A presumed blood sample is first collected with a swab. A drop of phenolphthalein reagent is added to the sample, and after a few seconds, a drop of hydrogen peroxide is applied to the swab. If the swab turns pink rapidly, it is said to test presumptive positive for blood. Waiting for periods over 30 seconds will result in most swabs turning pink naturally as they oxidize on their own in the air.
Optionally, the swab can first be treated with a drop of ethanol in order to lyse the cells present and gain increased sensitivity and specificity. This test is nondestructive to the sample, which can be kept and used in further tests at the lab; however, few labs would use the swab used for the Kastle–Meyer test in any further testing, opting instead to use a fresh swab of the original stain.