When is an hpt 100% correct?

No test is 100% . By hpt, i assume you mean home pregnancy test. Statistically speaking, there is no test that is 100% correct 100% of the time. We're just happy to get 90+% sensitivity (positive test when you have condition) & specificity (negative result when you don't) when used properly as directed. But you can get false positive if guy takes hpt but you'd know to ignore result. Be sure to use test at right tim.