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

¹ Department of Rheumatology and Clinical Immunology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands; ² Department of Nuclear Medicine, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands; ³ Department of Hematology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands; and ⁴ Department of Gastroenterology and Hepatology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands

Correspondence: For correspondence or reprints contact: Bouke P.C. Hazenberg, MD, Department of Rheumatology and Clinical Immunology, University Medical Center, University of Groningen, P.O. Box 30.001, 9700 RB Groningen, The Netherlands. E-mail: b.p.c.hazenberg{at}int.umcg.nl

Serum amyloid P component (SAP) binds to amyloid. ¹²³I-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 ¹²³I-SAP retention in systemic amyloidosis. Methods: Two hundred megabecquerels of ¹²³I-labeled human SAP was injected intravenously for scintigraphy in 20 controls and in 189 consecutive patients with systemic and localized amyloidosis. Extravascular retention of ¹²³I-SAP was quantified from serum and urine measurements after 24 h (EVR₂₄) and 48 h. Sensitivity and specificity were assessed, and retention was correlated with kidney, heart, liver, and nerve involvement and with survival. Results: The cutoff value representing a desired specificity of 90% of EVR₂₄ was 50%. The associated sensitivity of EVR₂₄ 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 EVR₂₄ increased with the number of organs involved (from a mean of 43% for 1 organ to a mean of 81% for 4 organs). The EVR₂₄ 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 EVR₂₄ (hazard ratio, 2.0; 95% CI, 1.1–3.9) were independent predictors of survival. Conclusion: In AL amyloidosis, the EVR₂₄ 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 EVR₂₄ has no additional value over ¹²³I-SAP scintigraphy in the detection of systemic amyloidosis.

Key Words: diagnostic accuracy • prognostic factor • serum amyloid P component • systemic amyloidosis

COPYRIGHT © 2007 by the Society of Nuclear Medicine, Inc.

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