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NELA & POSSUM Differences

The NELA Project Team have received queries as to why the NELA Risk number differs from P-Possum.

Below we provide an explanation of some of the greatest contributors to the difference.

These are:

1. ASA score - which is in NELA but not P-POSSUM.

2. Age - is modelled as a continuous variable in NELA and categorical in P-POSSUM and has a sizeable effect in NELA.

3. Gender - is included in NELA (not P-POSSUM).

4. The weight given to operative severity is very large in P-POSSUM, but less so in NELA. The latter only has a small boost in risk for the Major+ category (ie all emergency laparotomies are considered high risk).

5. Serum creatinine is included in NELA, only urea is in P-POSSUM.

6. Haemoglobin is included in P-POSSUM, not NELA.

7. Probably the single biggest reason for the difference is the move away from categorisation of variables (as happens in P-POSSUM) and towards continuous variables in NELA - so a small difference in values will be reflected, whereas P-POSSUM variables must cross a threshold to be noticed.