Zinc Blood Test: When It Makes Sense, How to Interpret the Result and Why It Is Imprecise

The test measures zinc concentration in serum or plasma — the two values are close, though not identical. According to the BOND review only about 1% of the total zinc in the body is in circulating blood; the rest is in muscle, bone and inside cells. In addition, albumin is the main carrier of zinc in plasma, so its level affects the result. The body defends the constancy of blood concentration at the expense of tissues, so plasma concentration is only an indirect indicator of what happens in cells.

On the other hand, the test is not worthless. The systematic review by Lowe et al. (2009), covering 32 potential biomarkers from 46 publications, showed that plasma zinc concentration responds to dietary changes in a dose-dependent way (in adults, women, men, pregnant and lactating women and the elderly), and that in healthy individuals plasma, urinary excretion and hair are reliable biomarkers of zinc status. The BOND experts regard plasma zinc concentration as one of three recommended indicators (alongside dietary zinc intake and growth in children), but stress its many limitations.

In practice this means several concrete phenomena that everyone should know when looking at a result. First, blood zinc falls after a meal and over the course of the day — which is why thresholds differ for morning fasting samples, morning after-meal samples and afternoon samples. Second, in inflammation and infection zinc is moved from the blood to the liver as part of the acute-phase response (metallothionein induction), which lowers its blood concentration despite adequate stores. Third, low albumin (malnutrition, liver disease, acute illness) lowers the result, because albumin is the main zinc carrier. Fourth, pregnancy, oral contraceptive use, severe stress and body position during sampling also affect the result.

These values (from Table 7 of the BOND review, adapted from IZiNCG documents) are thresholds of risk of deficiency in populations, not diagnostic thresholds for a patient. A result of 62 µg/dL drawn in the afternoon in a man may lie above the afternoon threshold, while the same result drawn in the morning fasting would be below the morning threshold. It is therefore reasonable for the laboratory or doctor to assess the result in light of the time of sampling and fasting status. The reference ranges given by the laboratory on the result can be different, depend on the method and reference population, and may be wider than the risk-of-deficiency thresholds.

The practical conclusion is subtle. Inflammation can lower the zinc result, especially in children, so with a low result in a person with an infection, injury, after surgery or with a chronic disease, interpretive caution is warranted, and measuring CRP alongside zinc helps judge whether the low result is simply a consequence of an inflammatory reaction. At the same time the data from adult women show that the association is not as clear-cut as is sometimes thought. The BOND review recommends measuring CRP and/or α1-acid glycoprotein together with zinc, but notes that the statistical correction method needs further validation and is not routine in the consulting room.

Zinc is everywhere: in rubber tube stoppers, in glove talc, in dust, on skin. Therefore, according to the guidance described in the BOND review, zinc measurement requires special care: blood should be drawn with a stainless-steel needle into trace-element-free tubes, haemolysis should be avoided, and the tourniquet should not stay on longer than a minute. Blood cells contain roughly ten times more zinc than plasma, so any haemolysis — in the body or in the tube — falsely raises the result. Leaving whole blood at room temperature before centrifugation also changes the result.

From the standpoint of the principles described in expert documents, the zinc test makes most sense when there is a clinically justified suspicion of deficiency — not as a screening test in a healthy person. BOND describes zinc as a nutrient whose deficiency gives non-specific signs (impaired growth, more frequent infections, skin changes), so there is no single characteristic clinical picture.

We have no evidence that routine zinc testing in healthy people without symptoms improves health outcomes, and falsely low results (after a meal, in inflammation, with haemolysis or contamination) can lead to unnecessary supplementation. In turn, in people with chronic illness and low albumin the result may be underestimated, so the assessment should take the whole picture into account.

The BOND review notes that plasma zinc concentration responds quickly — within 5–10 days — to supplementation in all population groups, regardless of baseline values. This means that a test done in someone who has taken a zinc preparation in recent days largely reflects the dose from the supplement and not the 'state of stores'. For this reason a normal or high result during supplementation does not prove that there was no deficiency, and a low result after a break of several weeks corresponds better than a result 'during' to baseline status.

The same principle applies to monitoring supplementation: if zinc was recommended to treat a deficiency, a check usually makes sense after the course ends, taking the time off into account, and with longer use of large doses, checking copper is often as important as the zinc level. The upper limit of zinc intake from supplements and food according to the US Food and Nutrition Board document cited in BOND is 40 mg a day for adults (based on a fall in the activity of a copper-dependent enzyme at doses above 50 mg/day); to our knowledge EFSA adopts a lower value, 25 mg a day. The limits do not apply to people treated with zinc under medical supervision.

Also pay attention to a low zinc result in a person with an infection, injury or chronic disease: it can reflect the general state rather than an isolated deficiency. In such a situation the doctor may propose a check after recovery and a CRP or albumin measurement, instead of immediately starting high doses of zinc.

To sum up: blood zinc is a useful test, but one that requires conditions and context. It is best drawn in the morning fasting, avoiding contamination and a long tourniquet, taking CRP and albumin into account, and not basing a supplementation decision on one borderline result. It is most valuable when it serves to check a real suspicion of deficiency in a risk group, and not as a routine element of a 'trace-element panel' in healthy people.