It is common practice in the evaluation of neonatal hypoglycaemic episodes, especially if unusual or prolonged, to perform a “critical sample”. This is performed to rule out underlying metabolic or endocrine disorders. But what should the critical sample consist of?

I have looked at several recommednations, and there is great variability in what is included in such samples; it is near universal that a lab glucose, a serum cortisol, insulin and growth hormone are included, but after that there is little consensus.

One exception to those inclusions is the American Pediatric Endocrine Society, who are quite minimalist, recommending that the critical sample includes a glucose, bicarbonate, Beta-hydroxy-butyrate and a lactate. They further recommend that plasma be kept in reserve for further tests (and give the following as examples: plasma insulin, FFA, and C-peptide : total and free carnitine and acyl-carnitine), they don’t even overtly mention cortisol and growth hormone as part of the critical sample. Their recommendation is meant for screening for neonates but is also meant for older children, and it therefore ignores the relative incidence of the various causes of hypoglycaemia in the neonatal period. With substrate deficiency (transitional hypoglycaemia) and hyperinsulinaemia being much more common than other causes, and endocrine causes being next and relatively speaking fairly uncommon.

Beta-hydroxy-butyrate is recommended by many to be included in initial screen, it is the most commonly measured of the ketone bodies, and is depressed in hyperinsulinaemic infants, but is low also in infants with transitional hypoglycaemia, so is really only helpful as a flag for endocrine deficiencies and for the rarer glycogen storage diseases, where it is raised during hypoglycaemia.

Ketones, such as beta-hydroxy-butyrate, are increased during hypoglycaemia due to growth hormone or cortisol deficiency. The diagnosis of those entities is sometimes tricky, growth hormone secretion being pulsatile and the range of normal serum cortisol being wide. Often, also, when an infant has a critical sample performed because the bedside glucose is low, the lab glucose will be taken a few minutes later and often ends up being low normal, either because of the delay, and/or because bedside glucose is inaccurate and usually lower than the lab glucose. So, a serum cortisol which is on the low side of normal on a critical sample with a lab glucose of 2.8 (for example) what does that mean? Knowing that the ketones were high would be a good clue that further endocrine evaluation is required.

Ketones are also elevated in ketotic hypoglycaemia, which is one justification for measuring them in some guidelines, but it is not on the differential of neonatal hypoglycaemia, becoming important in older children.

Free Fatty Acids are also in many recommendations, and mainly serve to distinguish fatty acid oxidation defects, where they are elevated during hypoglycaemia despite low ketone bodies, the total incidence of all those disorders is probably about 1:10,000 births, but they are treatable, and picking them up when an infant is hypoglycaemic in the first few days of life is probably beneficial, and may well improve outcomes. I think we should keep them in our critical sample, but I don’t know the proportion of babies with fatty acid oxidationn defects who present with hypoglycaemia in the first few days of life, so diagnosing them from a critical sample is likely to be very uncommon.

Some recommend including a C-peptide measurement, with the idea that a high insulin with a low C-peptide is evidence of exogenous insulin administration. As that is an extremely unlikely scenario in the neonatal ward or NICU I think we can drop the C-peptide, (especially as some varieties of commercial insulin do not even register with some insulin assays). It is mostly paediatric endocrinologists who seem to want a C-peptide, and as part of the work-up of an older child it might be more relevant.

Many recommendations suggest growth hormone and cortisol estimation in the critical sample, and these seem to be the next most likely to lead to a diagnosis, after high insulin concentrations. Inappropriately low concentrations have led to a diagnosis in several babies I have seen over the last few years.

The Canadian Paediatric Society recommends obtaining a critical sample, but does not mention what should be measured in the sample. Others have suggested IGF binding protein-1 levels, without a good explanation why, and then several suggest other tests in later work up depending on the initial findings, including ammonia, urine organic acids and serum amino acids, triglycerides, carnitine and acylcarnitine profiles.

The volume of blood required is an issue for most blood sampling in the newborn, so I think the tests required for specific diagnosis of other rare conditions that rarely present with neonatal hypoglycaemia can be left out of the initial “critical sample”.

The other consideration is that you can get an idea whether a hypoglycaemic infant is hyper-insulinaemic from the amount of glucose that they are requiring. Those that require very high glucose intakes to remain normoglycaemic (more than 8 to 10 mg/kg/min) are likely to be hyper-insulinaemic. That includes a proportion of those who are Small for Gestational Age, or with birth asphyxia as well as infants of diabetic mothers or LGA infants. Of course the uncommon babies with congenital hyperinsulinism syndromes will fit this picture.

Putting all this together, I think the most appropriate critical sample when it is required; that is, unexplained, prolonged, or recurrent hypoglycaemia, should measure the following, and if blood volume is a problem, start with the first 2 items on the list and work down:

After that, if you still don’t have an answer and hypoglycaemia is recurrent, a call to your helpful local paediatric endocrinologist would be a good idea!