Type A and B are diagnosed by measuring the amount of ASM in white blood cells. The test can be done using a blood or bone marrow sample. Such testing can tell who has the disease, but does not reveal who may be carriers. DNA tests can be done to diagnose carriers of Type A and B. A skin biopsy is usually used to diagnose Type C and Type D Niemann-Pick. Laboratory scientists will watch how the skin cells grow and study how they move and store cholesterol. DNA tests may also be done to look for the two genes that cause Niemann-Pick disease Type C.