Objective To explore the correlation between the replacing time of plasma drainage bag and the bacterial growth rate among orthopaedic surgery patients , and to control or reduce the risk of catheter-related infection.Methods The convenient sampling method was used to select 100 patients with postoperative serum drainage tube and disposable drainage bag , from September 2012 to August 2013.Patients were randomly divided into the control group (group A, 33 cases) and the observation group (group B, 32 cases; group C, 35 cases).The amount of drainage were recorded every 24 hours of the three groups of patients , and then the drainage bags were emptied .Group A replaced the drainage bag regularly every day; Group B replaced the drainage bag every 48 hours;Group C did not replace the drainage bag .The outside mouth of the drainage tube and the drainage liquid were taken to bacterial culture . The positive rate of bacterial culture , patients'postoperative body temperature and the rate of incision infection were compared within groups .Results Twenty-four hours after operation, the results of bacterial culture among three groups were negative .However, after 48 hours, six patients (18.1%) in group A, two patients (6.4%) in group B showed positive results of the bacterial culture .None of the patients in group C had the positive results .The comparison among three groups was statistically significant (χ2 =10.32,P<0.05).After 72 hours, eight patients (24.2%) in group A, two patients (6.4%) in group B showed positive results of the bacterial culture .None of the patients in group C had the positive results.The comparison among three groups was statistically significant (χ2 =11.82,P<0.05). The incidence of incision infection of group A , group B and group C were 15.1%(5/33), 6.3%(2/32) and 0.0%(0/35), respectively.There was a significant difference among three groups (χ2 =11.18,P<0.05).The body temperature of the three groups showed no significant difference (F=7.53,P>0.05).Conclusions With the increase of drainage bag replacement frequency , the positive rate of bacterial culture , as well as the drainage bag and drainage tube mouth bacteria culture positive rate increase .Therefore, for orthopaedic patients , the plasma drainage tube and disposable drainage bag should be emptied every 24 hours after the measurement of drainage liquid, but do not have to be replaced every 48 hours or every 24 hours.