Impact of muscle biopsy on the clinical decision-making process in patients with suspected idiopathic inflammatory myopathy.

BACKGROUND: The significance of muscle biopsy as a diagnostic tool in idiopathic inflammatory myopathies (IIM) remains elusive. We aimed to determine the diagnostic weight that has been given to muscle biopsy in patients with suspected IIM, particularly in terms of clinical diagnosis and therapeutic decisions. MATERIAL AND METHODS: In this retrospective multicentric study, we analyzed muscle biopsy results of adult patients with suspected IIM referred to a tertiary center between January 1, 2007, and October 31, 2021. Information regarding referral department, suspected diagnosis, biopsy site, demographic, clinical, laboratory data, and imaging results were extracted. Statistical analyses included the level of agreement between suspected and histological diagnosis and calculation of diagnostic performance (positive and negative predictive values, positive and negative likelihood ratios, sensitivity, and specificity of muscle biopsy in relation to clinical diagnosis and/or treatment initiation). Performance was tested in different strata based on clinical pre-test probability. RESULTS: Among 758 muscle biopsies, IIM was histologically compatible in 357/758 (47.1%) cases. Proportion of IIM was higher if there was a solid clinical pre-test probability (64.3% vs. 42.4% vs. 48% for high, medium and low pre-test probability). Sensitivity and specificity of muscle biopsy were highest (82%) when the diagnosis by the clinician was used as outcome scenario. Negative predictive value was only moderate (between 63% and 80%) and lowest if autoantibodies were positive (35%). CONCLUSION: In patients with clinically suspected IIM, approximately 50% of biopsies revealed features indicative of IIM. Diagnostic performance of muscle biopsy was moderate to high depending on clinical pre-test probability.

Transglutaminases as diagnostically relevant autoantigens in patients with gluten sensitivity.

BACKGROUND: Patients with gluten sensitivity, i. e. celiac disease and dermatitis herpetiformis have anti-endomysial antibodies recognizing transglutaminases, which are usually detected on appropriate tissue sections. It would be desirable to have available a reliable, tissue-independent serological diagnostic tool. We compared disease-specificity and sensitivity of tTG versus eTG-based detection systems for the diagnosis of anti-endomysial IgA-antibodies. PATIENTS AND METHODS: We examined 204 serum samples in duplicates with commercial human ELISA-kits: 54 healthy blood donors, 20 celiac disease, 29 dermatitis herpetiformis and 101 with other autoimmune dermatoses. RESULTS: The tTG-based ELISA proved to be very disease-specific (100 %) and sensitive for the diagnosis of gluten sensitivity (95 % celiac disease; 96.6 % dermatitis herpetiformis). The eTG-based ELISA was also perfectly specific (100 %), but only 15 % of celiac disease-sera and 44.8 % of dermatitis herpetiformis-sera yielded positive results. CONCLUSIONS: The human tTG-ELISA fulfills all criteria of a screening test and, because of being investigator-independent, inexpensive and highly reproducible, compares favorably with the current diagnostic gold standard (indirect immunofluorescence and biopsy) of celiac disease and dermatitis herpetiformis. The low sensitivity of the eTG-ELISA may have technical reasons, but could theoretically also be linked to disease activity or indicate the existence of an as yet undefined disease subset. Studies are currently under way to address these issues.