Blood tests. Severe combined immunodeficiency is diagnosed in children with a few different tests. Some states (like ny, wi, ma) include scid screening with a trec assay on all newborns. Blood tests for t-cells, b-cells, nk-cells can be done as well. Lastly genetic tests, based on the t/b/nk cell info can be done. Seek the help of an immunodeficiency expert if you know a child with scid.
History, blood test. Diagnosis is suspected when a child present with too many infections or failure to gain weight. It is confirmed by studying the number and function of t cells (special set of white blood cells). Genetic testing follows to classify the disease. In some states (wi, ma, ca, ny) newborns are currently screened for scid.