“Why can’t you just do a blood test?” is one of the most reasonable questions a parent asks me, and it deserves a proper answer.
Why a single sample often tells us nothing
Many hormones are not released steadily. Growth hormone in particular is secreted in short bursts, mostly during deep sleep, and between those bursts the level in the blood of a perfectly healthy child is close to zero. So a random daytime growth hormone level of almost nothing is completely uninformative. It is exactly what we would expect to find in a child who is entirely fine.
So instead of asking “how much is there right now?”, we ask “can the gland produce hormone when it is asked to?” That means giving a stimulus and then sampling repeatedly over the following hours to see what the gland does.
The same logic drives ACTH stimulation testing for the adrenal glands, GnRH testing for puberty, and the oral glucose tolerance test.
What the morning actually looks like
Details vary by test and by centre, but the shape is consistent.
| Time | What happens |
|---|---|
| Before you arrive | Fasting from midnight; usually water is allowed. Confirm this with the clinic. |
| Arrival, around 8 am | Height and weight, consent, questions answered. |
| First 15 minutes | A single cannula (a thin plastic tube) is placed in a vein. One needle for the whole test. |
| Baseline | The first sample is drawn through the cannula. |
| The stimulus | A medicine is given, by mouth or through the cannula depending on the test. |
| Next 2–3 hours | Samples through the same cannula at set intervals, often every 30 minutes. |
| Throughout | Your child rests, reads, watches something. Observation by staff. |
| End | Cannula removed, a meal, and usually home. |
The whole thing typically takes three to four hours. Results generally take several days, because the samples are batched and run together.
The part that helps most: the cannula
Tell your child this, because it changes everything about how the day feels:
There is one needle, right at the beginning. After that, a soft tube stays taped to your arm, and every other sample comes out through the tube. Nothing else will hurt.
Ask whether a numbing cream (lidocaine–prilocaine) can be applied 45–60 minutes before the cannula. In most centres it can, and it makes a substantial difference. It has to go on early, so ask about it the day before, not on arrival.
What to bring
- All previous reports, and the growth chart or old measurement records
- The appointment slip and consent form
- A fully charged device with downloaded content. Lab Wi-Fi is not a plan
- A book, drawing things, cards. A long morning is long
- A blanket or hoodie; test rooms are usually cold
- Food and drink for immediately afterwards, ready in the bag
- A second adult if at all possible, so one of you can step out
- For younger children, one comfort object that is not negotiable
What not to do
- Do not break the fast, even with tea or a biscuit. It invalidates several tests and the morning has to be repeated.
- Do not surprise your child. Children handle unpleasant things far better than they handle being lied to about them.
- Do not promise it will not hurt. Promise instead that there is exactly one uncomfortable moment, that you will be there for it, and that you will say when it is coming.
- Do not skip regular medication without asking. Thyroid tablets, hydrocortisone and some other medicines have specific instructions around testing, so check in advance.
Is it safe?
Stimulation tests are done in a supervised setting precisely because they involve pushing a system deliberately. Some tests can cause temporary low blood sugar, sleepiness or nausea; staff monitor for exactly this and treat it immediately. Serious complications are rare, which is why these tests are performed in a day-care or ward setting with staff present rather than at a walk-in collection centre.
If a test has been recommended for your child, ask what decision the result will change. There should always be a clear answer. A test that would not alter the plan is a test worth reconsidering.
Protocols differ between hospitals. Always follow the specific written instructions given by the centre performing your child’s test.