Introduction:
Amyloidosis is a heterogeneous group of conditions associated with tissue deposition of insoluble abnormal proteins that damage vital organs. Early diagnosis, when the deposits are minimal, determines the prognosis and requires histological confirmation. The commonly adopted gold standard technique is alkaline Congo red (ACR) staining, though its sensitivity is limited. There is a need for a simple and inexpensive screening method offering a better chance of detecting mini­mal amyloid deposits. The aim of this study was to compare amyloid detectability with ACR and phenol Congo red (PHCR) staining techniques for early detection of minimal deposits.

Material and methods:
We assessed 452 tissue specimens (including adipose tissue, gastrointestinal mucosa, labial salivary gland, myocardium, and bone marrow) from 425 patients with clinically suspected systemic or local amyloidosis, which had been sent to the Pathology Laboratory of the National Institute of Geriatrics, Rheumatology and Rehabilitation in Warsaw. Adjacent sections from each specimen were stained with ACR and PHCR. If amyloid was detected, immunohistochemical typing was conducted. The consistency of the two staining methods was expressed as Cohen’s κ coefficient.

Results:
A total of 169 tissue specimens (37%) yielded positive readings, with 93 cases ACR(+) and PHCR(+); 75 cases ACR(–) and PHCR(+), and 1 case ACR(+) and PHCR(–). The percentage agreement between the staining methods was 83%, with the Cohen’s κ coefficient value of 0.60 (95% CI: 0.52–0.69), which corresponds to moderate agreement according to Fleiss. Additional immunohistochemical amyloid typing, conducted in ACR(–) and PHCR(+) specimens, yielded conclusive results in 82% of cases.

Conclusions:
The use of PHCR staining as a screening method in suspected amyloidosis improves amyloid (light chain, transthyretin, and amyloid A protein) detectability in various tissues. The PHCR staining specificity should be verified via electron microscopy and/or mass spectrometry.