Final answer:
The effectiveness of a client's medication regimen for HIV is best determined by the viral load test, which measures the amount of HIV RNA in the blood.
Step-by-step explanation:
The lab report that provides information regarding the effectiveness of a client's medication regimen for HIV is the viral load test. This test measures the amount of HIV RNA in the blood and reflects how well the treatment is controlling the virus. The initial diagnosis of HIV may involve an ELISA test followed by a confirmatory Western Blot or PCR test. Once treatment starts, the viral load and CD4 count become important indicators. A successful treatment regimen typically results in a reduced viral load and maintenance or increase in CD4 T cell count. A CD4 count is also crucial, as it measures the number of CD4 T cells in the blood, indicating the person's immune system status, but the viral load is the direct measure of treatment effectiveness.