A client with tuberculosis is starting drug therapy with Isoniazid (INH). Before giving the client the first dose, the nurse should ensure that which baseline study has been completed?

A. Electrolyte levels.: Monitoring electrolyte levels is not routinely required before starting isoniazid therapy, as INH does not significantly affect sodium, potassium, or chloride balance. Electrolyte testing may be done for other medical reasons, but it is not INH initiation. B. Coagulation times.: Isoniazid does not commonly affect coagulation or interfere with clotting factors. Baseline coagulation studies would be necessary only if the client were on anticoagulants or had a known bleeding disorder, not for standard tuberculosis treatment. C. Liver enzymes (LFTs).: Baseline liver function tests are essential before initiating isoniazid because the drug is hepatotoxic. Elevated liver enzymes or preexisting hepatic disease increases the risk of severe hepatotoxicity, especially in older adults and alcoholics. D. Serum creatinine level.: Although renal function is important in general pharmacologic management, INH is primarily metabolized by the liver rather than excreted unchanged by the kidneys. Therefore, serum creatinine testing is less critical than assessing liver enzymes.