Clinical use
To investigate causes of spontaneous hypoglycaemia
Background
In order to maintain body energy requirements during periods of hypoglycaemia a switch to alternative substrates e.g. free fatty acids, glycerol, ketone bodies – is made. This will be reflected by predictable changes of these compounds in the blood. A specific deficiency in the cellular energy producing systems will prevent appropriate changes in glucose metabolites occurring.
An assessment of the metabolite profile obtained during the hypoglycaemic state in conjunction with levels of other biochemical markers (e.g. insulin, cortisol, organic acids) enables the lesion to be pinpointed and a diagnosis to be made. The free fatty acid/ketone body ratio is particularly important – for example this may be increased in disorders of fatty acid oxidation and decreased in ketone body utilisation defects – hence the requirement for a separate blood sample for NEFA (Non-esterified fatty acids).
Mitochondrial disorders can be identified in this manner e.g. pyruvate carboxylase deficiency, medium/long chain acyl CoA dehydrogenase deficiency. Pyruvate and lactate only may be requested by neurological or paediatric wards as increased pyruvate/lactate ratio may point to an inherited or acquired mitochondrial disorder. Glucose metabolites are collected into 5% PCA to prevent degradation of the more labile intermediates.
Reference ranges
| (ALL mmol/L) | Blood | CSF |
|---|---|---|
| Glucose | 4.18 – 5.49 | 2.2 – 4.2 |
| Lactate | 0.40 – 1.8 | 1.2 – 1.8 |
| Pyruvate | 0.035 – 0.15 | 0.04 – 0.15 |
| β-OH butyrate | 0.01 – 0.34 | 0.024 – 0.094 |
| Acetoacetate | 0.01 – 0.15 | 0.016 – 0.053 |
| Glycerol | 0.03 – 0.10 | 0.011 – 0.016 |
| Alanine | 0.20 – 0.50 | 0.01 – 0.05 |
| NEFA | 0.20 – 1.80 | |
| Lac/Pyr Ratio | 0.0 – 18.0 | 0.0 – 18.0 |
Patient preparation
Ideally samples should be collected during a hypoglycaemic episode or hypoglycaemia can be provoked as a consequence of a medically supervised fast before sample collection. Random non-fasting samples are also acceptable.
Specimen requirements
Blood
(A) 1 – 2 mL blood added to a previously weighed 10 mL screw top tube containing 5 mL 5% perchloric acid. Return tube on ice to laboratory immediately.
(B) Blood specimen in a Plain (red top), SST or Lithium Heparin (green top) tube for NEFA (Non-esterified fatty acids).
CSF
CSF collected into a previously weighed 10 mL screw top tube containing 5 mL 5% perchloric acid. Return tube on ice to laboratory immediately. A separate sample for NEFA is not required.
Turnaround time
4 weeks
Additional Information
Additional PCA tubes are kept in the fridge in PDU, these are made up by the laboratory staff upon request.
Referred test
Referred test
Location
Paediatric Metabolic Centre