Final answer:
The most common laboratory value observed in adults with HIV is a decreased CD4 T-cell count.
Step-by-step explanation:
In adults with human immunodeficiency virus (HIV), the most common laboratory value that is observed is decreased CD4 T-cell count. HIV gradually leads to a decline in the number of CD4 helper T cells, which impairs the immune system's ability to fight infections. Monitoring the CD4 T-cell count is crucial in managing HIV and adjusting treatment if necessary.