Bilirubin to Albumin (B:A) Ratio Calculator
Bilirubin to Albumin (B:A) Ratio Calculator
The bilirubin to albumin ratio in conventional units, against the AAP thresholds for exchange transfusion — an adjunct to the total bilirubin, never a replacement for it, and a test NICE explicitly advises against using.
Bilirubin:albumin ratio
Bilirubin + albumin → B:A ratioTotal bilirubin 340 µmol/L, albumin 30 g/L, SI units
Formula
SI units: B:A = [bilirubin (µmol/L) ÷ 17.104] ÷ [albumin (g/L) ÷ 10]
which is the same as B:A = 0.5847 × bilirubin (µmol/L) ÷ albumin (g/L)
- 17.104
- µmol/L per mg/dL for bilirubin, from a molecular weight of 584.66 g/mol for bilirubin IXα. Dividing by it converts an SI bilirubin into the conventional unit the thresholds are written in
- ÷ 10
- converts albumin in g/L to g/dL. A 30 g/L albumin is 3.0 g/dL — the same concentration written two ways, and the figure the 2022 AAP revision uses as a neurotoxicity risk threshold
- 0.5847
- 10 ÷ 17.104, the single factor that takes an SI bilirubin over an SI albumin straight to the conventional ratio. It is worth remembering, because it is the only arithmetic between a British report and an American threshold
- why the ratio exists
- bilirubin bound to albumin cannot cross the blood-brain barrier; free bilirubin can. Albumin's binding sites are finite, so the same total bilirubin is more dangerous in a baby with a low albumin. The ratio is a crude surrogate for how close those sites are to saturation
- the molar ratio
- a different quantity, bilirubin in µmol/L divided by albumin in µmol/L, where albumin at 66,500 g/mol gives about 15 µmol/L per g/L. A molar ratio of 1.0 means one bilirubin molecule per albumin molecule. It appears in the research literature; the clinical thresholds are not written in it
- what it is not
- a measurement of free bilirubin. Free bilirubin can be measured directly by peroxidase methods, and the ratio is a proxy for it that ignores binding affinity, competing drugs, acidosis and the albumin's own binding quality
Worked example
Total bilirubin 340 µmol/L, albumin 30 g/L, SI units
Bilirubin in conventional units: 340 ÷ 17.104 = 19.88 mg/dL
Albumin in conventional units: 30 ÷ 10 = 3.0 g/dL
B:A = 19.88 ÷ 3.0 = 6.63
By the shortcut: 0.5847 × 340 ÷ 30 = 6.63, the same figure
6.63 is below all three AAP 2004 thresholds — but a total bilirubin of 340 µmol/L at 30 hours of age sits only 10 µmol/L below the NICE exchange line of 350 µmol/L, and 128 µmol/L above the phototherapy line of 212 µmol/L. The ratio has not contradicted that and cannot
Now drop the albumin to 22 g/L with the bilirubin unchanged: 0.5847 × 340 ÷ 22 = 9.04, above every published threshold. Nothing about the bilirubin has changed; what has changed is how much of it is bound
AAP 2004 bilirubin:albumin ratios for considering exchange transfusion
| Baby | B:A ratio (mg/dL per g/dL) |
|---|---|
| 38 0/7 weeks or more, well | 8.0 |
| 35 0/7 to 36 6/7 weeks and well, or 38 0/7 weeks or more with a higher risk factor | 7.2 |
| 35 0/7 to 37 6/7 weeks with a higher risk factor | 6.8 |
What the two guidelines say about this ratio
| Guideline | Position |
|---|---|
| AAP 2004 | Publishes the 8.0 / 7.2 / 6.8 thresholds above, to be used alongside the total serum bilirubin and never instead of it |
| AAP 2022 revision | Requires serum albumin to be measured when care is escalated, and treats an albumin below 3.0 g/dL as a neurotoxicity risk factor in its own right |
| NICE CG98 | Do not use the albumin/bilirubin ratio when making decisions about the management of hyperbilirubinaemia |
| BARTrial (2014) | A randomised trial in preterm infants comparing B:A-guided with bilirubin-guided treatment found no difference in neurodevelopmental outcome |
Converting between the two unit systems
| Bilirubin | Albumin | B:A ratio |
|---|---|---|
| 340 µmol/L (19.9 mg/dL) | 35 g/L (3.5 g/dL) | 5.68 |
| 340 µmol/L (19.9 mg/dL) | 30 g/L (3.0 g/dL) | 6.63 |
| 340 µmol/L (19.9 mg/dL) | 25 g/L (2.5 g/dL) | 7.95 |
| 340 µmol/L (19.9 mg/dL) | 20 g/L (2.0 g/dL) | 9.94 |
| 425 µmol/L (24.8 mg/dL) | 30 g/L (3.0 g/dL) | 8.28 |
| 255 µmol/L (14.9 mg/dL) | 30 g/L (3.0 g/dL) | 4.97 |
Why the albumin matters as much as the bilirubin
Bilirubin circulates almost entirely bound to albumin. Bound bilirubin is confined to the plasma and is not available to cross into the brain; it is the small unbound, or free, fraction that produces bilirubin encephalopathy. Albumin's high-affinity binding sites are finite, so the same total bilirubin concentration represents a very different amount of free bilirubin in a baby with an albumin of 35 g/L and one with an albumin of 20 g/L. The bilirubin to albumin ratio is the simplest possible expression of that idea: how much bilirubin is there for each gram of the protein that has to carry it.
The ratio is a proxy, and a fairly crude one. It assumes that binding capacity is proportional to albumin concentration and identical between babies, which it is not — binding affinity varies with gestation and with illness, and is reduced by acidosis and by drugs that compete for the same sites, sulphonamides and ceftriaxone among them. It also takes no account of conjugated bilirubin, which binds differently. Free bilirubin can be measured directly by peroxidase methods, and where that assay is available it answers the question the ratio is only estimating.
Where the ratio earns its place is in flagging the baby whose total bilirubin looks manageable but whose albumin is low. A sick, preterm or septic newborn can have an albumin well under 25 g/L, and at that point a total bilirubin comfortably below a treatment line may still carry an unacceptable free fraction. That is the clinical situation the AAP thresholds were constructed for, and it is why the 2022 AAP revision requires an albumin to be measured whenever care is escalated, and treats an albumin below 3.0 g/dL as a neurotoxicity risk factor in its own right even though it does not turn on the ratio as such.
The disagreement between guidelines has to be stated plainly. AAP publishes ratio thresholds of 8.0, 7.2 and 6.8 stratified by gestation and risk, and says explicitly that they are used together with and not in lieu of the total serum bilirubin. NICE, by contrast, instructs that the albumin to bilirubin ratio should not be used at all in decisions about managing hyperbilirubinaemia, and a randomised trial in preterm infants comparing ratio-guided with bilirubin-guided treatment found no difference in neurodevelopmental outcome. A British unit working to NICE therefore measures the albumin, reads it as a risk factor, and makes the treatment decision on the total bilirubin against the gestation-specific threshold graph. This page computes the ratio so that a number quoted in the literature or on an international transfer can be understood — not so that it can be treated as a trigger.
Frequently asked questions
How is the bilirubin to albumin ratio calculated?
Divide the total bilirubin in mg/dL by the albumin in g/dL. From SI results, divide the bilirubin in µmol/L by 17.104 and the albumin in g/L by 10 first — or use the single shortcut, B:A = 0.5847 × bilirubin in µmol/L ÷ albumin in g/L.
What B:A ratio indicates exchange transfusion?
AAP 2004 gives 8.0 for a well baby of 38 weeks or more, 7.2 for a well baby of 35 to 36+6 weeks or a higher-risk baby of 38 weeks or more, and 6.8 for a higher-risk baby of 35 to 37+6 weeks. AAP states these are used together with the total serum bilirubin and never instead of it.
Does NICE recommend using the bilirubin to albumin ratio?
No. NICE CG98 states that the albumin/bilirubin ratio should not be used when making decisions about the management of hyperbilirubinaemia. A UK unit measures albumin as a risk factor but makes the treatment decision on the total bilirubin against the gestation-specific threshold graph.
Why does a low albumin make the same bilirubin more dangerous?
Because only unbound bilirubin crosses into the brain. Albumin's binding sites are finite, so at a lower albumin concentration a greater proportion of the same total bilirubin circulates free. A baby with a bilirubin of 340 µmol/L has a ratio of 6.6 at an albumin of 30 g/L and 9.9 at an albumin of 20 g/L.
Can the B:A ratio replace the total bilirubin?
No, and no guideline suggests it can. The ratio falls whenever albumin is high, so a baby with a bilirubin plainly above the exchange threshold can have a reassuring ratio. It is an adjunct that modifies how a bilirubin is read, not a substitute measurement.
Related calculators
References
- American Academy of Pediatrics Subcommittee on Hyperbilirubinemia. Management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. 2004;114(1):297–316 — B:A ratio thresholds, Figure 4 legend.
- Kemper AR, Newman TB, Slaughter JL, et al. Clinical practice guideline revision: management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. 2022;150(3):e2022058859.
- National Institute for Health and Care Excellence. Jaundice in Newborn Babies Under 28 Days. NICE clinical guideline CG98. London: NICE; 2010, updated 2023.
- Hulzebos CV, Dijk PH, van Imhoff DE, et al. The bilirubin albumin ratio in the management of hyperbilirubinemia in preterm infants to improve neurodevelopmental outcome: a randomized controlled trial — BARTrial. PLoS One. 2014;9(7):e99466.
- Ahlfors CE. Criteria for exchange transfusion in jaundiced newborns. Pediatrics. 1994;93(3):488–494.
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.