If SMA is suspected, a blood test will be requested, which will look for the presence or absence of the SMN1 gene. If the SMN1 gene is absent, SMA can be diagnosed. 

If the gene is present, other diagnostic tests will take place to rule out rare forms of SMA caused by changes to different genes, and other neuromuscular conditions. In two to five per cent of patients with SMA, the SMN1 gene is not missing, just inactive. Testing for this is more complicated and may take some time.