bleeding risk (platelet count) before any surgical procedure.
• D. Glucose is relevant if corticosteroids are also being taken, but it is not the primary concern with this drug pair.

Note: Renal function IS monitored with methotrexate, since the drug is renally cleared and impaired clearance drives toxicity — so it is a defensible secondary answer. But the CBC is the investigation that directly governs whether it is safe to operate. Standard methotrexate monitoring covers CBC, liver enzymes and renal function together.