Zinc contamination in serum samples
Cat Barr, PhD, DABT
Recently, the Texas A&M Veterinary Medical Diagnostic Laboratory’s (TVMDL) analytical chemistry section performed zinc testing on a small set of paired samples. Prior to testing, blood was collected from four clinically healthy animals and divided into two tubes with EDTA. One tube was a standard microtainer tube with a rubber o-ring buried in the groove of its screw-top and the other tube was an all-plastic microcentrifuge tube. Plasma zinc was measured in each of the four samples.
| Plasma Zn (µg/mL) | incr | incr | ||
| An ID | o-ring | plastic | diff abs | diff % |
| 1 | 1.73 | 1.60 | 0.13 | 0.08 |
| 2 | 1.61 | 1.55 | 0.06 | 0.04 |
| 3 | 1.87 | 1.79 | 0.08 | 0.04 |
| 4 | 1.53 | 1.39 | 0.14 | 0.10 |
Although the project was small in scale, there was a pattern of 0.04 to 0.10 % increase in zinc concentration of a 0.3 cc plasma sample by shipment in a microtainer tube with o-ring screw cap compared to similar volume plasma shipped in all-plastic.
For many years, research has suggested variable zinc contamination of serum samples from contact with red rubber stoppers. Most colors of rubber stoppers are manufactured with a zinc compound in the finishing step. The exception are royal blue rubber stoppers specifically made for use in trace mineral determination. These stoppers are not to be confused with powder blue stoppered citrate-containing tubes. For many conditions, it will make no difference which color stopper is on the tube when serum is collected.
When testing for zinc, either deficiency or toxicity, TVMDL recommends clients allow serum samples to clot in plastic or glass tubes without contact with rubber stoppers that are not royal blue. Once fully clotted, the client should centrifuge the sample and remove the serum to a second all-plastic or royal-blue-top trace mineral tube for shipping. This will ensure the zinc value detected accurately reflects the state of the animal.
For more information on analytical chemistry testing at TVMDL, visit tvmdl.tamu.edu or call 1.888.646.5623.