By Joseph E. Safdieh, MD

Vice Chair and Associate Professor, Weill Cornell Medical College

Dr. Safdieh reports no financial relationships relevant to this field of study.

SYNOPSIS: Cerebrospinal fluid analysis using second-generation, real-time, quaking-induced conversion has a high sensitivity and specificity for the diagnosis of Creutzfeldt-Jakob disease.

SOURCE: Foutz A, Appleby BS, Hamlin C, et al. Diagnostic and prognostic value of human prion detection in cerebrospinal fluid. Ann Neurol 2017;81:79-92.

Prion diseases are uncommon and invariably fatal neurologic disorders caused by propagation of misfolded proteins from the normal alpha-helix form to the pathogenic beta-pleated sheet form. Prion diseases in humans include Creutzfeldt-Jakob disease (CJD), variant CJD, fatal familial insomnia, and Gerstmann–Sträussler–Scheinker syndrome. Patients are suspected to have CJD when they present with a rapidly progressive dementia often associated with myoclonus and ataxia. Supportive diagnostic testing includes characteristic MRI changes (cortical ribbon and thalamic/basal ganglia hyperintensities on DWI) and associated lumbar puncture characteristics (elevation of the 14-3-3 protein). However, none of these tests provide enough sensitivity and specificity to firmly establish the diagnosis, often necessitating a brain biopsy.

Over the past few years, a novel testing method that can amplify tiny amounts of prion protein in the laboratory, called real-time quaking-induced conversion (RT-QuIC), has been proposed as a more sensitive and specific diagnostic test for CJD. RT-QuIC is performed by mixing recombinant prion protein with small amounts of pathogenic prion protein, which induces the formation of amyloid fibrils that can be detected by thioflavin T staining. In this study, the authors evaluated the sensitivity and specificity of RT-QuIC testing of cerebrospinal fluid (CSF) in patients with suspected CJD. The study had both a retrospective and prospective arm. The study was performed by the National Prion Disease Pathology Surveillance Center, which maintains a detailed database of all referred cases.

In the study, 2,141 CSF samples were tested with RT-QuIC. In the retrospective cohort of CJD cases (126 patients), 92% of the samples had positive RT-QuIC testing, as compared to 81.7% with elevated 14-3-3 protein. In the prospective cohort of CJD cases (65 patients), 95.4% of the samples had positive RT-QuIC testing, as compared to 81.5% with elevated 14-3-3 protein. Overall sensitivity of the RT-QuIC test (92-95%) was higher than assaying CSF 14-3-3 protein (81.2-81.5%). Of note, in both groups, approximately 76% of patients had characteristic MRI changes for CJD. Only 29% of patients in the retrospective cohort manifested characteristic EEG changes (periodic sharp waves).

To assess the specificity of the RT-QuIC assay, other rapidly progressive neurologic disorders were used as controls. Of note, the most common neuropathological diagnosis in the non-CJD cohort was Alzheimer’s disease. In the prospective cohort, none of the non-CJD cases demonstrated abnormal RT-QuIC assays (100% specificity), and in the retrospective cohort, there was one non-CJD case with abnormal RT-QuIC (98.5% specificity). The specificity of the 14-3-3 protein assay was much lower (62% in the retrospective cohort and 48% in the prospective cohort). In the prospective cohort the positive predictive value of RT-QuIC was 100% and of 14-3-3 was 86.9%. The negative predictive value was 82.4% for RT-QuIC and 50% for 14-3-3.

COMMENTARY

This study contributes valuable additional experience with RT-QuIC testing in the diagnostic workup of suspected CJD. Other studies have assessed RT-QuIC in CSF and even in nasal brushings, demonstrating a high degree of sensitivity and specificity. This study is extremely important because it confirms the diagnostic value of CSF RT-QuIC testing in both a retrospective and a prospective cohort. The test is demonstrated to have higher sensitivity and specificity than 14-3-3 protein and total tau. In the prospective cohort, specificity was 100%, suggesting that this test is highly unlikely to be positive in a non-CJD case. This is a significant advance over 14-3-3, which is sensitive but not very specific. In the setting of an appropriate clinical picture, a positive RT-QuIC test in CSF has a strongly positive predictive value, although it is not 100% sensitive, so a negative test may not exclude the disease. The hope for the near future is that by using a combination of imaging modalities with CSF analysis, we will be able to confidently diagnose CJD without resorting to a brain biopsy.