STUDY OF THE DISAPPEARANCE OF CONGO RED FROM THE BLOOD OF NON-AMYLOID SUBJECTS AND PATIENTS WITH AMYLOIDOSIS

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Abstract
Studies were conducted to re-evaluate the accuracy of the Congo Red test currently used for the diagnosis of amyloidosis. As a result of these studies, a modification in the method is proposed for the more accurate and earlier diagnosis of amyloidosis. This modification eliminates 2 major sources of error in the old test, namely, (1) that involving the assumption that complete mixing occurs in a 2 or 4 minute specimen used as a standard for comparison with a 60 min. specimen, and (2) the assumption that little or no absorption of injected Congo Red occurs in amyloidosis before the 2 or 4 min. specimen is obtained. The modification proposed involves the prepn. of a standard Congo Red absorption curve using a spectrophotometer and determining the theoretical concn. of injected Congo Red by the extrapolation to zero time (Co) of a curve which is plotted from a large number of detns. obtained in the 1st hr. This Co value is then compared with the concn. found in the plasma at the end of 30 min., since it was found that the major portion of injected dye is removed from the blood stream in amyloids in the first 30 mins. This method was applied to a study of 74 subjects, which included 10 controls, 64 with various diseases of which 6 were found to have secondary amyloidosis. The use of body wt. and surface area relationships to plasma vol. for the estimation of the concn. of injected Congo Red at zero time is also presented. The values obtained with this method were compared with those obtained from extrapolation with a good degree of correlation. This obviates the necessity of withdrawing frequent blood specimens for purposes of extrapolation. From the results obtained, it is suggested that with the use of the proposed method, the removal of more than 35% of injected Congo Red from the blood in 30 mins. is presumptive evidence of amyloidosis.