Biochemical genetic testing involves the study of enzymes in the body that may be abnormal in some way. The enzymes may be deficient, absent, or unstable, or they may have altered activity which can lead to clinical manifestations in a child (for example, birth defects). These types of disorders are usually called "inborn errors of metabolism" since they are present at birth and affect how the body's metabolism works. Metabolism is a term that describes how the body converts food to energy and then gets rid of the waste products.
There are more than 200 enzyme defects that can be studied in humans. Sometimes, rather than studying the gene mutation which is causing the enzyme to be defective in the first place, it is easier to study the enzyme itself (which is the product of the gene). The testing approach depends on the disorder. For example, mutations in the gene that causes Tay-Sachs disease can be studied. Although DNA mutation testing is available, the current methods do not detect all possible mutations. By performing a laboratory test for the enzyme that is deficient in Tay-Sachs, you can detect more cases than through DNA testing alone. Biochemical genetic studies may be done from a blood sample, urine sample, spinal fluid, or other tissue sample.