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Diagnosis of X-linked hypogammaglobulinemia can be especially difficult because recurrent infections are common even in normal infants (many of whom don’t start producing their own antibodies until age 18 to 20 months). Immunoelectrophoresis and quantitative immunoglobulins (nephelometry) confirm decreased levels, or a total absence, of IgM, IgA, and IgG in the serum. IgG is usually less than 200 mg/dl, and IgA and IgM are almost unmeasurable. However, diagnosis by this method usually isn’t possible until the infant is 9 months old. Antigenic stimulation confirms an inability to produce specific antibodies, although cellular immunity remains intact.
Source: Professional Guide to Diseases (Eighth Edition), 2005
A specific genetic test (polymerase chain reaction) can also be performed to diagnose this disease. This test looks for an expanded mutation (called a triplet repeat) in the FRAXA gene.
Before identification of the FMR1 mutation, a special cytogenetic (chromosome) blood test was used to microscopically detect the fragile site on the long arm of the affected X chromosome. It's now common knowledge that a full FMR1 mutation doesn't always result in a cytogenetically detectable fragile site. Therefore, chromosome analysis alone can provide false-negative results. Chromosome analysis still has utility together with FMR1 mutation analysis when performing a genetic evaluation on a male with mental retardation of unknown etiology.
In addition to diagnosing fragile X syndrome, genetic testing can determine whether the mother of a diagnosed individual is a carrier of the FMR1 premutation or has a full mutation. This information can be used for preconceptional genetic counseling by a trained professional and prenatal testing if the woman so chooses. FMR1 mutation analysis can also be subsequently performed on at-risk family members. It should be noted, however, that communication of genetic test results to at-risk family members constitutes a breech of patient confidentiality and privacy unless prior written permission to communicate results has been obtained from the previously tested patients.
Source: Professional Guide to Diseases (Eighth Edition), 2005
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