ALT For a disease present in 2% of the population using a screening test with 95% accuracy. Of 1,000 people, 20 have the disease (of which 19 test positive) and 1 tests negative (false negative). Of 980 people without the disease, 49 test positive (false positive) and 931 test negative. The total number of 'positive' results is 68, but only 19 of these are correct, so the chance you have the disease if you have received a positive result is actually only 28%. A 28% chance is higher than the 2% chance you had before taking the test, but for most diseases this isn't enough to start treatment and further investigation would be needed.