The gender specific risk factors for prolonged hospitalization due to acute pyelonephritis in a Japanese tertiary emergency center. 
The aim of this study is to characterize the potential differences between male and female patients with acute pyelonephritis (AP) and to predict the severity of AP based on the length of hospital stay.  We conducted a retrospective medical chart review of 172 consecutive adult patients who were hospitalized in Tsuyama Central Hospital due to AP from January 2007 through June 2012.  We analyzed the length of hospital stay by the proportional hazard model.  A total of 172 patients were identified who were admitted to our hospital with a diagnosis of AP.  Of them, 62% (106/172) were female.  Except for urological malignancy, there was no significant difference between men and women in underlying disease.  Out of 26 variables, univariate analysis in male showed that only urolithiasis (OR 1.75, p = 0.0294) was significantly associated with longer hospital stay, while septic shock (OR 3.18, P = 0.003), urological malignancy (OR 2.94, P = 0.002), age over 65 (OR 1.66, p = 0.018) and neurogenic bladder (OR 1.92, p = 0.014) were all associated with longer hospital stay in female patients.  This is the first report to identify the risk factors for prolonged hospital stay for the patients who were admitted with AP in the Japanese population.  The risk factors causing prolonged hospital stay were totally different between males and females.  Reviewing the medical history based on sex gender might enable a clinician to predict the severity of acute pyelonephritis during the initial evaluation.