Associations between postoperative anaemia and unplanned readmission to hospital after major surgery: a retrospective cohort study †

Summary Background Anaemia following major surgery may be associated with unplanned readmission to hospital. However, the severity‐response relationship between the degree of anaemia at discharge and the risk of unplanned readmission is poorly defined. We aimed to describe the severity‐response relationship between haemoglobin concentration at the time of discharge and the risk of unplanned readmission in a cohort of patients undergoing different types of major surgery. Methods We performed a retrospective cohort study in a single tertiary health service, including all patients who underwent major surgery (orthopaedic, abdominal, cardiac or thoracic) between 1 May 2011 and 1 February 2022. The primary outcome was unplanned readmission to hospital in the 90 days following discharge after the index surgical procedure. These complex, non‐linear relationships were modelled with restricted cubic splines. Results We identified 22,134 patients and included 14,635 in the primary analysis, of whom 1804 (12%) experienced at least one unplanned readmission. The odds of unplanned readmission rose when the discharge haemoglobin concentration was < 100 g.l ‐1 (p < 0.001). On subgroup analysis, the haemoglobin threshold below which odds of readmission began to increase appeared to be higher in patients undergoing emergency surgery (110 g.l ‐1 ; p < 0.001) compared with elective surgery. Declining discharge haemoglobin concentration was associated with increased odds ratios (95%CI) of unplanned readmission in patients undergoing orthopaedic (1.08 (1.01–1.15), p = 0.03), abdominal (1.13 (1.07–1.19), p < 0.001) and thoracic (1.12 (1.01–1.24), p = 0.03) procedures, but not cardiac surgery (1.09 (0.99–1.19), p = 0.07). Conclusions Our findings suggest that a haemoglobin concentration < 100 g.l ‐1 following elective procedures and < 110 g.l ‐1 following emergency procedures, at the time of hospital discharge after major surgery, was associated with unplanned readmission. Future interventional trials that aim to treat postoperative anaemia and reduce unplanned readmission should include patients with discharge haemoglobin below these thresholds.