Zinc is an essential element of established importance in humans: numerous enzymes are zinc metalloproteins which control several stages of both protein and nucleic acid synthesis. Severe deficiency of zinc occurs in acrodermatitis enteropathica, an inherited defect in which zinc absorption from the intestine is impaired. It results in retarded growth, hypogonadism, skin and ophthalmic lesions, and a deficiency in cell-mediated immunity.
Determination of plasma zinc concentration is by far the most commonly used procedure for investigation of suspected zinc deficiency. However, the interpretation of small changes in the concentrations observed in respect to a reference population is complicated by several factors which affect plasma zinc concentration independently of zinc status. These include the concentration of plasma albumin, diurnal variation, the time of sampling in relation to meals, and a non-specific decrease following the acute phase response. Plasma zinc should therefore be measured around two weeks after an acute phase response has resolved.
In this country, significant zinc deficiency is most likely to occur in patients on TPN feeding, and those with malabsorption, burns, or prolonged diarrhea. This can result in poor appetite, delayed wound healing, dermatitis, mental lethargy, and growth retardation.
Increased urinary zinc may be due to muscle breakdown and/or infusion of chelating agents. Such factors make the use of urine zinc unreliable as a test for zinc deficiency in hospital practice, although urinary excretion is reported to be lower than normal in chronic zinc deficiency.
Sample Requirements and Reference Ranges for Zinc
| Sample Type | Plasma or serum, urine |
|---|---|
| Container | Plasma/serum: lithium heparin, plain, or ‘Trace Metal’ (plain or heparin). SST and EDTA unsuitable. |
| Precautions | NB. Zinc tends to leak from red cells so plasma should be separated as soon as possible (cut-off: 4 hours; or within 24 hours if refrigerated). NB. Plasma zinc concentrations are stable. NB. Zinc and castor oil ointment applied to the skin may cause substantial contamination. Please mix 24 hour urine well before taking an aliquot. |
| Minimum volume* | Plasma: 250 µL Urine: 1 mL |
| Reference ranges | |
| - plasma | 12 to 18 µmol/L (adult) 10 to 21 µmol/L (0 to 1 year) 10 to 18 µmol/L (1 to 5 years) 13 to 21 µmol/L (6 to 9 years) 12 to 15.5 µmol/L (10 to 13 years) 10 to 18 µmol/L (14 to 19 years) < 5.0 µmol/L (zinc deficiency likely) |
| - urine | < 3 to 21 µmol/24h (normal) |
| Turnaround time | 3 days |
| Method | Inductively coupled plasma – mass spectrometry |
* This is the absolute minimum volume; these volumes are insufficient to carry out a repeat analysis in the event of an analytical problem.