Conditions for bloodwork


I've been reading up on timing and conditions for drawing blood for ACTH and insulin. And am unsure what to do. Lots of factors involved.

Time of day and feeding/fasting:
Rusty's first ACTH test was done before I'd read up on this topic: early morning, after he had his little supplement mash, his 1 lb TCTBC soaked, and his first hay after the night, during which he finishes up what's in the hay bag. I'm guessing there's a long gap between munching hay and getting his morning supplements and hay. So blood taken within an hour or so of eating after his gap in eating. From what I'm reading up on now, that's not the best for accurate results.

This coming Thursday, our vet will be out first thing again, to do bloodwork. If I change nothing of our routine, his eating will be the same: supplements then soaked TCTBC then hay bag before vet arrives. Should I instead put out extra hay bags overnight so he'll have no fasting time? Should I let his hay run out overnight but not feed him before the blood draw? Should I do something else?

Air Temperature:
Meanwhile, predicted temperature on Thursday morning will be closer to 40F than ~55F (November blood draw).

It is not an option for the vet to come later that day, in fact I'm always scheduled first thing, something to do with distance and sequence of appointments, etc. Should I advocate for a later appointment another day if possible? And if so, am I looking for a time when the air temps will be close to first blood draw, and/or looking for a time when I do the feed/not feed recommendations without being up in the middle of the night? 

As ever, trying to figure things out and do it right, and needing help to understand what's good enough versus what's necessary. Thanks in advance.
Lasell (luh-SELL) ~ Virginia USA ~ September 2022
Fairlane Rusty
Sam - likely IR, Case History to come

Eleanor Kellon, VMD

Just put out extra hay and don't feed anything else until after the blood draw. Don't worry about the temperature.
Eleanor in PA  BOGO 2 for 1 Course Sale Through End of January
EC Owner 2001
The first step to wisdom is "I don't know."

Sherry Morse

Hi Lasell,

You want to do bloodwork at least 4 hours after breaking a fast (fast in this case being 6 hours without food in front of them) for insulin and glucose.  ACTH is not affected by feeding to the extent that insulin and glucose are.

If the vet is coming 'first thing' and you can't feed at least 4 hours before or be sure that the overnight feed will not run out more than 4-6 hours before it's probably best to do the bloodwork 'fasting' (no feed or hay until after the vet has taken blood) and know that you need to account for that when looking at the insulin results.  

It's almost impossible to control for temperature at time of the blood draw, especially this time of year so you'll just have to keep that in mind when looking at results.

Trisha DePietro

Hi Lasell.  As far as the timing of the blood draw to the feeding....I give extra hay through the night and then get up at 5am and keep hay in front of the horses until the blood draw....usually we draw between 9am and 10am. I looked at your case history and I would draw the insulin, glucose and the ACTH all at the same time. I didn't see a glucose drawn on your last one, the Glucose:insulin ratio is a number to look at ...

Glucose:Insulin Ratio (G:I Ratio)

The upper limit of normal for glucose in a horse that has not been fed anything but hay prior to testing is around 100 mg/dL (5.6 mmol/L) by definition. It is important to also know that the horse could have an insulin result inside the lab's normal range and still have EMS. This is determined by the use of calculations called proxies and supported by abnormal leptin or adiponectin levels.

Based on human studies and data from our group members' horses, we have been using the G:I ratio proxy for many years now. It's very easy to do. Simply divide the glucose result in mg/dL by insulin in uIU/mL. (If your insulin is reported in pmol/L you have to convert it to uIU/mL by dividing by 6.0.) A normal value is 10:1 or higher. The lower the ratio, the higher the laminitis risk.

Air temperature is important in the sense that the results can be elevated from the cold. But if you live in a cold climate it is what it is, you can't really change your temperature, but you can make note of the temperature at the time of the draw and include that temp in your case history. This way, if there is anything unusual about your results, we have the temp for that day. for instance, insulin does go up from the cold weather, but its all relative. It is all factored into the big picture of Diagnosis, Diet, Trim and Exercise....Hope this helps.

If the blood is drawn at the beginning of the vets day...when will they get it back to the office to be spun? You might have already read this part but its a good review because not only do you want your horse non fasted and as calm as possible, you also need the blood sample to be handled appropriately....Here is the link to that detail if you haven't seen it yet.  --
Trisha DePietro
Aug 2018
Primary Responder
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Thanks to each of you. Very helpful.

I can provide enough hay so there won't be a fasting interval before the blood draw. 

I am checking with my vet about the "when spun" factor, and have requested the glucose test be done in addition to insulin and ACTH. 

Lasell (luh-SELL) ~ Virginia USA ~ September 2022
Fairlane Rusty
Sam - likely IR, Case History to come