Validation of the Nottingham Hip Fracture Score (NHFS) for the prediction of 30-day mortality in a Swedish cohort of hip fractures

BACKGROUND

Hip fracture is a common osteoporotic fracture with great morbidity and mortality. The utility of ASA classification is limited, as most patients are ≥ASA 3. A reliable predictor of mortality risk could support decision-making. We aimed to evaluate Nottingham hip fracture score (NHFS) for the prediction of 30-day mortality and then to recalibrate the formula converting NHFS to risk of 30-day mortality.

METHODS

All patients >60 years with surgically treated hip fracture surgery during 2015-16 were assessed. Data was extracted manually from routinely collected clinical data in registry and medical records. Discriminative performance of NHFS and ASA was assessed with C-statistics. The conversion formula from NHFS to risk of 30-day mortality was recalibrated using logistic binominal regression. Observed vs expected ratios of 30-day mortality were compared with the 2012 NHFS-formula and recalibration was performed in a split dataset.

RESULTS

1864 patients were included, with 213 deaths within 30 days. C-statistic were 0.64 for NHFS and 0.62 for ASA. Comparing expected values from the 2012-revision with our observed deaths gave a ratio of 1.37. Relating predicted levels of 30-day mortality based on 70% of our cohort vs. 30% test portion of our Swedish dataset gave a ratio of 0.97.

DISCUSSION

NHFS underestimated mortality in our cohort and showed poor discrimination. Revision of the formula based on a split dataset improved calibration. We suggest NHFS to be routinely implemented to support clinical judgements, expand preoperative assessment and escalate intraoperative monitoring.

Independent validation of the Nottingham Hip Fracture Score and identification of regional variation in patient risk within England

The Nottingham Hip Fracture Score (NHFS) was developed to assess the risk of death following a fracture of the hip, based on pre-operative patient characteristics. We performed an independent validation of the NHFS, assessed the degree of geographical variation that exists between different units within the United Kingdom and attempted to define a NHFS level that is associated with high risk of mortality. The NHFS was calculated retrospectively for consecutive patients presenting with a fracture of the hip to two hospitals in England. The observed 30-day mortality for each NHFS cohort was compared with that predicted by the NHFS using the Hosmer-Lemeshow test. The distribution of NHFS in the observed group was compared with data from other hospitals in the United Kingdom. The proportion of patients identified as high risk and the mortality within the high risk group were assessed for groups defined using different thresholds for the NHFS. In all 1079 hip fractures were included in the analysis, with a mean age of 83 years (60 to 105), 284 (26%) male. Overall 30-day mortality was 7.3%. The NHFS was a significant predictor of 30-day mortality. Statistically significant differences in the distribution of the NHFS were present between different units in England (p < 0.001). A NHFS ≥ 6 appears to be an appropriate cut-point to identify patients at high risk of mortality following a fracture of the hip.