Absolute Lymphocyte Count (ALC)

ALC = WBC count x 1000 x % lymphs

Absolute lymphocyte count is a hematologic principle that has been validated in an emergency setting; an ALC <1,000 cells/mm³ predicted CD4 counts less than 200 cells/mm³ with 67% sensitivity (95% confidence interval 62-72%), specificity of 96% (94-98%), positive predictive value 91% (87-85%), and a negative predictive value of 81% (78-84%). An ALC <2,000 cells/mm³ predicted CD4 counts less than 200 cells/mm³ with sensitivity 97% (95-99%), specificity 41% (37-45%), positive predictive value 52% (48-56%), and negative predictive value 95% (92-98%).

A more recent study that looked into the PPV and NPV of the total lymphocyte in 184 HIV positive patients. Sensitivity, specificity, positive and negative predictive values of lymphocyte count <1,200 cells/mm³ to predict CD4 count <200 cells/mm³ were 72.2%, 100%, 100%, and 95.7% respectively.

Predicted CD4 from ALC (van der Ryst E 1998)

Serves as a surrogate for CD4 count in HIV patients who are naive to antiretroviral therapy in resource-limited areas.

• Frequently used in the ED setting because CD4 tests take longer to come back and need patient consent.

• Used in limited-resource settings where CD4 tests are costly or unavailable.

• If the ALC is low, the CD4 will usually be low (Shapiro 1998).

• Currently evidence is mostly around HIV-1 treatment-naive patients.

• Other viral infections such as influenza or EBV can cause lymphopenia.

• Does not aid in the diagnosis of HIV.

Does not change management, but helps support the suspicion of HIV infection with low CD4.

• In the right clinical context, consider undiagnosed HIV as a cause of low lymphocyte count.

• If available, obtain a CBC with differential and CD4 count for confirmation.