Which laboratory test is most critical for a client with HIV receiving zidovudine (AZT)?

The most critical laboratory test for a client with HIV receiving zidovudine (AZT) is the complete blood count (CBC). Zidovudine, an antiviral medication, is known to potentially cause hematological side effects, particularly affecting bone marrow function. This can lead to conditions such as anemia, leukopenia, and thrombocytopenia, which are identifiable through a CBC.

Monitoring the CBC allows healthcare providers to keep a close watch on the patient’s red blood cell, white blood cell, and platelet counts. Early detection of any abnormalities enables timely interventions, which can be crucial in managing side effects and maintaining the client's overall health and ability to continue HIV treatment.

While monitoring electrolyte levels, liver function tests, and renal function tests are important for comprehensive patient care, the specific concerns related to zidovudine's toxicity and its impact on blood cell production place the CBC as the most relevant test in this context.