Table 2 Second tier testing methods in KD NBS

30 kb deletion testing

Known pathogenic mutation, common in IKD patients

Low complexity, rapid assay. When found homozygous indicates IKD.

Rare mutation, whose presence is more likely to indicate carrier status (i.e. “false positive”) and where the absence will still not avoid possible IKD (“false negative”)

Psychosine testing

Appears to be associated with active disease in KD patients

Rapid test that when elevated indicates IKD.

Requires MS/MS equipment with higher sensitivity than that typically used in NBS labs but testing can be regionalized while still ensuring rapid turnaround time.

GALC Genotyping

With 30kbDel, it is traditionally considered the “gold standard” 2nd tier testing in KD-NBS, but there may still be GALC deletions missed, leading to false negative results.

Can identify those infants at highest risk for IKD. Provides some reassurance to those who are carriers, have only enzyme lowering polymorphisms, or known “mild” mutations.

Instrumentation and expertise required are beyond the capabilities of most NBS labs.

Many GALC mutations identified through KD-NBS are of uncertain clinical significance. Database of all known KD genotypes not available to support genotype interpretations.