FIND-A1 Biochemical Tests
If AATD is being considered in a differential diagnosis a quantitative serum analysis should be conducted. This measurement is inexpensive. It is recommended that C-reactive protein (CRP) is assessed at the same time because AAT that may be elevated during inflammation and so appear deceptively normal in patients with medium grade deficiency. Simultaneous measurement of CRP will help identify this possibility. The normal serum concentration in adults is 1.5 to 2.5 g/L. Sometimes the concentration is expressed in micromoles per litre – these figures are roughly 20 times the value when expressed in grams per litre. Homozygotic PiZZ patients typically have a serum level of AAT less than 0.3 g/L. The minimum protective level is generally accepted to be 0.8 g/L
Phenotyping is a more complex analysis, which measures the electrophoretic mobility of the AAT molecules. The conventional names of the alleles are based on this test: F (fast: anodic), M (medium: moderate mobility), S (slow anodic mobility), Z (cathodic).
The more common M, S and Z alleles may be detected by genotyping using a polymerase chain reaction (PCR) to amplify the point mutations which are then detected by various means. This process is becoming more popular and can be cheaper than IEF. If a rare allele is suspected and a suitable PCR probe is not available then genetic sequencing may be performed. This is a more expensive option and only needed when the clinical picture is confused.