Which lab finding should the nurse report immediately for a client receiving phenytoin?

The correct answer highlights the importance of monitoring phenytoin levels due to the drug’s narrow therapeutic index. A phenytoin level of 8 mcg/mL is considered subtherapeutic for most patients, particularly those with seizure disorders. This low level indicates that the drug may not be effectively controlling seizures, which is a critical aspect of therapy for individuals requiring phenytoin.

Therefore, it is vital to report this finding immediately to ensure the client’s seizure management is addressed promptly. Adjustments in dosage or monitoring for additional therapeutic strategies may be necessary to achieve a therapeutic range and prevent potential complications related to uncontrolled seizures.

Other lab finders provided in the options, such as the digoxin level, INR, and lithium level, while relevant to their respective therapies, do not present an immediate danger or require urgent intervention as the phenytoin level does in this context.