Assessing the consistency of mass spectrometry, a clinical-laboratory model, and immunohistochemistry in amyloid subtyping: a Brazilian experience.

Abstract

Background: Systemic amyloidosis is a potentially fatal protein misfolding disorder usually underdiagnosed in low- and middle-income countries, where limited awareness and restricted access to diagnostic tools contribute to prolonged diagnostic journeys and delayed diagnoses. Accurate identification of the precursor protein is essential but remains a challenge, particularly in resource-limited settings. This study aimed to perform mass spectrometry (MS) for amyloid subtyping and to use it as the reference method to evaluate the consistency of a clinical-laboratory model (CLM) and immunohistochemistry (IHC) in determining the amyloid subtype.

Methods: In this retrospective, observational, single-center study, MS was performed on tissue biopsies from patients diagnosed with systemic amyloidosis between 2009 and 2018 at a public university hospital in Brazil. An IHC panel of four antibodies (anti-kappa, -lambda, -serum amyloid A, -transthyretin) was performed on samples with sufficient material. Review of medical records assessed the amyloid subtype determined by the clinical-laboratory model (CLM), which was based on clinical presentation, laboratory and imaging data, genetic testing, and pathological findings available at the time of the initial diagnosis.

Results: From 127 patients, 48 were excluded due to unavailable biopsies or insufficient material for MS analysis. The final cohort consisted of 79 patients, 61% male, with a median age of 61 years. Biopsies from 13 different tissues were analyzed by MS, revealing the following amyloid subtypes: AL (56%), ATTR (25%), AA (6%), AFib (3%), AH (1%). Seven cases (9%) remained inconclusive. IHC correctly subtyped amyloid in 28% of cases but failed in 66%. In 80% of patients the CLM correctly identified the amyloid subtype. However, it generated incorrect typing leading to inappropriate treatments.

Conclusion: The consistency analysis between the CLM, IHC and MS demonstrated the superiority of MS in amyloid subtyping from tissue biopsies. While the CLM failed in 20% of cases and resulted in inappropriate treatments due to false-positive results, IHC showed very limited diagnostic performance, contrasting with results from reference centers, with less than one-third of cases correctly classified. These findings reinforce the role of MS as a more accurate and cost-competitive method for amyloid subtyping in middle-income countries.