The 3-day rule for stool cultures: should all patients with haematological malignancies be excluded?

The ‘3-day rule’ for stool culture ordering suggests that only selected inpatients with nosocomial diarrhoea should have stool cultures for enteropathogenic bacteria (EPBs). Patients with haematological malignancies are not included in this group. We have analysed the ordering of stool cultures at Laikon Hospital to investigate whether all patients with haematological malignancies should be excluded from the 3-day rule. We have retrospectively analysed all inpatient stool specimens sent to the microbiology laboratory for enteropathogenic bacteria culture at Laikon Hospital, Athens, Greece, between January 1, 2014 and December 31, 2014. We classified stool cultures sent after the third day as ‘appropriate’, ‘excluded’ with standard rule, ‘excluded’ with haematological malignancies, and ‘inappropriate’. During the study period, 1101/1593 inpatient stool cultures (69.1%) had been ordered after the third day of hospitalization. The total yield for inpatient EPB stool cultures was 0.7% (11/1593). The yield for ‘appropriate’ cultures was significantly higher than the yield of all ‘excluded’ specimens (3.7% (3/81) versus 0.3% (2/585), p 0.018) and to ‘inappropriate’ orders (3.7% (3/81) versus 0.0% (0/485), p 0.0028). There was no difference in the yield between specimens ‘excluded’ with the standard rule and ‘excluded’ with haematological malignancies. In our hospital, the yield of stool cultures from patients with haematological malignancies is similar to that of patients ‘excluded’ from the standard 3-day rule. If patients with haematological malignancies were not excluded from the rule, we would reduce the inpatient stool cultures by 13.6% (217/1593) at the cost of missing one positive stool culture.