Interlaboratory Evaluation of Short Tandem Repeat Triplex CTT

An interlaboratory comparison of typing results for Short Tandem Repeats (STRs) at the GenBank loci HUMCSF1PO, HUMTPOX, HUMTH01, and HUMVWFA31 using the "CTT triplex" and "CTTv quadruplex" has been evaluated. These STRs all have a nominal four basepair (bp) repeat. Seven different samples were distributed to 41 laboratories. The 34 laboratories that returned results used a wide variety of analytical systems. Comparable results were obtained for all samples at all loci when results were reported as an allelic name. Raw sizing results obtained from internal-lane sizing standards differed by nearly five bp at some loci. Many different factors contribute to this observed sizing variability, including choice of sizing standards and matrix composition. Although sizing results can be made more comparable by locus-specific offsets or calibration to a comprehensive set of alleles at each locus, samples typed to the allelic name can now be validly compared regardless of analytical method. Interlaboratory comparison of raw allelic size remains problematic.