Split-hand sign is a useful clinical and electrophysiological feature in the diagnosis of amyotrophic lateral sclerosis (ALS). We proposed a novel split-hand index (SI) using F-wave persistence (FP) and assessed its diagnostic utility in ALS. Eighty-three consecutive ALS patients were recruited, and 50 healthy, age-, and height-matched volunteers were used as a control group. Compound muscle action potentials (CMAP) and FP were recorded from the abductor pollicis brevis (APB), first dorsal interosseous (FDI), and the abductor digiti minimi (ADM) muscles. The SI derived from FP was calculated using the following formula: SI = (FP ×FP)/FP. The sensitivity and specificity of SI and SI in differentiating ALS from healthy controls (HCs) were derived from receiver-operating characteristic (ROC) curve analysis. Both SI and SI were significantly reduced in ALS patients. The ROC curve analysis indicated that both SI and SI reliably differentiated ALS from HCs [0.92 (95% CI: 0.88-0.95); 0.86 (95% CI: 0.82-0.91)], but SI showed better diagnostic accuracy than SI ( = 0.04), with a high sensitivity (81.2%) and specificity (97%). In subgroup analyses, SI appeared to be a better variable than SI for differentiation of ALS patients with normal CMAP from HCs, with an area under the curve of 0.87 (95% CI: 0.80-0.93), sensitivity of 69.4%, and specificity of 94%. The SI reliably distinguished ALS patients from HCs and may be more sensitive for determining the split-hand pattern of ALS than SI, particularly in the early stage of the disease.