NUTRITIONAL LABORATORY
 TESTING


 NOTE and DISCLAIMER:

These articles are for general interest of qualified family physicians only. They are NOT for diagnostic or therapeutic use.

The articles are definitely NOT for any public use whatever, nor intended in any way to be taken as advice for any medical or health condition.



There are no routine tests for nutritional status.
Chose according to the clinical situation.

The laboratory can be a powerful tool in confirming your nutritional diagnosis, but one that must be used with care. Laboratory testing tends to be less precise in nutrition than in many other fields of medicine, and there are no `routine screening tests' for nutritional status. The cost and lack of sensitivity of many nutrient assays means that the laboratory should only be used selectively to confirm a suspected deficiency, rather than taking a shotgun testing approach.

Many nutrients are hard to test accurately, because blood samples are not representative of the sites (such as enzyme systems, bone marrow etc.) where the nutrient is stored or acts. This can mean that a blood test remains normal even in deficiency states (example: plasma zinc), or that a mildly abnormal test result has little clinical meaning (example: vitamin B6 ).

To help solve this problem, tests have been devised to measure nutrients at the site where they act. For example, red cell folate assay. Other tests assess the functional effects of a nutrient deficiency, rather than measuring nutrient concentration directly. A common example is the measurement of haemoglobin to assess iron status (unfortunately not a very sensitive assessment). Another group of tests which uses this approach is red cell enzyme reactivation assays. They measure body status of certain nutrients (e.g. some of the B group vitamins) by testing how well a particular enzyme, which is dependant on the nutrient, is functioning in red cells.

Whatever test is used, check what foods and supplements the patient has been taking recently. Serum iron, for instance, is strongly influenced by the iron content of recent meals. Vitamin C supplements can lower serum B12 levels. However, in most cases fasting samples are not required for nutrient testing.

Blood tests

Screening tests

Although there are not true `screening tests' for nutritional status, nutritional problems can be sometimes be suggested by abnormalities in commonly performed routine assays. A full blood count and routine biochemistry may give early indication protein-energy deficiency through low serum albumin or a depressed lymphocyte count. Other proteins such as serum transferrin may help to confirm this. Red cell indices can also reflect alcohol abuse, iron, vitamin B12 or folate deficiency. Low serum potassium might be the only indicator of diuretic excess, and therefore should prompt a follow-up serum magnesium assay since the two deficiencies often coexist.

Specific nutritional blood tests

Amongst the most frequently ordered nutritional tests in Australia are serum ferritin and serum vitamin B12 . Both are very sensitive to deficiency states, but serum ferritin can be falsely normal in patients with iron deficiency who have coexisting infection or inflammatory disorders. Red cell B12 is available but is rarely used. (see also article on vitamin B12 . Red cell folate, on the other hand, is a useful addition to serum folate testing where there is a possibility of coexistant B12 deficiency, for example in the elderly psychogeriatric patient on anticonvulsants.

Many laboratories now offer red cell reactivation tests are available for certain of the B group vitamins: thiamin (TPP reactivation test), riboflavin (GRAC reactivation) pyridoxine (SGOT reactivation). Of these, SGOT reactivation has proved of limited clinical use as it is hard to interpret clinically.

Difficult nutrients

Some nutrients are particularly hard to test for. Zinc is one of the most important examples. Although zinc deficiency is relatively common, serum zinc assay is quite insensitive to the condition, so that the key to diagnosis of zinc deficiency remains a good history (see also article on zinc . Hair analysis has been used to test for zinc and for toxic levels of heavy metal elements such as cadmium and mercury. However at present this method is susceptible to external interference and is not yet reliable enough for routine clinical use.

Practice tips

  • Where possible, assess function as well as nutrient concentration.
  • Find out what supplements the patient is taking.
  • Use lab tests to confirm a specific diagnosis, not as a fishing expedition


Return to clinical articles page