CSF Xanthochromia Net Bilirubin Calculator
CSF Xanthochromia Net Bilirubin Calculator
Net bilirubin absorbance from a CSF spectrophotometric scan, against the UK national threshold of 0.007 AU. The sample must be protected from light and taken at least 12 hours after headache onset, or the result cannot exclude anything.
CSF Xanthochromia Net Bilirubin
Two absorbances → net bilirubin AUAbsorbance at 476 nm 0.0195, baseline tangent at 476 nm 0.0075, net oxyhaemoglobin absorbance 0.045 AU, tap at 14 hours after onset, sample protected from light
Net bilirubin absorbance
NBA > 0.007 AU → consistent with subarachnoid haemorrhage
Corroborated by NOA > 0.02 AU at 415 nm with a visible oxyhaemoglobin peak
NOA above about 0.1 AU makes bilirubin unmeasurable
- the tangent
- a straight line drawn to the spectrum between 350–400 nm and 430–530 nm, which models what the baseline would have been without bilirubin. Net bilirubin absorbance is the vertical gap between that line and the curve at 476 nm
- 476 nm, not 450 nm
- bilirubin absorbs maximally around 450–460 nm, which is uncomfortably close to the oxyhaemoglobin peak. The revised method reads at 476 nm, where oxyhaemoglobin interferes less, at the cost of reading off the shoulder of the bilirubin peak rather than its top
- 0.007 AU
- the national cut-off, derived from control patients undergoing CT myelography whose net bilirubin absorbances ran from 0 to 0.007 AU. It is a small number measured on a small signal, which is why the method needs an external quality assessment scheme behind it
- 12 hours
- bilirubin is made in vivo by macrophages breaking down haemoglobin and takes hours to appear. Before 12 hours the scan can be falsely negative, which is the single commonest way this test misleads
- protect from light
- bilirubin is photolabile. Every hour of light exposure destroys signal, and the error runs one way only — towards a negative result in a patient who has bled
Worked example
Absorbance at 476 nm 0.0195, baseline tangent at 476 nm 0.0075, net oxyhaemoglobin absorbance 0.045 AU, tap at 14 hours after onset, sample protected from light
NBA = 0.0195 − 0.0075 = 0.0120 AU
0.0120 is above the national threshold of 0.007 AU
Net oxyhaemoglobin absorbance is 0.045 AU — above the corroborating 0.02 AU and comfortably below the 0.1 AU at which bilirubin becomes unmeasurable
The sample was taken at 14 hours and protected from light, so both preconditions for interpreting a scan are satisfied
Reported as consistent with subarachnoid haemorrhage: discuss with neurosurgery and arrange vascular imaging
Before acting on it as a bleed, check the paired serum bilirubin and the CSF total protein — a jaundiced patient or a markedly raised CSF protein can raise the net bilirubin absorbance without a haemorrhage
Reading the two absorbances together
| Net bilirubin absorbance | Net oxyhaemoglobin absorbance | Interpretation |
|---|---|---|
| ≤ 0.007 AU | ≤ 0.02 AU | No spectrophotometric evidence of subarachnoid haemorrhage — provided the sample was taken at ≥12 hours and protected from light |
| ≤ 0.007 AU | 0.02 – 0.1 AU | Oxyhaemoglobin without bilirubin. Usually a traumatic tap or in-vitro lysis rather than a bleed, but a sample taken too early looks exactly like this |
| > 0.007 AU | > 0.02 AU, visible peak | Consistent with subarachnoid haemorrhage — the guideline combination, and it stands regardless of CSF total protein |
| > 0.007 AU | ≤ 0.02 AU | Bilirubin without oxyhaemoglobin. Check the paired serum bilirubin and CSF total protein before attributing it to a bleed |
| Any value | > ~0.1 AU | Uninterpretable. Oxyhaemoglobin swamps the spectrum and bilirubin cannot be measured beneath it. Repeat on a cleaner sample |
Sample handling — the part that decides whether the result means anything
| Requirement | Why | What goes wrong if it is missed |
|---|---|---|
| Lumbar puncture at least 12 hours after headache onset | Bilirubin is formed in vivo by macrophages over hours; it is not present at the moment of the bleed | A falsely negative scan in a patient who has bled — the commonest and most dangerous failure of this test |
| Protect from light from collection to analysis | Bilirubin is photolabile and degrades on exposure | Signal is lost. The error runs one way only: towards negative |
| Deliver promptly; centrifuge without delay and analyse the supernatant | Red cells continue to lyse in the tube, generating oxyhaemoglobin that was never in the patient | Rising oxyhaemoglobin obscures the bilirubin and can render the scan uninterpretable |
| Use the last tube collected | A falling red cell count across sequential tubes points to a traumatic tap | Blood introduced by the needle is misread as blood that was already there |
| Send a paired serum bilirubin and a CSF total protein | Both can raise CSF bilirubin without a haemorrhage | A jaundiced patient is investigated for an aneurysm they do not have |
| Scan on a spectrophotometer, not by eye | Visual inspection for a yellow tinge misses small amounts of bilirubin | Naked-eye xanthochromia has been shown repeatedly to be unreliable and should not be reported |
A negative scan on the wrong sample excludes nothing
Computed tomography detects subarachnoid haemorrhage in the great majority of patients scanned within six hours of onset, and its sensitivity falls steadily thereafter. CSF spectrophotometry exists to cover the patients CT misses: the ones who present late, and the ones whose scan is equivocal. It looks for bilirubin, because bilirubin can only be made in vivo — haemoglobin is broken down to it by macrophages over hours — whereas oxyhaemoglobin appears equally readily from a needle that has nicked a vein on the way in, or from red cells lysing in a tube on the way to the laboratory.
The measurement itself is a subtraction. A tangent is drawn to the spectrum between 350 and 400 nm and between 430 and 530 nm, modelling the baseline the scan would have had without bilirubin, and the net bilirubin absorbance is the vertical distance from that line up to the curve at 476 nm. The reading is taken at 476 nm rather than at the bilirubin peak nearer 450 nm because it sits further from the oxyhaemoglobin peak and so suffers less interference. The UK national threshold is 0.007 AU, corroborated by a net oxyhaemoglobin absorbance above 0.02 AU with a visible oxyhaemoglobin peak; above roughly 0.1 AU of oxyhaemoglobin, bilirubin cannot be measured beneath it at all and the scan should be reported as uninterpretable rather than as negative.
Two conditions of the sample decide whether any of that arithmetic means anything, and they matter more than the numbers do. The lumbar puncture must be at least 12 hours after headache onset, because bilirubin takes hours to form and a scan taken earlier can be negative in a patient who has bled. And the sample must be protected from light from collection through to analysis, because bilirubin is photolabile and every hour of exposure destroys signal. Note that both errors run the same way: towards a negative result. A scan that fails either condition has not excluded subarachnoid haemorrhage, and reporting it as though it had is how this test causes harm.
Two things can raise CSF bilirubin without a bleed — a serum bilirubin high enough to cross into the CSF, and a very high CSF total protein — so both are sent alongside every scan. And a negative scan on a perfect sample still only answers one question. It says nothing about a cerebral venous sinus thrombosis, a cervical artery dissection, a reversible cerebral vasoconstriction syndrome or a sentinel bleed already resorbed. A patient whose story is still that of a thunderclap headache needs vascular imaging whatever the absorbance says. The number informs the clinician’s judgement; the story is what should drive it.
Frequently asked questions
What net bilirubin absorbance indicates subarachnoid haemorrhage?
A net bilirubin absorbance above 0.007 AU, with a corroborating net oxyhaemoglobin absorbance above 0.02 AU and a visible oxyhaemoglobin peak, is reported under the UK national guidelines as consistent with subarachnoid haemorrhage — and the evidence indicates that this stands regardless of the CSF total protein.
Why must the lumbar puncture be at least 12 hours after headache onset?
Because bilirubin is formed in vivo, by macrophages breaking down haemoglobin, and takes hours to appear. A sample taken earlier may contain oxyhaemoglobin but no bilirubin, so the scan can be negative in a patient who has genuinely bled. This is the commonest way CSF spectrophotometry misleads.
Does a negative CSF spectrophotometry result exclude subarachnoid haemorrhage?
Only if the sample was taken at least 12 hours after onset, protected from light and analysed promptly. If any of those failed, the negative result excludes nothing. Even on a perfect sample it addresses only subarachnoid haemorrhage, and a thunderclap headache still warrants vascular imaging for dissection, venous sinus thrombosis or vasoconstriction syndromes.
Why does the sample have to be protected from light?
Bilirubin is photolabile and degrades on exposure to light, so a tube left on a bench or a windowsill loses the very signal the test is looking for. The error is one-directional: light exposure can turn a positive scan negative, but never the reverse.
What does oxyhaemoglobin without bilirubin mean?
Usually a traumatic tap or red cells lysing in the tube before analysis, since oxyhaemoglobin is generated readily in vitro while bilirubin is not. But a sample taken too soon after a real bleed looks identical, so the timing of the tap has to be known before that pattern can be called reassuring.
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References
- Cruickshank A, Auld P, Beetham R, et al. Revised national guidelines for analysis of cerebrospinal fluid for bilirubin in suspected subarachnoid haemorrhage. Ann Clin Biochem. 2008;45(3):238–244.
- UK NEQAS for Immunochemistry Working Group. National guidelines for analysis of cerebrospinal fluid for bilirubin in suspected subarachnoid haemorrhage. Ann Clin Biochem. 2003;40(5):481–488.
- Chalmers AH. Cerebrospinal fluid xanthochromia testing simplified. Clin Chem. 2001;47(1):147–148.
- Beetham R, Egner W, Patel D. The UK NEQAS scheme for cerebrospinal fluid haem pigments: a paradigm for service improvement. Ann Clin Biochem. 2011;48(6):529–536.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.