The patient-ventilator breath contribution (PVBC) index estimates the relative contribution of the patient to total tidal volume (VTinsp) during mechanical ventilation in neurally adjusted ventilator assist mode and has been used to titrate ventilator support. The reliability of this index in ventilated patients is unknown and was investigated in this study. PVBC was calculated by comparing tidal volume (VTinsp) and diaphragm electrical activity (EAdi) during assisted breaths (VTinsp/EAdi)assist and during unassisted breaths (VTinsp/EAdi)no-assist. VTinsp was normalized to peak EAdi (EAdipeak) using 1) one assisted breath, 2) five consecutive assisted breaths, or 3) five assisted breaths with matching EAdi preceding the unassisted breath ( N1PVBC 2 X5PVBC 2, and X5PVBC EAdi-matching 2 , respectively). In addition, PVBC was calculated by comparing only VTinsp for breaths with matching EAdi (PVBC 2). Test-retest reliability of the different PVBC calculation methods was evaluated with the intraclass correlation coefficient (ICC) using five repeated PVBC maneuvers performed with a 1-min interval. In total, 125 PVBC maneuvers were analyzed in 25 patients. ICC [95% confidence interval] values were 0.46 [0.23– 0.66], 0.51 [0.33– 0.70], and 0.42 [0.14 – 0.69] for N1PVBC 2 X5PVBC 2 X5PVBC EAdi-matching 2 , respectively. Complex waveform analyses showed that insufficient EAdi filtering by the ventilator software affects reliability of PVBC calculation. With our new EAdi-matching techniques reliability improved (PVBC 2 ICC: 0.78 [0.60 – 0.90]). We conclude that current techniques to calculate PVBC exhibit low reliability and that our newly developed criteria and estimation of PVBC— using VT insp of assisted breaths and unassisted breaths with matching EAdi—improves reliability. This may help implementation of PVBC in clinical practice.