Goal: To evaluate the analytical parameters of a lateral flow (LF) pepsin immunoassay (Peptest) and assess its suitability in the diagnostics of gastroesophageal reflux disease (GERD). Background: Peptest is a noninvasive assay to analyze pepsin in saliva, intended for use in GERD diagnostics. Although commercialized, fundamental studies on its performance are missing. The assay therefore requires basic analytical parameter evaluation to assess its suitability in clinical practice. Study: Assay reaction’s time dependence, reader device repeatability, and individual LF devices and longitudinal pepsin concentration reproducibility in individual subjects was evaluated. Salivary pepsin was analyzed in 32 GERD patients with extraesophageal reflux symptoms and 13 healthy individuals. Results: The assay’s signal increase is not completed at the recommend readout time and continues to increase for another 25 minutes. The relative standard deviation of measurement was good when using the same LF device, ranging from 2.3% to 12.9%, but the reproducibility of 10 different individual LF devices was poor. The random error when analyzing the same saliva sample on 10 LF devices was as high as 36ng/ml and this value is thus suggested as the positivity cut-off. Pepsin concentration in individual subjects during a 10-day period varied significantly. The sensitivity of the Peptest was 36.8% in the group with acid reflux and 23.1% in the group with weakly acid reflux. The specificity was 61.5%. Conclusions: The Peptest assay's sensitivity and specificity is low, the results are highly variable and it should not be used as a near-patient diagnostic method in primary care.