The Blood Test That Cannot Tell You Your Baby Will Be Fine

What a review of 28 studies and nearly 21,000 newborns found about the Coombs test and jaundice treatment — and why a negative result is not a clean bill of health

A common newborn blood test called the Coombs test looks for one specific cause of jaundice: antibodies from the mother attacking the baby's red blood cells. A large 2025 review pooling 28 studies and 20,935 newborns found that when the test is positive it does raise the odds a baby will need treatment, but when it is negative it rules very little out — roughly one in nine at-risk babies with a negative test still went on to need light therapy for jaundice.

Why Jaundice Is Watched So Closely

That finding comes from a 2025 review that gathered and combined the results of every suitable study published on the question [1]. To see why it matters, it helps to start with why newborn jaundice is watched at all.

Almost every newborn baby is a little yellow in the first days of life. The yellow colour comes from bilirubin, a pigment produced when red blood cells are broken down and recycled. Adults clear bilirubin through the liver without noticing. Newborns are less efficient at this, so the pigment builds up for a few days before their liver catches up. In most babies this is entirely harmless and fades on its own.

In a minority it does not. If bilirubin climbs high enough, it can cross into the brain and cause lasting injury. That outcome is rare in places with good newborn care precisely because it is looked for so diligently — with skin measurements, blood tests, and treatment when levels approach a threshold. The usual treatment is phototherapy, in which the baby lies under blue light that changes bilirubin into a form the body can excrete without the liver's help. Very occasionally a baby needs an exchange transfusion, in which their blood is gradually replaced with donor blood. This is the reason your baby's jaundice is checked and rechecked, and why a follow-up appointment after discharge matters even when everyone looks well.

Where the Coombs Test Comes In

Some babies become jaundiced faster than others because their red blood cells are being actively destroyed. The most common reason is a mismatch between the mother's blood type and the baby's. If a mother has blood group O and her baby has group A or B, or if the mother is Rhesus (Rh) negative and her baby is Rh positive, the mother's immune system may make antibodies against the baby's red cells. Those antibodies cross the placenta before birth, coat the baby's red cells, and mark them for destruction.

The direct antiglobulin test — almost always called the Coombs test after the researcher who developed it — detects those antibodies sitting on the surface of the baby's red cells [2]. It is a good test at that job. It is often done on blood taken from the umbilical cord at birth, so the result is available before jaundice has had a chance to develop.

That timing is exactly why it became so widely used as a warning system. When American paediatric guidance on newborn jaundice was written in 2004, a positive Coombs test with blood-group mismatch was listed as a major risk factor, and testing babies of group O mothers was recommended [3]. The revised guidance published in 2022 kept antibody-driven red cell destruction among the factors that lower the level at which treatment should start [4]. Over the years, some hospitals began testing every at-risk baby routinely; others tested more selectively and found they could do far fewer tests without apparent harm [5]. Researchers also tried grading the test — asking whether a strongly positive result meant more trouble than a weakly positive one [6]. What nobody had done was gather all the evidence together and ask a plain question: how well does this test actually predict which babies will need treatment?

What the Researchers Did

A team of neonatal doctors working in India and the United Kingdom, with a medical librarian at a Canadian university, set out to answer exactly that. They did not run a new study on babies. Instead they carried out what is called a systematic review and meta-analysis: they searched five large medical research databases from their very beginning up to February 2024, registered their plan publicly in advance so they could not change the question after seeing the results, and gathered every study that reported both a Coombs test result and whether the baby subsequently needed treatment [1].

They screened 1,409 studies, read 73 in full, and included 51 in the review. Of those, 28 contained enough numerical detail to be combined statistically, covering 20,935 babies born at 34 weeks of pregnancy or later, in eighteen countries, in studies published between 1978 and 2024. They then used a statistical method that produces a pooled estimate along with a range showing how much uncertainty surrounds it — a crucial feature, because a number without its uncertainty can be badly misleading.

What They Found

Two numbers describe how any medical test performs. Sensitivity is how often the test is positive in babies who genuinely go on to need treatment — its ability to catch problems. Specificity is how often the test is negative in babies who do not need treatment — its ability to avoid false alarms.

For babies with an ABO blood-group mismatch, the largest group studied, the Coombs test had a sensitivity of about 56% and a specificity of about 84%. In plain terms: it caught a little over half of the babies who ended up needing light therapy, and it correctly stayed quiet in about five out of six babies who did not.

The researchers translated this into a worked example that is worth sitting with. Imagine 1,000 babies with an ABO mismatch in a hospital where 20 out of every 100 such babies need phototherapy. About 243 of them will have a positive Coombs test — but 131 of those, more than half, will never actually need treatment. Of the 757 babies with a negative test, about 88 will need phototherapy anyway. That is roughly one in nine babies with a reassuring test result who still ends up under the lights.

For Rh mismatch the test performed worse at catching problems, though only three studies with 491 babies were available. For exchange transfusion — the rarer, more serious treatment — the numbers looked better on the surface but came from just two or three studies each, with such wide uncertainty ranges that the researchers concluded nothing dependable could be built on them. The pattern matched what individual hospitals had already reported: babies with negative tests still needing light therapy [7], and results that varied from one centre to the next [8],[9],[10].

The researchers were also careful to grade how much confidence the evidence deserves, using two established systems for assessing study quality and certainty [11],[12]. Their verdict was sobering: confidence in every sensitivity estimate was rated very low. The main reason is that in almost all the studies, the doctors deciding whether to treat already knew the Coombs result — which can quietly influence the decision and make the test look better or worse than it truly is. Notably, the laboratory performance of the test itself was rated as high quality in every single study. The assay is not the problem; the question being asked of it is.

The authors' conclusion was direct: they do not recommend using the Coombs test as a screening test to predict which babies will need phototherapy or an exchange transfusion.

What This Means for Your Baby

The most useful takeaway for families is about what a negative result does and does not mean. It means that one particular cause of rapid jaundice was looked for and not found. It does not mean your baby will not become jaundiced, and it does not mean the bilirubin checks or the follow-up appointment can be skipped. If a clinician has told you the Coombs test was negative and you took that as an all-clear, this review is the reason to keep the follow-up appointment anyway.

A positive result is worth taking seriously without alarm. It shifts the odds — roughly, it moves a one-in-five chance of needing light therapy to something closer to a coin flip — but it is not a diagnosis and not a prediction. Many babies with positive tests never need any treatment at all. What a positive result should prompt is closer watching: earlier bilirubin measurements, more of them, and a lower threshold for coming back to be checked.

Either way, the decision to treat is made on your baby's actual bilirubin level measured over time, compared against thresholds set for their age in hours and their individual risk factors — not on the blood type result. Bilirubin can also be estimated with a skin device that avoids a blood draw, though that measurement has its own accuracy limits, particularly around phototherapy [13]. It is the trend in the numbers, not any single test, that guides care.

What Researchers Are Working On Next

Three things would move this forward. The first is a study in which the treating team does not see the Coombs result, so that the test can be judged without influencing the decision it is being measured against. The second is treating the test as a scale rather than a yes-or-no answer — asking whether a strongly positive result predicts more than a weakly positive one, a question this review could not answer because the underlying studies did not record it. The third is combining the Coombs result with an early bilirubin measurement into a single risk estimate, which is likely to work better than either alone.

There is also a geographic gap. How common these blood-group mismatches are, and how severe their consequences, varies considerably between populations and healthcare systems [14],[15]. A test that behaves one way in one country may behave differently in another, and most of the evidence here comes from a small number of well-resourced settings.

None of this changes the fundamentals of newborn jaundice care, which work well: watch, measure, treat when the level calls for it. What it changes is how much weight one early blood test should carry in that process — and the honest answer, on this evidence, is less than it has been given.

References

  1. Kumar Krishnegowda V, Vadakkencherry Ramaswamy V, Abiramalatha T, et al. Direct antiglobulin test for the prediction of neonatal hyperbilirubinemia needing an intervention: a systematic review and diagnostic test accuracy meta-analysis. Front Pediatr. 2025;12:1475623. doi:10.3389/fped.2024.1475623 ↩
  2. Keir A, Agpalo M, Lieberman L, Callum J. How to use: the direct antiglobulin test in newborns. Arch Dis Child Educ Pract Ed. 2015;100(4):198–203. doi:10.1136/archdischild-2013-305553 ↩
  3. 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. doi:10.1542/peds.114.1.297 ↩
  4. 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. doi:10.1542/peds.2022-058859 ↩
  5. Shahid R, Graba S. Outcome and cost analysis of implementing selective Coombs testing in the newborn nursery. J Perinatol. 2012;32(12):966–969. doi:10.1038/jp.2012.26 ↩
  6. Kaplan M, Hammerman C, Vreman HJ, Wong RJ, Stevenson DK. Direct antiglobulin titer strength and hyperbilirubinemia. Pediatrics. 2014;134(5):e1340–e1344. doi:10.1542/peds.2014-1290 ↩
  7. Gabbay JM, Agneta EM, Turkington S, et al. Rates of phototherapy among ABO-incompatible newborns with a negative direct antiglobulin test. J Perinatol. 2023;43(11):1357–1362. doi:10.1038/s41372-023-01650-3 ↩
  8. AlKhater SA, Albalwi RA, Alomar SA, et al. Value of the direct antiglobulin test in predicting the need for phototherapy in newborns. J Blood Med. 2021;12:53–61. doi:10.2147/JBM.S291606 ↩
  9. Chowdhary S, Devi U, Giridhar S. Predicting significant hyperbilirubinemia in ABO incompatibility: is cord direct antiglobulin test useful? Indian J Hematol Blood Transfus. 2022;38(3):591–595. doi:10.1007/s12288-021-01513-x ↩
  10. Mehta R, Petrova A. Direct antiglobulin test in the prediction of hyperbilirubinemia and predischarge bilirubin levels in infants with mother-infant blood type incompatibility. Pediatr Neonatol. 2021;62(4):406–411. doi:10.1016/j.pedneo.2021.04.002 ↩
  11. Whiting PF, Rutjes AW, Westwood ME, et al. QUADAS-2: a revised tool for the quality assessment of diagnostic accuracy studies. Ann Intern Med. 2011;155(8):529–536. doi:10.7326/0003-4819-155-8-201110180-00009 ↩
  12. Yang B, Mustafa RA, Bossuyt PM, et al. GRADE guidance: 31. Assessing the certainty across a body of evidence for comparative test accuracy. J Clin Epidemiol. 2021;136:146–156. doi:10.1016/j.jclinepi.2021.04.001 ↩
  13. Ten Kate L, van Oorschot T, Woolderink J, Teklenburg-Roord S, Bekhof J. Transcutaneous bilirubin accuracy before, during, and after phototherapy: a meta-analysis. Pediatrics. 2023;152(6):e2023062335. doi:10.1542/peds.2023-062335 ↩
  14. Zonneveld R, Lamers M, Schonewille H, et al. Prevalence of positive direct antiglobulin test and clinical outcomes in Surinamese newborns from D-negative women. Transfusion. 2017;57(10):2496–2501. doi:10.1111/trf.14229 ↩
  15. Shin KH, Lee HJ, Song D, et al. Characteristics of bilirubin according to the results of the direct antiglobulin test and its impact in hemolytic disease of the newborn. Lab Med. 2019;50(2):138–144. doi:10.1093/labmed/lmy050 ↩