Cord Blood Gas Interpreter
Cord Blood Gas Interpreter
Paired umbilical arterial and venous results read together: whether the pair is genuinely arterial and venous at all, and whether the arterial sample meets the conventional definition of significant metabolic acidaemia.
Umbilical cord blood gas
Paired pH, base excess, lactate → interpretationArterial pH 6.96, venous pH 7.12, arterial base excess −14.5 mmol/L, arterial lactate 9.4 mmol/L
Normal umbilical cord blood gas values at term
| Measurement | Arterial median (5th–95th centile) | Venous median (5th–95th centile) |
|---|---|---|
| pH | 7.27 (7.12 – 7.35) | 7.35 (7.23 – 7.44) |
| pCO₂ | 7.3 kPa / 55 mmHg (5.6 – 9.8 kPa) | 5.4 kPa / 40 mmHg (3.8 – 7.1 kPa) |
| Base excess | −3.0 mmol/L (−9.3 to +1.5) | −3.0 mmol/L (−8.3 to +2.6) |
| Lactate | 3.7 mmol/L (2.0 – 6.7) | — |
Validating the pair before interpreting it
| Criterion | Requirement | Why |
|---|---|---|
| Arteriovenous pH difference | More than 0.02 units, artery lower | Two samples from the same vessel differ only by analytical imprecision |
| Arteriovenous pCO₂ difference | More than 0.5 kPa (about 3.75 mmHg), artery higher | The second criterion, applied together with the pH difference |
| Both vessels sampled | Two syringes, taken separately | A single sample cannot be shown to be arterial at all |
| Cord segment double-clamped at delivery | Isolated promptly | A clamped segment is stable for around an hour; an unclamped loop continues to metabolise |
| Which vessel is which | Two arteries — smaller, thicker-walled, muscular. One vein — larger, thin-walled | Identifying them before sampling prevents the error the validation criteria are there to catch |
The criteria for significant metabolic acidaemia
| Both limbs required | Threshold |
|---|---|
| Umbilical arterial pH | Below 7.00 |
| Umbilical arterial base deficit | 12 mmol/L or more, that is a base excess of −12 or lower |
| What it is not | Not a diagnosis of hypoxic-ischaemic encephalopathy, which is clinical |
| What it does not exclude | Deterioration after delivery. The gas describes the moment of clamping and nothing later |
Two samples, or no answer
A cord gas is the only objective record of the fetus’s metabolic state at the moment of delivery. Everything else about a birth — the cardiotocograph, the Apgar scores, the impression of the room — is an inference or a judgement. The gas is a measurement, and it is the measurement that has to stand up years later when the events of a delivery are examined. That is why the sampling technique matters as much as the numbers.
Umbilical arterial blood is fetal blood on its way to the placenta; umbilical venous blood is oxygenated blood on its way back from it. Only the arterial sample reports the fetus. A venous sample reports the placenta, and because placental blood is less acidotic it reports it reassuringly. The classic error is therefore not a mislabelled tube but a mis-sampled vessel: the umbilical vein is single, large and thin-walled and it is the easy one to enter, while the two arteries are smaller, thicker and muscular. Two syringes drawn from the vein produce a pair of normal-looking results, and the baby they look normal for is precisely the baby whose arterial result would not have been.
The defence against that is to sample both vessels and then to prove that both were sampled. The accepted criteria are an arteriovenous pH difference of more than 0.02 units, with the artery the lower, together with a pCO2 difference of more than 0.5 kilopascals, with the artery the higher. Two samples taken from the same vessel cannot generate both differences; they will agree to within analytical imprecision. A pair that fails the criteria has not produced a normal cord gas — it has produced an uninterpretable one, and saying so in the record is more useful than a reassuring number that cannot be substantiated. Double-clamping a segment of cord immediately after delivery isolates both vessels and keeps the values stable for around an hour, which is long enough for a resuscitation to take priority over a blood gas.
Once the pair is validated, the interpretation is comparatively simple. Significant metabolic acidaemia is conventionally defined as an umbilical arterial pH below 7.00 together with a base deficit of 12 millimoles per litre or more, and it is the combination that matters: a pH below 7.00 with a preserved base excess is a respiratory acidaemia from acutely accumulated carbon dioxide, which resolves rapidly once the baby ventilates, while a large base deficit reflects sustained anaerobic metabolism. Lactate is the same information arriving earlier, and rises before the pH falls. None of it is a diagnosis of hypoxic-ischaemic encephalopathy, which is made clinically over the following hours; most babies who meet the biochemical threshold never develop it. And none of it says anything about what happens after the cord is clamped — a normal cord gas does not exclude sepsis, metabolic disease or postnatal collapse.
Frequently asked questions
What cord blood gas result defines significant metabolic acidaemia?
An umbilical arterial pH below 7.00 together with a base deficit of 12 mmol/L or more. Both limbs are required — a pH below 7.00 with a near-normal base excess is a respiratory acidaemia with a quite different prognosis, and a large base deficit with a pH above 7.00 does not meet the definition either.
Why must both the umbilical artery and vein be sampled?
Because only the arterial sample reports the fetus; venous blood is arriving from the placenta and is less acidotic. Without a venous sample to compare against, an arterial result cannot be shown to be arterial — and the commonest sampling error is filling both syringes from the larger, easier umbilical vein, which produces a falsely reassuring pair.
How do you confirm a cord sample is genuinely arterial?
By the arteriovenous differences: the arterial pH must be more than 0.02 units below the venous pH, and the arterial pCO2 more than 0.5 kPa above the venous pCO2. Two samples from the same vessel cannot show both. A pair failing these criteria should be reported as unvalidated rather than as a normal cord gas.
What are normal umbilical cord blood gas values?
Umbilical arterial pH has a median of about 7.27 with a 5th to 95th percentile range of 7.12 to 7.35, base excess about −3 mmol/L, and lactate a median of 3.7 mmol/L. The vein is less acidotic, with a median pH of about 7.35.
Does a normal cord gas exclude hypoxic-ischaemic encephalopathy?
It makes an acute intrapartum hypoxic-ischaemic cause much less likely, but it describes only the moment the cord was clamped. Encephalopathy is a clinical diagnosis made over the hours after birth, and postnatal deterioration, sepsis and metabolic disease all occur despite a normal gas.
Related calculators
References
- Armstrong L, Stenson BJ. Use of umbilical cord blood gas analysis in the assessment of the newborn. Arch Dis Child Fetal Neonatal Ed. 2007;92(6):F430–F434.
- Westgate J, Garibaldi JM, Greene KR. Umbilical cord blood gas analysis at delivery: a time for quality data. Br J Obstet Gynaecol. 1994;101(12):1054–1063.
- American College of Obstetricians and Gynecologists. Umbilical cord blood gas and acid-base analysis. Committee Opinion No. 348. Obstet Gynecol. 2006;108(5):1319–1322.
- Yeomans ER, Hauth JC, Gilstrap LC 3rd, Strickland DM. Umbilical cord pH, PCO2, and bicarbonate following uncomplicated term vaginal deliveries. Am J Obstet Gynecol. 1985;151(6):798–800.
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.