OBJECTIVE: The purpose of this study was to assess the value of the serum amyloid P (SAP) component scintigraphy in patients with primary amyloidosis (AL). MATERIAL AND METHODS: Pure human SAP labeled with iodine-123 (123I-SAP) was given intravenously to 24 patients with biopsy-proven systemic amyloidosis (15 without multiple myeloma = group 1, and 9 with multiple myeloma = group 2) and to 6 patients with multiple myeloma without any clinical or biological signs of amyloidosis (group 3). Whole-body images as well as regional views and tissue retention levels were obtained after 24 hours. Our study was approved by the institutional review committee and all individuals gave informed consent and were prospectively studied (median 13 months, range 1 to 47 from the date of the scintigraphy to May 1995). RESULTS: Organ localization of 123I-SAP, indicating the presence of substantial visceral amyloid deposits, was observed in all patients in group 1 and 2. The organ uptake of 123I-SAP included the spleen (1 patient was splenectomized) in 20 of 23 cases (87%), the liver in 15 of 24 (60%), and the kidneys in 6 of 24 (25%). Myocardial 123I-SAP was never seen although 13 out of the 24 patients had clinical or echographic data for amyloidosis. Twenty-four hour tissue retention was significantly elevated in all patients (group 1 and group 2): 55.66% +/- 19.16% in group 1 and 34.37% +/- 24.92% in group 2, as compared with normal levels < 24%. The sensitivity of the technique was 79% when only organ uptake was considered but reached 100% when tissue retention was also considered. The 24-hour tissue retention might be correlated with the severity of the amyloidosis: mean survival in patients with tissue retention greater than 50% was 11.3 months versus 24.5 months in patients with levels less or equal to 50%. Five of the 6 patients with multiple myeloma without evidence of amyloidosis had abnormal 123I-SAP imaging and 24-hour tissue retention levels. In 2 of them, amyloidosis was secondly detected. In the 9 patients who had two scintigraphies, variations in 24-hour tissue retention values were in accordance with the clinical evaluation. CONCLUSIONS: Spleen and liver distribution of amyloidosis is mostly revealed by 123I-SAP scintigraphy in patients with AL amyloidosis. The uptake of 123I-SAP appeared in proportion to the quantity of amyloidosis present in different tissues, and the relative quantity of amyloid deposits in the myocardium, carpal tunnel, digestive tract, and kidneys was often small and seldom visualized by 123I-SAP scintigraphy. In contrast 24-hour tissue retention levels were abnormal in all cases of known AL amyloidosis. This may be a positive argument for the diagnosis of amyloidosis when histopathological tests are normal. Tissue retention levels appear important as they may be correlated with survival.