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Why is overnight fasting necessary for a blood test?

collectingrocks

Well-Known Member
Messages
241
As in the title really. I have been called up for a fasting blood test which is only done in the morning.

If I didn't eat from (say) 5pm the night before, wouldn't the body dump glucose into the system overnight meaning it would read high? That's before I'm given the glucose drink....

Wouldn't it be better to fast during the day and have the fasting test done late afternoon so I've had a chance to burn up the glucose by being active all day?

Am I missing something here?
 
Is this for a full diabetic profile, or a single test?

HbA1c tests don't require fasting, but fasting blood glucose and lipid, including the important breakdown with triglycerides do require a fasting test.

I would guess its simpler, and more comfortable to fast for 10-12 hours when mainly asleep, than trying to tolerate it in the working day. I'm also guessing that fasting through the day could be particularly tricky for those who do manual work.

I imagine it's also much easier for T1s overnight, although that's a semi-educated guess which could be very wrong.
 
This is a fasting HbA1c and lipid (specifically a fasting cholesterol test).

I don't do manual work and can easily fast during the day.

Doesn't the body dump glucose into the bloodstream overnight meaning that glucose will be high first thing in the morning?
 
Your liver dumps when your levels drop, so this would happen during the day too, probably. In fact, it might happen more as you would be burning more energy, even driving a desk, than you would asleep.
 
I may have completely misunderstood your point but my understanding is that your liver will only dump glucose if your blood sugar falls too low, regardless of whether this is during the day or night. You might be referring to the dawn phenomenon whereby your body dumps glucose ready for activity when you wake. This doesn't happen in everyone (I think?) but your background insulin should (!) take this into account although I know it doesn't always work that way.

As far as HBA1C tests, these don't look at your bg on the spot but look at your general control over the previous 6 weeks.

Does this help?
 
Thanks guys. The reason I am asking is that the surgeries I know (and hospitals!) only do fasting blood tests in the morning. They don't do them in the evening which is more convenient for me
 
collectingrocks said:
Thanks guys. The reason I am asking is that the surgeries I know (and hospitals!) only do fasting blood tests in the morning. They don't do them in the evening which is more convenient for me
Click to expand...
If you feel strongly enough, could you ask for a exception? But you'd probably spend more time fighting for the convenience than dealing with the inconvenience of attending for the standard fasting window.

In the bigger picture, I imagine they might have more attendees who had sort of fasted if they did intraday fasting tests.
 
Yes I've asked for an exception but always come up against a brick wall. I want something that's convenient for "me" without having to take time off etc.

I've had one of those letters for a "free NHS health check" as I'm over a certain age and in the second paragraph it states "I must have a fasting cholesterol test"

Although slightly OT, I am interested to see whether they feel I need to be put on Statins so that practice targets can be met and practice managers get their commission.

But I will also use this opportunity to ask for another HbA1c too
 
To be honest I think the NHS are a bit obsessive about fasting tests. I was once told by a phlebotomist not to go beyond 12 hours due to the dawn phenomenon yet some are told 10 - 14 hours. I'm sure there are occasions when fasting is the right thing but fasting tests just for BG are not of much value. I note that in common with many other surgeries recently my annual tests are no longer fasting. It means the cholesterol measure is a bit less precise but it takes the pressure off the pathology clinics in the morning
 
The fasting requirement is for your cholesterol test, not your HbA1c. A true cholesterol test has to be fasting 10 plus hours. An HbA1c doesn't require fasting.
 
The problem I have is that my local hospital will only take samples from warfarin patients before 10:00 and therefore my fasti tends to be around 15 hours by the time my blood sample is taken.
 
I can accept that a fasting test is necessary for a cholesterol test but I have never understood the fasting blood glucose test. Maybe we patients have misunderstood what is going on.

I have taken reading when I get up. I have taken readings after I have been out for my morning walk. I have taken readings after I have done the walk and done the shopping in Tesco.

When I get the blood test done at the surgery it is at a time which is at their convenience. I have no idea what the result means.
 
They only do fasting blood glucose tests for diagnostic purposes at my surgery. HbA1c tests are not fasting, and they don't bother with the glucose tests, presumably because they mean nothing once diagnosed.
 
The more I learn about my body's fasting BG, the more I realise what nonsense the surgery's tests are.

During the day, if I go more than 4 hrs without food (or liver dump), my BG is around 5.3
On waking, after 8-10 hrs fasting, it'll be around 6.3
From then on, due to liver dump/dawn phenomenon, it will rise steadily all morning eventually reaching 8 to 9
Then it will drop sharply, giving me hypo, or false hypo, symptoms at 4.5 ish.

So when would they like to do their 'fasting glucose' test?

They might as well stick the tail on the donkey!
 
@Brunneria
And that is how much of this diabetes problem is. No one size fits all and while we can all, and we do, benefit from the knowledge shared on here it is a bit of an unknown quantity as is pinning the tail on a donkey.




Late onset T1, several auto immune issues.
Humalogmix25 twice a day, Methotrexate 25mg once per week, FolicAcid 5mg once per week, prednisolone 5mg daily, Allopurinol 300mg, Calcichew-D3 800iu, Levothyroxine 50mcg, Atenolol 50mg, Losarten 100mg, Aspirin 75mg, Nicorandil 20mg, Nitrolingual GTN spray, Metformin 2000mg, Allimemazine 10mg, Lanzoprazole 30mg, Atorvastatin 20mg, Co Codamol 8/500mg, Depo Medrone (Methylprednisolone) or double Prednisolone for 7 days in case of RA flare.
 

As far as I know the fasting BG test is only for initial diagnosis.

This is not a competition to try and burn glucose off before the test by exercising, it is to try and establish if you have normal BG control under standard conditions, or if you have some kind of problem.
If you have an overnight liver dump and your BG does not come down to non-diabetic normal by the time of the test this is a pretty good pointer to diabetes, and very valuable information.
If it does come down, however, it does not completely rule out diabetes.
This may be where the glucose drink you mention comes in.
The aim is not to "beat the test" because diabetes won't go away just because you manage to frig the results.
The aim is to be as standard as possible so you and your medical team get an accurate picture of your current health.

I know that it can be a nuisance, but especially where there are big clinics handling warfarin testing and all other blood tests then they tend to organise them to fit in with the warfarin tests.

Hope this helps.

LGC
 
I have no desire to "beat the test" but my surgery appears to be more interested in measuring my fasting cholesterol even though the letter states about a health check. This will be my first ever blood test at my own surgery...my last ones were at the hospital and my last cholesterol test there 18 months ago was well within normal so I am not expecting any surprises - unless they are keen to push statins on me.

But I have also requested a HbA1c for my peace of mind which I'm going to have done elsewhere. I have a blood form for this which says:

4.5ml EDTA EB HbA1c separate sample required

then....

Glucose (fasting)

So does this mean they'll do an HbA1c and an OGTT?
 
One reason they do the random BG test is because some of the other tests they do can be invalidated by a high BG. I think it is also a random test to detect people who have terrible control. Like a police breathalyser test at the side of the road. It's pretty unscientific. It may just be a holdover from the old days when they didn't have HBa1c. If you think about it, I suppose it would take a lot of guts for them to decide to see a diabetic for a checkup and NOT do a random BG.

Sent from the Diabetes Forum App
 
collectingrocks said:
I have a blood form for this which says:

4.5ml EDTA EB HbA1c separate sample required

then....

Glucose (fasting)

So does this mean they'll do an HbA1c and an OGTT?
Click to expand...

No, It sounds like a standard HbA1c ans a Fasting BG test. A OGTT would involve a 2 hr sit around which they would warn you about (I hope)!
 
What's a "false hypo"? :>
 
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