Diagnostic Performance and Prognostic Value of Extravascular Retention of 123I-Labeled Serum Amyloid P Component in Systemic Amyloidosis

Abstract

Serum amyloid P component (SAP) binds to amyloid. (123)I-SAP scintigraphy is used to evaluate the extent and distribution of amyloid in systemic amyloidosis and has great clinical value in the detection of systemic amyloidosis. The aim of the study was to assess during scintigraphy the diagnostic performance and prognostic value of a simple parameter describing extravascular (123)I-SAP retention in systemic amyloidosis. Two hundred megabecquerels of (123)I-labeled human SAP was injected intravenously for scintigraphy in 20 controls and in 189 consecutive patients with systemic and localized amyloidosis. Extravascular retention of (123)I-SAP was quantified from serum and urine measurements after 24 h (EVR(24)) and 48 h. Sensitivity and specificity were assessed, and retention was correlated with kidney, heart, liver, and nerve involvement and with survival. The cutoff value representing a desired specificity of 90% of EVR(24) was 50%. The associated sensitivity of EVR(24) for detecting reactive systemic, immunocyte-derived (AL), and hereditary amyloidosis was 65%, 61%, and 22%, respectively, using a cutoff point of 50%. In AL amyloidosis, the EVR(24) increased with the number of organs involved (from a mean of 43% for 1 organ to a mean of 81% for 4 organs). The EVR(24) correlated with serum alkaline phosphatase (r = 0.63) and with creatinine clearance (r = -0.36). In AL amyloidosis, both cardiac involvement (hazard ratio, 3.9; 95% CI, 2.0-7.8) and EVR(24) (hazard ratio, 2.0; 95% CI, 1.1-3.9) were independent predictors of survival. In AL amyloidosis, the EVR(24) is strongly associated with organ involvement and with prognosis and might serve as an indicator of the body amyloid load. Quantification of SAP retention using the EVR(24) has no additional value over (123)I-SAP scintigraphy in the detection of systemic amyloidosis.