How this instrument works
CURB-65 sums five criteria checked at a pneumonia diagnosis: new Confusion, a raised blood Urea, a Respiratory rate of 30 breaths per minute or higher, low Blood pressure (systolic under 90, or diastolic 60 or under), and Age 65 or older. Each contributes one point, for a total running from 0 to 5, and that total is meant to sort patients by how severe the pneumonia is likely to run.
The rule comes from Lim and colleagues, who pooled data from three prospective pneumonia studies across the UK, the Netherlands, and New Zealand and published the result in Thorax in 2003. Their urea criterion is defined only in the units their labs used — greater than 7 millimoles per litre. The roughly 19 to 20 milligrams per decilitre figure often quoted for US labs, which measure blood urea nitrogen rather than urea itself, is a later approximate conversion added for convenience; it does not appear anywhere in the original paper and carries its own rounding.
Every criterion here is something already recorded at the bedside or on a basic metabolic panel — nothing this instrument measures on its own. In Lim's derivation cohort, higher totals tracked with higher 30-day mortality, and clinicians commonly use the score alongside their own judgment to help weigh outpatient treatment against a hospital ward or a higher level of care. It does not replace an assessment of the specific patient in front of a clinician, including factors the five criteria don't capture.
- Set New confusion to Yes if the patient shows acute disorientation, No otherwise.
- Set Urea over 7 mmol/L to Yes if a lab result crosses that line, or roughly 19-20 mg/dL BUN.
- Set Respiratory rate to Yes if breathing runs 30 breaths per minute or faster.
- Set the blood pressure field to Yes if systolic reads under 90 or diastolic reads 60 or under.
- Set Age 65 or older to Yes or No, then read the total CURB-65 score, 0 through 5.
Worked example — three points on the 0-to-5 scale
No criterion present — no confusion, urea within range, a normal breathing rate, normal blood pressure, and under 65: 0 + 0 + 0 + 0 + 0 = 0, the bottom of the scale and, in Lim's derivation data, a group with a low 30-day mortality rate often considered suitable for outpatient treatment.
Confusion, raised urea, a fast respiratory rate, and age 65 or older all present, blood pressure normal: 1 + 1 + 1 + 0 + 1 = 4, near the top of the scale and a total that generally points toward hospital admission, often with consideration of a higher level of care.
Age 65 or older present on its own, with confusion, urea, respiratory rate, and blood pressure all normal: 0 + 0 + 0 + 0 + 1 = 1 — a reminder that age by itself contributes a full point in this scoring system, regardless of how the other four criteria read.
Questions
What does a CURB-65 score of 0 mean?
It means none of the five criteria were present — no confusion, urea within range, a normal breathing rate, normal blood pressure, and under 65 years old. In the original derivation cohort, patients scoring 0 had a low 30-day mortality rate, and this group is often considered appropriate for outpatient treatment, though a clinician still weighs factors outside the five criteria before making that call.
Is the CURB-65 urea cutoff the same in US mg/dL labs?
Not exactly. Lim's 2003 paper defines the threshold only in millimoles per litre — greater than 7 mmol/L — because that is the unit UK, Dutch, and New Zealand labs used in the original data. The roughly 19 to 20 mg/dL blood urea nitrogen figure commonly cited for US labs is a later approximate conversion, not a number that appears in the original study, and BUN and urea are not quite the same measurement to begin with.
Why does age 65 alone count as a full point?
Because age was one of the four features Lim's team found consistently linked to worse pneumonia outcomes across all three cohorts they studied, alongside confusion, urea, and breathing rate — a link strong enough to earn equal weight in the total rather than a fraction of one. It is a deliberate choice from the original derivation, not an add-on tacked on afterward.
Can CURB-65 replace a clinician's judgment on hospital admission?
No. It is a decision-support aid built from five criteria, meant to sit alongside everything else a clinician knows about a specific patient, including oxygen levels, other illnesses, and how the patient looks in the room. The original paper stratified mortality risk by total score, but it never proposed the number as a stand-alone rule for where a patient should be treated.
What counts as low blood pressure in CURB-65?
Either a systolic reading under 90 mmHg or a diastolic reading of 60 mmHg or under counts as one point — the criterion is met if either threshold is crossed, not both at once. This mirrors the original Thorax 2003 paper's definition of the B in CURB-65.
How is CURB-65 different from a full pneumonia severity index?
CURB-65 uses five yes-or-no criteria and a simple sum, built to be worked out quickly at the bedside without extra lab panels beyond a basic metabolic result. Other tools, like the Pneumonia Severity Index, weigh many more variables, including specific lab values and coexisting illnesses, and tend to take longer to complete but can sort risk more finely. Lim's team designed CURB-65 for speed and simplicity in an emergency setting, not to replace more detailed indices where time allows for one.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.