New Injury Severity Score (NISS)
Calculates NISS from AIS codes to quantify overall trauma severity
For use by health professionals and trauma registries only. This tool assumes that individual injuries have already been coded with Abbreviated Injury Scale (AIS) scores. It does not replace clinical judgement, trauma team assessment, or local protocols.
Enter AIS severity scores
Enter AIS scores (1–6) for up to six injuries. The calculator will automatically use the three highest AIS scores (regardless of body region) to compute the NISS.
AIS scale (simplified): 1 = minor, 2 = moderate, 3 = serious, 4 = severe, 5 = critical, 6 = maximal (currently untreatable).
NISS: 0
Severity category: Not yet calculated
Interpretation: —
Enter AIS scores for the patient’s injuries and click “Calculate NISS” to see the score and severity category.
NISS is defined as the sum of the squares of the AIS scores of the three most severe injuries, regardless of body region. Scores are conventionally limited to 0–75. Many studies group trauma severity as: 1–8 mild, 9–15 moderate, 16–24 severe, ≥25 very severe. Actual management must always follow local trauma system protocols.
The New Injury Severity Score (NISS) is an anatomical trauma scoring system used to quantify overall injury severity and help predict outcomes such as mortality, ICU admission, and length of stay in trauma patients.
It is a modification of the classic Injury Severity Score (ISS) and is based on the Abbreviated Injury Scale (AIS), where each individual injury is graded from 1 (minor) to 6 (maximal, currently untreatable).Wikipedia+1
What is the NISS and how is it calculated?
The original ISS sums the squares of the highest AIS score in each of the three most severely injured body regions. In contrast, the New Injury Severity Score (NISS) ignores body regions and instead simply takes the three most severe injuries anywhere in the body.SciSpace+1
Definition:
NISS = (highest AIS)² + (second-highest AIS)² + (third-highest AIS)²
using the three most severe injuries, regardless of body region.
If fewer than three injuries are present, only the available AIS scores are squared and summed. NISS values are conventionally limited to the range 0–75, similar to ISS.ScienceDirect+1
This change particularly improves severity estimation in patients who have multiple serious injuries within the same region (e.g. several severe head injuries or multiple limb fractures), which ISS tends to underestimate.
Why use NISS?
Multiple studies have compared ISS and NISS:
- In several trauma cohorts (including penetrating and ICU trauma patients), NISS often outperforms ISS in predicting mortality and complications.ijccm.org+3PMC+3PubMed+3
- NISS shows better discrimination in patients with multiple injuries in one region, where ISS may underweight overall severity.Lippincott Journals+1
However, more recent work suggests that while NISS is generally at least as good as ISS, the difference may be modest and context dependent, and more complex scores (e.g. TRISS, EISS) may perform even better in some settings.PLOS+2ResearchGate+2
Interpreting NISS
There is no single universal set of bands, but many studies group trauma patients approximately like this:PLOS+2ResearchGate+2
- NISS 1–8: Mild injury
- NISS 9–15: Moderate injury
- NISS 16–24: Severe injury (often used as a “major trauma” threshold)
- NISS ≥25: Very severe / critical injury, associated with high mortality and ICU use
Higher NISS = more severe overall trauma, but:
Important: NISS is a research and risk-stratification tool, not a treatment algorithm by itself. Decisions about surgery, ICU admission, and interventions must always rely on clinical judgement and local trauma protocols.
