Guthrie test is a bacterial inhibition assay. p2- Thienylalanine is placed in the medium and normally causes the inhibition of Bacillus subtilis growth. However, in the presence of excess of phenylalanine, this inhibition is test is the least expensive screening method v overridden and bacterial growth occurs. Thi available for determining excess phenylalanine in the blood but other tests are used to confirm findingsGuthrie Test • is a bacterial inhibition assay. β2- Thienylalanine is placed in the medium and normally causes the inhibition of Bacillus subtilis growth. However, in the presence of excess of phenylalanine, this inhibition is overridden and bacterial growth occurs. This test is the least expensive screening method available for determining excess phenylalanine in the blood, but other tests are used to confirm findings