Allergy Blood Tests
	UniCAP RAST
	Limits of UniCAP RAST
	Other blood tests

A special blood test will pick up the commonest type of allergic reaction.  This is the kind of allergy that involves production of and antibody (immune globulin E or IgE) specific to the substance in question.

Allergy blood test - Specific IgE

The test is called specific IgE but sometimes referred to as a RAST test.  This method of testing is excellent for diagnosing the allergic cause of certain conditions (those associated with immediate allergy) and for picking up allergy to certain allergens (those known to induce specific IgE), although it is less exact for others.  In other the test is of the greatest help where allergy to items like pollens, dust mites and animals is suspected and in the case of true (immediate) food allergy.

However, even in these cases, careful comparison between the results of specific IgE blood tests and the results of challenge tests, either by inhalation (bronchial challenge) or by ingestion (food challenge) have shown that the test may in some cases over-diagnose (false positive) or in some cases under-diagnose (false negative) the presence of allergy.

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However, careful analysis of the data received from various scientific reports shows that agreement between allergy skin tests and allergy blood tests varies according to the allergen being tested and can range from greater than 80% agreement for some allergens to less than 20% agreement for other allergens.  For this reason it is important that any diagnosis based on this test should consider the result alongside the patients own experience (the allergy history), and in some cases the results of challenge tests with the suspected item.  It is a strong argument against making allergy blood tests available in the high street (for example, through a pharmacy) other than on the recommendation of a doctor.

Specific IgE Panels
Test panels are available for a large number of pollens, animals and fungal spores.  In the case of foods, over 150 different food allergens can be tested.  For simplicity, panels of common allergens have been devised, that may be especially helpful where allergy to a number of foods may be suspected:

	fx1	Brazil, Hazel, Almond, Coconut, Peanut
	fx2	Cod, Shrimp, Mussel, Tuna, Salmon
	fx3	Wheat, Oat, Maize, Sesame, Buckwheat
	fx5	Milk, Egg White, Wheat, Soya, Peanut, Cod
	fx7	Tomato, Yeast, Garlic, Onion, Celery

Foods such as milk and fruits contain a large number of different possible allergens and specific IgE tests may not be available to test all of these.  This could mean that someone sensitised to a less common allergen in a particular food may have a negative specific IgE test to a food to which they are sensitive.

It is also important to understand that the delayed kind of food sensitivity called food intolerance does not show up using the specific IgE test method.  Such  delayed-in-onset food reactions only show up by the use of an elimination diet followed by food challenge tests.  The failure of the standard  allergy tests to pick up this type of food reaction may be due either to the fact that the food reaction under test is not an allergy at all (for example it may be a toxic effect or an enzyme deficiency as in lactose intolerance) or it is a type of allergy with a mechanism that is not yet fully understood.

Food intolerance

IgG Blood Test

Food intolerance IgG test

This is a popular, widely advertised, and quite expensive blood test that is claimed to diagnose food intolerance. The customer collects his or her own blood sample and posts it to a laboratory where measurements of an antibody called immunoglobulin G (IgG) are taken. The level of IgG to each food is measured, for a wide range of foods. 

Scientific study
Scientists at the University of Manchester have studied the test by using it to draw up treatment diets for patients suffering from the irritable bowel syndrome (IBS). Everyone was tested at the outset, and those in the active treatment group had a diet based on their IgG result, while those in the control group had a sham diet that didn't exclude any foods to which they had high IgG levels. This trial has received a lot of publicity and has been quoted in relation to the marketing of IgG tests. So it is worth looking at it in some detail.

Now, we know that a good elimination diet (a diet which cuts out all regularly eaten foods at the beginning, and performs challenge tests with normal portions of food) will identify food intolerance in about 67% of IBS patients There have been several well-run trials that show this kind of result. We also know that when patients respond to an elimination diet they do so quickly and in an unmistakable way. Within 7-10 days of starting the diet (when they are eating a very limited range of foods) they are substantially better. Then they test foods by eating them, one by one, to see which ones were responsible for their symptoms. If IgG testing could give patients as good an answer as an elimination diet, what would you expect from a trial using a diet based on IgG levels? You would expect that about 67% of those in the treatment group would get substantially better within two weeks of starting the diet. The actual result is nothing like as good. People took a couple of months to improve, and most of them didn't improve substantially. The number who were very much better was only 11%. This compares very badly with 67% who were very much better from an elimination diet.

In the publicity stemming from this trial of IgG testing, a figure of 40% of patients improving has sometimes been given. However, this relied on assuming that many of the people who didn't stick closely to the diet would have done well if only they had followed the diet properly for 12 weeks. You just can't assume this. It is much more likely that the people who gave up on the diet did so because they hadn't experienced any improvement in the first week or two, and couldn't see the point in continuing with it for 12 weeks. Normal scientific procedure requires that you always assume that the ‘drop-outs' didn't benefit from the treatment. To assume anything else is a serious source of innaccuracy.

Comparing studies
What about the actual foods identified? To take just one example, the number of people in this study who had a high level of IgG to yeast was no less than 87%. If we look at two large-scale studies of IBS using diagnostic elimination diets, one carried out in Cambridge and the other in Oxford , the percentages of people who had a reaction to yeast, when they tested it by eating it after the avoidance period, were just 12% and 5.5%. The average of those two figures is close to 9% This is a huge difference from 87%. It cannot be explained by differences in the people studied because all these studies were carried out in England, and all included a large number of patients - so you would expect to get a roughly similar percentage of people responding. This huge discrepancy in the figures makes it very unlikely that high levels of IgG to yeast tell you anything about yeast as a cause of IBS symptoms. And it is a similar story for several other foods. Chocolate, for example, is quite a common offender in IBS as shown by elimination diet studies - affecting 22% in the Cambridge study and 27% in the Oxford study. Only 1% of patients gave a positive IgG test to cocoa beans.

Another discrepancy is that everyone who was tested for IgG antibodies had some positive reactions and so was told to avoid some foods. In elimination diet studies, a third of IBS patients turn out not to have any food intolerance at all. An IgG-based diet would be a waste of time for these people. All this confirms what most doctors suspect: that IgG antibodies don't generally play a part in causing the symptoms of food intolerance. There may, for various reasons, be some partial overlap between foods causing food intolerance and foods showing high IgG levels, but it occurs by chance and it isn't a close match. It certainly isn't close enough to make IgG testing useful in diagnosing food intolerance.

The patients in the treatment group in the IgG trial did do a little better than those in the control group. This is almost certainly because many of them were told to avoid wheat or milk (common offenders in food intolerance), while few of those in the control group cut out these foods. Wheat and milk are known to be major offenders in food intolerance.

Wheat and milk also provoke high levels of IgG in quite a few people, but this doesn't mean that the IgG levels cause the food intolerance. Generally speaking, people have higher IgG levels to foods they eat very regularly, and most of us eat wheat and milk several times a day. It would have been very interesting to have had a third control group in this study, who were given a diet sheet based on someone else's IgG results, not their own. It is a fair guess that some of them would have improved, just because they were avoiding milk or wheat.

Professor John Hunter of Addenbrooke's Hospital, an acknowledged expert on IBS and diet, remarked in a letter about this trial:

"The same diet sheets, distributed randomly to the patients, regardless of IgG levels, would probably have produced the same overall result."

Prof Hunter, one of the medical experts who wrote to the journal where this trial was published pointing out the many flaws in the design of the trial also commented:

"Before this trial was begun, it would have made sense to try to answer the more basic research question: do high levels of IgG against a food predict an adverse reaction to that food? Only one very small trial has so far done this."

The trial he refers to tested IgG in IBS patients and compared the results with those from food challenges (eating that food following a period of avoidance). If IgG testing works, you would have expected people to react to the same foods that came up positive in the IgG test. In fact there was no match between the foods identified.

This work needs to be repeated with a larger group of patients. It would give specific and unmistakable answers to the question of whether IgG testing is of any value. In the meantime, there is little to suggest that it is.

Other “food allergy tests”

There are numerous “food allergy tests” available in the High Street or by mail order, ranging from those based on reasonable science (e.g. ALCAT, FACT, CAST, cytokine measurements) to others that are frankly weird. Methods in the first category are often supported by reasonable evidence of utility and may be published in peer-reviewed learned journals whilst those in the latter generally claim success by quoting case-histories and glowing testimonials from patients. Evidence of the latter sort cannot be evaluated because it seldom includes letters from dissatisfied customers! We therefore have no idea of how often these methods fail.

Lactose tolerance tests

There are a number of tests available to doctors to determine whether or not you may be lactose intolerant. Lactose requires the enzyme lactase for its proper digestion and if absent from the gut (not uncommon) digestive problems arise when milk and milk products are consumed. The condition is called lactose intolerance and it has nothing to do with food allergy

Three kinds of test are commonly used to diagnose the condition, the lactose tolerance test, the hydrogen breath test and the acid stool test.

The Lactose Tolerance Test

Lactose is normally broken down by lactase in the small intestine, converting it into glucose. This is then absorbed, and blood glucose level will consequently rise. The lactose tolerance test involves drinking a measured test dose of lactose. Several blood samples are then taken over a two-hour period in order to measure the level of glucose in the blood.

If you are lactose intolerant there is not enough lactase to break down the lactose. The lactose is therefore not converted into glucose and your blood glucose level will consequently not rise.

The Lactose Hydrogen Breath Test

The lactose hydrogen breath test also involves drinking a measured test dose of lactose. The patient fasts (water only) for 12 hours and at the start of the test, a sample of breath is taken and the hydrogen level in the sample breath measured.

Samples of breath are then collected every 15 minutes for several hours. They are then analysed to measure the levels of hydrogen. If hydrogen is found in your breath samples, this indicates lactose intolerance.

The Lactose Stool Acid Test

If lactose is undigested it ferments in the lower bowel and creates lactic acid, which can then be measured. This test is particularly useful for children because the other tests require test doses of sugars that may cause troublesome diarrhoea in small children.