Utility of the Predictive Summary Index for Comparison of Diagnostic Protocols in Myasthenia Gravis With Repetitive Nerve Stimulation and Concentric Needle Jitter.

introductionAs sensitivity of a test can be increased at the expense of specificity, a quality measure taking into account both parameters is desirable. The "predictive summary index" (PSI) is calculated from the prevalence of the disease, sensitivity and specificity. We characterize diagnostic yield of repetitive nerve stimulation (RNS) and stimulated concentric needle jitter (CNJ) in the diagnosis of myasthenia gravis (MG) with PSI. From PSI, the number of tests that is required for the prediction of one correct diagnosis ("number needed to predict" [NNP] in analogy to the number needed to treat) can be calculated as 1/PSI.

methodsAmong patients consecutively referred to the electrophysiology laboratory for a workup of a disorder of the neuromuscular junction (NMJ), we identified 35 patients with MG (clinical diagnosis supported by antibody findings or treatment response) and 85 patients with diagnosis unrelated to the NMJ in whom both RNS and stimulated CNJ had been performed.

resultsThe PSI was 0.82 and 0.53 for CNJ and RNS, respectively. The NNPs were 1.22 for CNJ and 1.89 for RNS. This means that 1.49 patients have to be tested with CNJ instead of RNS to arrive at one correct diagnosis. In data from the literature, PSI illustrates that 7% as a cut-off in RNS can be helpful and that amplitude decrement is more reliable than area decrement.

conclusionCNJ was more reliable in the diagnosis of MG than RNS. PSI provides a rational quantitative framework to discuss both the validity of different techniques and their cost in terms of time spent to arrive at a correct diagnosis.