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Can someone please explain blood tests for allergies to me?

(15 Posts)
OHforDUCKScake Mon 08-Oct-12 13:39:02

I understand how skin prick tests work. But how do blood tests work? Will it roughly tell us the severity of the allergy?

Will the results be in numbers?

OHforDUCKScake Mon 08-Oct-12 14:02:56

Also, he'd had oral steroids a few days before. Would that effect the outcome? It was about a week or so before...

freefrommum Mon 08-Oct-12 15:44:05

This is my understanding of allergy blood tests (or RAST tests). The test measures levels of allergen-specific IgE or antibodies in the blood. Like skin prick tests, they are NOT able to predict how severe a person's reaction will be as some may have high levels of IgE yet only react mildly on contact with a specific allergen whereas others may have low IgE but react severely if they actually eat the food. They are therefore just another 'tool' in the doctor's kit to try and gauge the severity of a person's allergies. They are most useful when trying to work out if a person's allergy is improving over time and whether or not it might be worth trying a food challenge. My DS has blood tests annually and his consultant said that if the results improve (numbers come down) then he will do skin prick tests and possibly food challenge.

The results will normally be given in one of two ways: micrograms per millilitre (μg/mL) or a level between 0 and 5 or 6 (depending on the scale used) where 0 indicates a very low likelihood of specific allergy and 5 or 6 indicates a very high probability of allergy. My DS's results were all within the 5 or 6 category last time he had them but we're waiting for the results of his most recent tests to see if these have come down at all. Here's a really good article about RAST tests: foodallergies.about.com/od/diagnosingfoodallergies/f/rastresult.htm

I'm not 100% certain about the effect of steroids on the results but I do know that, unlike SPTs, they are definitely not affected by antihistamines so I would assume that they are not effected by steroids either.

OHforDUCKScake Mon 08-Oct-12 18:49:37

Thats exactly the answer I needed.

Much appreciated, thank you.

tweety89 Tue 09-Oct-12 12:35:59

OHforDUCKScake have you been told that DS needs the blood test? has he had the prick test yet? I remember that you cannot have antihistamines before any allergy test, not sure about steroids though.
freefrommum when I saw the second consultant even though DD has had 2 anaphylactic shocks he just asked the nurse to do the prick test which showed that she has a severe allergy to all nuts and eggs. The first consultant went straight for the blood test that didn't show any allergy whatsoever (back then though we thought she was allergic to fish) and wasn't keen on the prick test. Should I ask the second consultant on her next app to do the blood test as well? We changed consultants because we moved and we brought her to another hospital on her second anaphylactic shock.

OHforDUCKScake Tue 09-Oct-12 12:54:54

tweety they are doing bloods as opposed to SPT because he is very atopic. Whilst the amount they put on is tiny, the consultants arent willing to risk it.

The results would are just to see if it goes down in time, we know what he is allergic to so if it came up negative we'd not go with that. Unfortunately I hear the blood tests are as unreliable as the SPT's.

Most importantly though they are testing his calcium levels, vitamin d and iron. This is what I want to know the most.

freefrommum Tue 09-Oct-12 13:41:12

Hi tweety just wanted to clarify that you CAN have antihistamines before blood tests but not skin prick tests. Also, the blood tests are for specific allergens only so if they tested for fish then it would only show a reaction to fish, nothing else. I think it would be worth asking for blood tests in addition to the skin prick tests if only as another way of trying to judge whether or not the allergies are improving.
Duck my DS reacts quite badly to SPTs which is why consultant decided this time not to put him through it unless the blood tests show a significant improvement.
There is another blood test result that gives a general IgE level score which is not specific to any particular allergen but just gives a general indication of how 'allergic' a person is. This tends to be highest in those with allergies AND eczema like my DS. I think normal range is up to around 140 - DS's was 3000!!! Think this is probably why consultant didn't hold out much hope of scores coming down much yet sad

OHforDUCKScake Tue 09-Oct-12 14:05:33

Ds had a blood test at 5 months. They gave me a number of 80 and said it was quite high. I never knew what to make of that and they never explained. At the time he was covered head to toe in terrible urticaria.

What happens to your son when he had a SPT? How old is he? I literally (at least once a fortnight) dream of hearing his allergy levels are going down. Can you imagine that? Hearing their getting better?

Unfortunately this is a long way off for us, DS's situation is rather extreme.

freefrommum Tue 09-Oct-12 14:18:05

DS is 5 and yes I do dream that one day his allergies will magically disappear but this is a long way off for us too sadly. When he has SPTs his whole body gets red and itchy and he has to have Piriton then sit in the waiting room for a few hours before they let us go home.

Funnily enough I had a call from school yesterday to say they'd had to give him Piriton because he'd come out in hives on his leg in PE. It would appear that they hadn't cleaned the floor properly after lunch so when he sat down in his shorts his leg must've come into contact with some food residue (I'm guessing something with milk in as this tends to be his worst). The school have been very good with managing his allergies though. I was worried they'd either not take them seriously enough (which would have been awful) or they'd be so nervous they'd be asking me to take him home every time he came out in hives but they've been very sensible and seem to handling it all well (she says, touching wood and crossing her fingers...)

OHforDUCKScake Tue 09-Oct-12 16:11:11

Good to hear the school is good. Did he go to nursery before school?

tweety89 Tue 09-Oct-12 17:05:47

thank you both for your answers. DD has eczema too so I'd be curious to see the findings.

tweety89 Tue 09-Oct-12 17:05:51

thank you both for your answers. DD has eczema too so I'd be curious to see the findings.

freefrommum Wed 10-Oct-12 09:18:11

Yes he did go to nursery at age 3 for 5 afternoons a week. I chose a very small local nursery that only had about 10 children and went in to speak to them about his allergies beforehand. The allergy nurse also went in to give them some information and training. It all worked out very well and the staff were fantastic. Yes I was very worried especially initially but they always made me feel like they were taking his allergies seriously and they also bent over backwards to make sure he was included in all activities, including baking and even having 'school dinners' on Fridays - I took in freefrom pasta, sausages etc and they cooked his separately but he always had something similar to the other kids so he never felt left out.

OHforDUCKScake Wed 10-Oct-12 10:24:41

Thats great. I think I will hunt far and wide for one similar for my son.

I fear him not going to nursery at all and having to 'cope' with school being the first thing he's away from me with. Id hate that.

mumat39 Thu 18-Oct-12 00:08:55

Freefrom mum, I've never really understood the blood results before. You're explanation is brilliant and really helpful so thanks for that.

My DD is also 5 but has spt's annually but her consultants only does blood tests every 2 years so far. Last year he mentioned that each allergen tested costs £12.00 so I guess that's why they only do them every 2 years. I think we're on a long road with her allergies too. sad

Not sweeping up after lunch is a pain eh?! Dd has reacted to something in her eye from a dirty hall floor before. I hope you're DS's reaction didn't last too long and that he's better now. Xxx

OhforDucksSake, when my dd first started nursery I tried to deal with the handover of info myself and the staff were fine but I never felt that they really understood.

Dd has recently started full time school, and this time I asked her allergy nurse if she could come and meet them with me. I think her being there has really helped and the school have taken things on board and seem to be genuinely 'on the same page' if that makes sense. When you get to that stage with either nursery or school, definitely try and get your allergy nurse to go and meet the staff with you. I think having a medical person there added a certain amount of gravity to the meeting which has helped. The school also asked the nurse to go in as they needed more training on managing allergies and administering Epipens. The nurse also trained them on Jext pens which Dd has been given to replace the home set we have. DD's allergy nurse is excellent and really lovely so we're very lucky to have her.

Also, we are always told that they can't see dd in the clinic if she has had to have any antihistamine in the 7 days before the appointment. The nurse says they say that but would do tests if had been at least 5 days before.

I hope your little one does start to outgrow their allergies soon. I also dream and imagine that day.

Take care
Xxx

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