Why it matters in the Letby case
Two Letby counts turn on the inference that low blood sugar was caused by exogenous insulin. The expert dispute is about whether the laboratory method used could reliably support that inference.
Medical explainer
Neonatal hypoglycaemia — plain-English explainer
Neonatal hypoglycaemia (low blood sugar in a newborn) has many natural causes — inadequate substrate, prematurity, transient hyperinsulinism, sepsis. Diagnosing exogenous (administered) insulin from blood ratios alone requires careful methodology.
Scientific literature2Expert/professional
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Prosecution position
Two counts turn on the inference that low blood sugar in Children F and L was caused by insulin administered from outside the body, evidenced by the laboratory's insulin and C-peptide results.
Expert challenge / post-conviction reading
Neonatal hypoglycaemia has many natural causes — inadequate substrate, prematurity, transient hyperinsulinism, sepsis. The Panel attributed both children's hypoglycaemia to natural and iatrogenic causes rather than to administered insulin, and independent biochemists argue the laboratory method could not reliably distinguish the two.
What remains uncertain
Treatment for hypoglycaemia suppresses C-peptide, and the samples were drawn after repeated dextrose infusions — so the ratio was measured against a baseline that treatment had already moved. What the untreated values would have been is not recoverable.
Common questions
What causes low blood sugar in a newborn?
Commonly inadequate substrate, prematurity, transient hyperinsulinism or sepsis. It is a frequent and well-recognised problem in preterm and growth-restricted babies.
Can low blood sugar alone show that insulin was administered?
No. The inference depends on the insulin-to-C-peptide ratio, and on that ratio having been measured by a method able to support a forensic conclusion.
Why does prior treatment complicate the reading?
Because treating hypoglycaemia with dextrose suppresses C-peptide. A sample taken after treatment does not reflect the baby's baseline, which is the value the inference actually depends on.
What did the Panel conclude?
That the ratios were within the range normal for preterm infants, and that the hypoglycaemia in both children is explained by prematurity, growth restriction, sepsis and management factors.
Related reading
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