Blood tests in PNH show changes consistent with hemolytic anemia: low hemoglobin, raised lactate dehydrogenase, raised reticulocytes (immature red cells released by the bone marrow to replace the destroyed cells), raised bilirubin (a breakdown product of hemoglobin) and decreased levels of haptoglobin. The direct antiglobulin test (DAT, or direct Coombs' test) is negative, as the hemolysis of PNH is not caused by antibodies.

A sugar or sucrose lysis test, in which a patient's red blood cells are placed in low ionic strength solution and observed for hemolysis, is used for screening. A more specific test for PNH, called Ham's acid hemolysis (after Dr Thomas Ham, who described the test in 1937) test, is performed if the sugar test is positive for hemolysis. More sensitive modern methods include flow cytometry for CD55 and CD59 on white and red blood cells. Dependent on the presence of these molecules on the cell surface, they are classified as type I, II or III PNH cells. Type I cells have normal levels of CD55 and CD59, type II have reduced levels and type III have absent levels. The higher the number of type III cells, the higher the risk of hemolysis and thrombosis.