## 4 Discussion  
All examinations performed with the ONIRY system were non-invasive, painless for the patient, and lasted no more than one minute. No adverse events were reported during the study. The use of the specially designed ONIRY Probe resulted in an increase of measurement sensitivity compared to previous studies [45; 46]. The compatibility of our results with the reference ultrasound method, when assessed using the OASI classification, was at the level of about 93% and almost 100%, for the raw and re-analyzed datasets, respectively. Such compliance with the reference ultrasound method (remaining the gold standard in medical practice) allows us to assume that the impedance-based method of detecting anal sphincter injuries has the potential to be widely used in early diagnostics, or even screening in a large population of women giving birth. Similarly, a combination of k-NN and Random Forest methods allows differentiating between cases without any symptoms, or with some symptoms, reported during clinical evaluation using the Wexner scale. Sensitivities of over 83% for OASI classification, and over 74% for the Wexner scale, obtained for raw (strongly imbalanced) data already indicate that bioimpedance may be a long-sought marker for linking structural findings and clinical manifestations of the problem. So far, their results have often not been consistent, which is a serious weakness of anal sphincter diagnostics. This makes the ONIRY method a promising tool for assessing anal sphincter condition (this is also indirectly indicated by the increase in sensitivity when analyzing more balanced data). The current results are consistent with both ultrasound and physical examination, which appears to be unique in the diagnosis of sphincter injury and is strongly awaited in clinical practice. Furthermore, the software application installed inside the ONIRY Meter may get even more robust once further measurements refine its specificity and sensitivity, as long as the retraining process is continuously applied. Thanks to the application, we deal with the biggest problem of obstetric diagnostics, which is the availability of endoanal ultrasound in maternity wards. This excellent, highly sensitive diagnostic tool requires specialized staff for result interpretation, which limits its use in early detection of obstetric injuries. Meanwhile, early diagnosis is crucial for integrating proper treatment. In the case of a sphincter rupture, only the first-day-diagnosis allows its surgical primary repair, which provides the best distant outcome. In addition, minor injuries, those that are functional and recognized immediately, can be treated with the most efficient early rehabilitation. Appropriate obstetric procedures for these cases, such as proper diet and anti-constipation drugs, can be included, helping to avoid deepening the damage. We decided to only assess performance of the OASI classification and Wexner scale prediction based on impedance data (calculation of the features is based on the measured raw values; nevertheless, the parameter set could be extended for the next measurement phase and future analyses by adding parameters that can be extracted from the impedance diagram, such as resistive plateaus, frequency of Beta dispersion, membrane capacitance, or phase extrema. However, in our opinion, this would need more samples). The relationships between impedance and specific, more detailed, endoanal ultrasonography or anorectal manometry parameters (e.g., resting rectal pressure, and others listed in Table 1) were not evaluated at this stage of clinical development due to the small sample size. However, those results have been recorded and will be taken into account for future studies, when the number of patients (also with a grade of 3 and 4 in the OASI classification) will increase. Taking into account that injury non-detection rates can reach up to 80% (according to the literature [9; 10]), the new tool for injury diagnostics is strongly expected.