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Background: Muscle biopsy is a common diagnostic marker for myopathy assessment; however, it has a relatively low pathologic yield of less than 60%. Additionally, both diagnostic and non-diagnostic muscle biopsies can provide guidance for treatment, i.e, provide therapeutic usefulness., Purpose: We designed a study to determine if having a documented definitive preoperative differential diagnosis would affect the pathologic yield and therapeutic usefulness of muscle biopsies for myopathy.  Methods: This was a retrospective, single institution chart review of 106 consecutive muscle biopsies in adult patients, which looked at the presence or absence of a definitive preoperative differential diagnosis and relation to diagnostic yield and therapeutic usefulness of muscle biopsies.  Results: Of 106 muscle biopsies, 50 biopsies (47%) had a definitive preoperative differential diagnosis, 52 biopsies (49%) returned definitive pathology, and 93 biopsies (88%) provided therapeutic information. The presence of a documented differential diagnosis increased the odds of pathologic yield by 3.73 (p-value < 0.01) and therapeutic usefulness by 3.40 (p-value 0.08). If pathology was diagnostic then the therapeutic usefulness of the biopsy was 4.54 times more likely (p-value < 0.01).  Conclusion: Documentation of a definitive preoperative differential diagnosis, when pursuing muscle biopsy for myopathy, is associated with an increased pathologic diagnostic yield. Definitive pathology was associated with an increase in the therapeutic usefulness of the muscle biopsy., Competing Interests: The authors have declared that no competing interests exist.

Introduction Requests for muscle biopsy for evaluation of myopathy originate from a variety of different specialties. It is unknown whether the specialty of the referring service affects the likelihood of diagnostic biopsy or the therapeutic usefulness of a biopsy. Methods We reviewed 106 consecutive muscle biopsies requested by healthcare providers in neurology, rheumatology, family medicine, oncology, and gastroenterology. We tested for an association between referring service and whether the biopsy yielded a definitive pathology result or provided therapeutically useful results. Results Half of the biopsies (49%) returned definitive pathology and 88% of the biopsies provided information that contributed to therapeutic decisions. The diagnostic yield for muscle biopsies referred by different services was not statistically significant (p-value 0.1344) nor was the therapeutic yield statistically significant for muscle biopsies referred by different services (p-value 0.5525). Discussion The specialty of the service that requests a muscle biopsy does not influence the likelihood of obtaining a definitive pathological diagnosis or therapeutically useful information. Other factors may be more important in determining the likelihood of obtaining a clinically useful biopsy result., Competing Interests: The authors have declared that no competing interests exist.