MON-334 Hooke Effect for Chromogranin A in Immunofluorescence Assay, a Pitfall Revealed by Targeted Proteomics

Abstract

Introduction: Chromogranin A (CGA) is a glycoprotein secreted by several neuroendocrine tissues; therefore, it is a good marker for neuroendocrine tumors, with high specificity for diagnosis and good correlation with disease progression, recurrence and treatment response. Several conditions may lead to "falsely elevated" values ​​of this marker, but reports of occurrence of "falsely low" values ​​due to the hook effect are rare. We report this effect detected in the laboratory evaluation of a patient with neuroendocrine tumor who presented conflicted results of CGA using two different techniques: a traditional immunofluorescence assay and a new directed proteomics method using liquid nanocromatography associated with high-resolution mass spectrometry. Case report: A 64-year-old male patient with chronic renal insufficiency undergoing treatment for a neuroendocrine lung tumor, during treatment progression the patient presented CGA of 359 ng / mL (RR <93 ng / mL) and 179 ng / mL (RR less than 93 ng / mL), both performed using immunofluorescence assay. Later on the follow up a new CGA measurement using guided proteomics methodology resulted in 204,538 ng / mL (RR less than 93 ng / mL), this same sample measurement performed using immunofluorescence assay was 261 ng / mL. This result discrepancy led to the suspicion of a hook effect, what was confirmed after the dilution of the samples (1: 25 and 1: 100) showed results higher than 80,000 ng / mL, which clearly demonstrated that the immunofluorescence assay underestimated the result by more than two orders of magnitude. Discussion and conclusion: The hook effect is a technical pitfall, characterized by the presence of falsely low levels of a substance, which is present in a very elevated concentration, much higher than expected. The most characterized hook effect is with prolactin and to the best of our knowledge; this is the first description of this effect in relation to CGA. The detection of falsely low value of CGA was only possible due to the use of this new technique. Targeted proteomics method is innovative, and proved to be able to give greater accuracy to the CGA result, leading to more appropriate treatment and follow-up for this patient.