Men who have sex with men (MSM) are very good at predicting if they will not have sex the next day, but not so accurate at anticipating whether they will, aidsmap reports. Such findings have important implications for research into intermittent or “on-demand” pre-exposure prophylaxis (PrEP) dosing strategies.

Publishing their findings in the Journal of Acquired Immune Deficiency Syndromes, researchers asked 92 MSM in New York City to keep daily online sexual diaries for 30 days, 1,688 days altogether. The men gave a percentage likelihood of their having anal intercourse with a casual partner the next day, and then reported each time such sex occurred.

The men tended to over-predict whether they would have sex the following day. When claiming there was a 100 percent likelihood of having intercourse the next day, 58 percent actually wound up having sex as predicted. Among those predicting a 50 percent likelihood, 20 percent had sex the next day. After a 20 percent predicted chance, 10 percent had sex. When they predicted a very low chance, of 1 to 10 percent, the men were more on point with their guesses: About 5 percent wound up having sex. When the men said they did not anticipate having sex, this prediction proved accurate.

The researchers calculated that if men were advised to take PrEP when they thought they would have sex the next day, there would be so many “false positives” that only 20 percent of those PrEP doses would actually fit the protocol. On the other hand, 3.8 percent of the skipped doses in such a scenario would be false negatives—or a missed chance to take PrEP. The only way to avoid false negatives was to instruct men not to take PrEP when they believed there was a zero percent chance of their having sex the following day.

To read the aidsmap story, click here.

To read the study abstract, click here.