Immunological methods are used to determine serum AAT concentrations, and serum standards are used to determine the concentration of the antigen in an unknown sample. Neonates have significantly lower serum concentrations compared to adults. In one study, the mean concentration observed in neonates for PI M was 23.8 ±5.5 mM, compared with the PI M adults, where the concentration was 32.7±8.7 mM. A concentration of less than 11 mM can be used as a cutoff for deficiency states, and provides a useful screening test. It is important to ensure that these concentrations are observed in the basal state in the absence of an acute-phase reaction. It is therefore useful to measure concurrently serum C-reactive protein (CRP), one of the most sensitive markers for the acute-phase response, to exclude an acute-phase reaction. The value of screening newborns and the general population for deficiency remains controversial.
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