Objective: To summarize facioscapulohumeral muscular dystrophy (FSHD) diagnostic testing results from the University of Iowa Molecular Pathology Laboratory. 

Methods: All FSHD tests performed in the diagnostic laboratory January 2015-July 2019 were retrospectively reviewed. Testing was by restriction enzyme digestion and Southern blot analysis with sequencing of SMCHD1, if indicated. Cases were classified as FSHD1 (4q35 EcoRI size ≤40kb; 1-10 D4Z4 repeats), FSHD2 (permissive 4q35A allele, D4Z4 hypomethylation, and  pathogenic SMCHD1 variant), or non-FSHD1,2. We also noted cases with borderline EcoRI fragment size (41-43kb; 11 D4Z4 repeats), cases that meet criteria for both FSHD1 and FSHD2, somatic mosaicism, and cases with hybrid alleles that add complexity to test interpretation. 

Results: Of the 1594 FSHD tests included in the analysis, 703 (44.1%) were diagnosed with FSHD. Among these positive tests, 664 (94.5%) met criteria for FSHD1 and 39 (5.5%) met criteria for FSHD2. Of all 1594 cases, 20 (1.3%) had a 4q35 allele of borderline size, 23 (1.5%) were somatic mosaics, and 328 (20.9%) had undergone translocation events. Considering only cases with at least one 4q35A allele, D4Z4 repeat number differed significantly among groups: FSHD1 cases median of 6.0 (IQR: 4-7) repeats, FSHD2 cases 15.0 (IQR: 12-22) repeats, and non-FSHD1,2 cases 28.0 (IQR: 19-40) repeats. 

Conclusion: FSHD1 accounts for 94.5% of genetically confirmed cases of FSHD. The data show a continuum of D4Z4 repeat numbers with FSHD1 samples having the fewest, FSHD2 an intermediate number, and non-FSHD1,2 the most.