Pellecchia MT et al. (2006) Multiple system atrophy is distinguished from idiopathic Parkinson's disease by the arginine growth hormone stimulation test. Ann Neurol 60: 611–615

Multiple system atrophy (MSA) can be difficult to distinguish from Parkinson's disease (PD). Reduced release of growth hormone (GH) by the pituitary gland in response to arginine has been proposed as a marker of MSA, but current evidence is conflicting. To evaluate the diagnostic accuracy of the arginine GH stimulation test, Pellecchia et al. measured the response of GH to arginine in 69 patients with MSA (43 of whom had parkinsonism as the main motor feature and 26 of whom had predominant cerebellar features), 35 patients with PD, and 90 healthy controls. Following the collection of baseline blood samples, 30 g of intravenous arginine was given to each participant over a period of 30 min; 3 blood samples were subsequently taken at 30 min intervals.

The increase in GH concentration in response to arginine was significantly lower (P <0.01) in patients with MSA than in either patients with PD or control subjects. Using receiver operating curve analysis, the researchers calculated that the sensitivity and specificity of the arginine GH stimulation test were highest at a GH concentration of 4µg/l. At this optimum cutoff, the test had a positive predictive value for MSA of 95%, and a sensitivity and specificity of 93% and 91%, respectively. GH response to arginine did not correlate with disease duration, MSA motor subtype or response to dopamine. The authors conclude that a reduced GH response to arginine is an accurate diagnostic marker of MSA.