Lower CD4 counts and high viral loads, but not antiretroviral (ARV) therapy, are associated with an elevated risk of having a blood clot, according to a study published in the November issue of AIDS Patient Care and STDs.

A number of small studies during the past two decades examined the risk of clots in blood vessels and the heart in people living with HIV. Early in the epidemic, these studies suggested that blood clots may be more common in people living with HIV than in the general public. A number of factors, including the higher rate of systemic infections and elevations in inflammatory proteins in people with advanced HIV disease, were offered as explanations. More recent studies have suggested that the rate of blood clots had actually increased since the introduction of potent combination ARV therapy in 1996.

To determine the rate of blood clots in people with HIV and the factors associated with them, Nancy Crum-Cianflone, MD, MPH, from the Naval Medical Center in San Diego, and her colleagues examined the medical records of 465 HIV-positive patients from the Naval Medical Center. Dr. Crum-Cianflone’s team first noted that 3.7 percent of the patients developed a blood clot between 1996 and 2007. Compared to age-matched men in the general population, the rate of development of blood clots was four times higher in the HIV-positive patients.

Though the rate of new blood clots was somewhat higher between 2001 and 2007 than it had been between 1996 and 2001, the difference was not statistically significant, meaning that it was small enough to have occurred by chance. Moreover, upon deeper analysis, neither ARV use overall, nor specific classes of ARVs or duration of HIV treatment was associated with having a blood clot. Factors that were associated with blood clots included currently having a lower CD4 count (153 cells on average), ever having had a particularly low CD4 count, or having a high viral load. Other factors included high cholesterol, systemic infections, having a central line catheter and smoking tobacco.

Crum-Cianflone’s team also searched medical literature and found a similarly elevated rate of blood clots among people living with HIV that was from two to 10 times higher than in the general population.