The critical sample is not a reason to delay glucose. Its value is that insulin, ketones, and free fatty acids describe the child’s physiology while glucose is low—and can change quickly after carbohydrate or IV dextrose. If a child is seizing, has altered consciousness, cannot swallow safely, or is otherwise unstable, draw the sample only if it can be done immediately alongside treatment. Do not hold up rescue to complete a lab list.
Make the draw at the right moment
For evaluation of a persistent hypoglycemia disorder, a laboratory plasma glucose around or below 50 mg/dL (2.8 mmol/L) is commonly used as the point at which a critical sample is informative. This is a diagnostic sampling threshold, not a universal treatment threshold: age, especially the first 48 hours after birth, and the child’s condition matter. Never wait for glucose to fall further just to obtain a more textbook sample.
Use a bedside glucose as a rapid signal, then obtain a laboratory-quality plasma glucose with the other time-sensitive specimens when feasible. Do not wait for the lab result before treating a clinically urgent episode. Record the collection time, last feed, illness context, and whether oral carbohydrate, glucagon, or IV dextrose came before or after the draw. That timing is part of the interpretation.
A practical collection plan is to use the local hypoglycemia order set, alert the laboratory if specimen handling is uncertain, and collect urine if available. Tube types, sample volumes, and transport requirements vary by laboratory; do not guess them at the bedside.
What belongs in the time-critical sample?
The most useful panel captures the glucose level, the body’s alternative fuels, and the hormonal response. Local protocols differ, so use this as a checklist to discuss with the team—not as a reason to delay treatment while searching for a particular tube.
| Sample | What it helps assess |
|---|---|
| Laboratory plasma glucose | Confirms the biochemical context in which the other results were obtained. |
| β-hydroxybutyrate and free fatty acids | Show whether ketones are being made and fat is being mobilized. Interpret them together. |
| Insulin and C-peptide | Assess insulin secretion and action; C-peptide helps interpret whether insulin is being secreted by the child. |
| Bicarbonate and lactate | Identify acid–base or metabolic clues that may direct further evaluation. |
| Cortisol and growth hormone | Capture counter-regulatory hormones during the episode, but do not diagnose deficiency from one low result alone. |
| Urine ketones; urine organic acids when indicated | Provide additional fuel and metabolic clues. Save an episode-time urine sample when practical and follow the local pathway. |
Depending on the presentation and local protocol, add ammonia, liver-function tests, an acylcarnitine profile, free and total carnitine, plasma amino acids, or drug testing. These are particularly relevant when the child is very ill, the episode is unexplained or recurrent, or the history raises concern for a metabolic disorder or medication exposure. The broader tests can add important clues, but they should not delay rescue. Some targeted tests, including plasma amino acids and acylcarnitines, may still contribute after glucose correction; collect urine organic acids during or soon after the episode when practical. Timing and handling vary by test and suspected disorder, so ask the laboratory or metabolic team. If hyperammonemia is suspected, measure ammonia promptly rather than treating it as a deferred add-on.
Read results in linked pairs, not as isolated answers
Insulin and C-peptide: Insulin should be suppressed during significant hypoglycemia. An insulin result that is measurable or inappropriately unsuppressed may support excess insulin, but a low or undetectable result does not always exclude it. Assay sensitivity and the evidence of insulin action matter. C-peptide helps distinguish the child’s own insulin secretion from some forms of external insulin exposure; suspected medication exposure may require targeted toxicology testing. In a planned, supervised evaluation for suspected hyperinsulinism, a glucagon-response test after collection of the critical blood sample may add evidence of excess insulin action. It is not a reason to postpone urgent rescue.
β-hydroxybutyrate and free fatty acids: Insulin suppresses both fat release and ketone production. Suppression of both during hypoglycemia supports inappropriate insulin action. Low ketones with free fatty acids that are not suppressed can point toward impaired ketone production. These are patterns that guide the next tests, not diagnoses by themselves.
Cortisol, growth hormone, lactate, and bicarbonate: These results add context, but a single low cortisol or growth hormone value during an episode does not establish hormone deficiency. Likewise, an abnormal lactate or bicarbonate may narrow the differential without naming the cause. Interpret each alongside the child’s baseline growth, development, feeding and fasting history, examination, and the rest of the sample.
If the sample opportunity is missed
Treat the child first. A sample drawn after glucose correction may no longer show the original insulin, β-hydroxybutyrate, or free-fatty-acid response, so do not label it a true critical sample or use that post-treatment snapshot to exclude a disorder. Record that treatment came first, preserve useful urine or other specimens if available, and discuss the next diagnostic step with pediatric endocrinology or a metabolic specialist.
A supervised fast may sometimes be considered when no spontaneous sample was obtained, but it is a planned, monitored diagnostic procedure—not an improvised way to recreate an episode. The practical win is to have an emergency-department or inpatient pathway ready before the next event.
Practical takeaways
- Draw time-sensitive blood tests while glucose is low, when feasible—but never delay urgent treatment to complete the panel.
- Pair laboratory glucose with β-hydroxybutyrate, free fatty acids, insulin, and C-peptide; include hormonal and metabolic tests according to the local protocol and presentation.
- Interpret paired fuels and hormones as physiologic patterns, not stand-alone diagnoses.
- Document whether treatment preceded collection; a post-treatment sample cannot recreate the original metabolic state.
