When a two-year-old child presented at the Children's Hospital at Westmead in Sydney last year with a decreased level of consciousness, no one was quite sure what was wrong with him.

He had spent the morning playing in the backyard with his siblings and appeared to be fine. Suddenly, however, he began walking with a staggered gait, as if drunk, and fell into a deep sleep from which he was difficult to rouse. Seven hours after he presented, he was back to normal.

As is normal practice for a child with a reduced level of consciousness, a full range of tests were conducted, including toxicology screens that all came back normal.

The emergency doctors then considered the possibility of a metabolic disorder, as in children this can happen very quickly. To rule this out, they ordered a urine metabolic screen, which is where Dr Kevin Carpenter came in.

Carpenter trained initially in chemical pathology in his native UK and then specialised in paediatric chemical pathology, which led to the study of inborn errors of metabolism. He is one of a small community of inborn error specialists in Australia and is now principal scientist at the NSW Biochemical Genetics Service at Westmead.

The urine metabolic screen, which includes organic acid testing, came back positive for gamma-hydroxybutyrate (GHB), which was unusual only in a sense.

While GHB is best known as a recreational drug, it is also a naturally occurring substance in the body.

It is excreted in a certain inborn error called succinic semialdehyde dehydrogenase (SSADH) deficiency, also known as gamma-hydroxybutyric aciduria. SSADH is an enzyme whose job is to convert succinic semialdehyde into GHB, which acts as a neurotransmittor and is a metabolite of GABA.

If you have an SSADH deficiency, GHB levels build up and you see it in the urine, Carpenter says.

"But as well as seeing GHB in the urine of the patients who have the inborn error, you see another compound as well - 4,5-dihydroxyhexanoate lactone - and that wasn't there. I looked at the clinical information on request and I thought it was unusual with a child of this age, with this presentation, that it would be that particular inborn error."

This is where the real sleuthing began. If the raised levels of GHB in the child's urine were not due to an inherited disorder then ingestion was the obvious suspect. The child was still in hospital as he had picked up an infection, so another urine sample was taken and the GHB was absent.