Objective: To measure the correlation between single breath counting (SBC) and forced vital capacity (liters, FVC_L) in amyotrophic lateral sclerosis (ALS) patients and to define the utility of SBC for determining when patients meet the threshold for initiation of noninvasive positive pressure ventilation (FVC < 50% predicted [FVC_pred]). Methods: Both patient paced (SBCpp) or externally paced (SBCep) counting along with FVC_L+pred and standard clinical data were collected. Linear regression was used to examine SBCpp and SBCep as a predictor of FVC_L. Receiver operating characteristic curve analysis evaluated the sensitivity and specificity of SBC categorically predicting FVC_pred of ≤50%. Results: In 30 ALS patients, SBC explained a moderate proportion of the variance in FVC_L (SBCpp: R²= 0.431, p < 0.001; SBCep: R² = 0.511, p < 0.01); this proportion improved when including covariates (SBCpp: R²= 0.635, p < 0.01; SBCep: R²= 0.657, p < 0.01). Patients with minimal speech involvement performed similarly in unadjusted (SBCpp: R² = 0.511, p < 0.01; SBCep: R²= 0.595, p < 0.01) and adjusted (SBCpp: R² = 0.634, p < 0.01; SBCep: R²= 0.650, p < 0.01) models. SBCpp had 100% sensitivity and 60% specificity (area under curve (AUC) = 0.696) for predicting FVC_pred <50%. SBCep had 100% sensitivity and 56% specificity (AUC = 0.696). With minimal speech involvement SBCpp and SBCep both had 100% sensitivity and 76.1% specificity (SPCpp: AUC = 0.845; SBCep: AUC = 0.857). Conclusions: SBC explains a moderate proportion of variance in FVC and is an extremely sensitive marker of poor FVC. When FVC cannot be obtained, such as during the current COVID-19 pandemic, SBC is helpful in directing patient care.

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