3 dynamic blood tests every vet nurse should be able to perform
What is a dynamic blood test - and what do they tell us?
A dynamic function test is one where we either stimulate or suppress a body process in order to evaluate that organ’s function.
There are loads of different dynamic function tests. Most of them are used to assess the endocrine system. Examples include:
The ACTH stimulation test
The low-dose dexamethasone suppression test
The high-dose dexamethasone suppression test
The T3 suppression test
We also commonly perform dynamic tests to evaluate liver function - via the bile acid stimulation test.
These dynamic tests are commonly performed in medical patients suspected to have endocrine or hepatic disease. This means that it’s vital for the veterinary nurse to know how these tests are performed, what impacts their results, and what they tell us.
In this post, I’m going to talk you through the three most common dynamic function tests we perform in practice - the ACTH stimulation test, the low-dose dex suppression test, and the bile acid stimulation test. We’ll chat through what they tell us, why they’re used, and how they’re performed, so that you’re a pro at performing these on your own patients!
If you’re not already running these as a nurse - what’s stopping you?! You can absolutely run these tests from start to finish, including calculating and administering the medications needed under veterinary direction.
Outpatients in for follow-up testing can even be booked in with nurses via a consultation, so we can see the patient and client, get an updated history, and start the test! Our clients can then either wait until the time of the second sample, leave the patient with us, or bring them back for repeat sampling at the appropriate time.
If you want to know more about the common biochemistry tests performed and what they mean for our patients, join me next week for the Biochemistry 101 workshop - exclusively for members of the Medicine Nurse Academy. Doors to the Academy open in 2 days (at the time this post goes live) - so make sure you’re inside!
The ACTH Stimulation Test
The adrenocorticotrophic hormone (ACTH) stimulation test is a commonly-used diagnostic test for both Cushing’s and Addison’s disease. It evaluates the hypothalamic-pituitary-adrenal-axis - the communication between the brain, pituitary gland and adrenal glands.
The HPAA
In a normal patient without disease, cortisol is regulated by the following process:
When cortisol levels are low, corticotrophin-releasing hormone (CRH) is released by the hypothalamus, and stimulates the pituitary gland
ACTH is released from the pituitary gland, and stimulates the adrenal glands
Cortisol is released from the cortex of each adrenal gland
Circulating cortisol levels increase
The hypothalamus detects this, shutting off the release of CRH and stopping the cycle. This is known as a ‘negative feedback’ mechanism.
The stimulation test
An ACTH stimulation test is used to:
Definitively diagnose hypoadrenocorticism (Addison’s disease)
Screen for hypercortisolism/hyperadrenocorticism (Cushing’s disease)
It is performed by taking a baseline serum sample for cortisol, injecting 5mcg/kg of synthetic ACTH (either IM or IV), and then collecting a follow-up sample 60-90 minutes later.
If you’ve given your ACTH IV, take your sample at 60 minutes post-injection. If you’ve had to give it IM, collect your sample at 90 minutes post-injection.
What do the results tell us?
An Addisonian patient will have low results, demonstrating a failure of the adrenal glands to respond to the ACTH.
High results are indicative of Cushing’s disease - however, this test cannot tell us the type of Cushing’s disease our patient has.
We can also use ACTH stimulation tests to monitor a Cushingoid dog’s response to treatment, and to detect iatrogenic hyperadrenocorticism.
The Low-Dose Dexamethasone Suppression Test
The LDDST is a more sensitive method of assessing adrenal function in patients with suspected Cushing’s disease, and provides some indication of whether pituitary or adrenal-dependant disease is present.
To perform a LDDST:
Collect a baseline serum cortisol sample
Administer 0.01-0.015mg/kg dexamethasone IV
Collect a second serum cortisol sample 4 hours after injection
Collect a third serum cortisol sample 8 hours after injection
What do the results mean?
If the patient’s 8-hour sample is <40 nmol/L, they are negative for hyperadrenocorticism
If the patient’s baseline sample is normal, their 4-hour sample is <40 nmol/L or < 50% of the baseline, and their 8-hour sample is >40 nmol/L, it indicates pituitary-dependant disease
If the baseline sample is normal, their 4-hour sample is >50% of the baseline, or >40 nmol/L, and their 8-hour sample is >40 nmol/L but <50% of the baseline, this indicates either pituitary or adrenal-dependant disease.
Top tips for adrenal function sampling
We want to keep these patients calm as far as possible, to avoid false positive results associated with stress.
Ideally, patients should be fasted prior to their test, to minimise the effect of lipaemia.
If a patient has been recently treated with steroids, or progestagen medications, their test may be inaccurate. Ideally, the test should be delayed until at least 2-4 weeks after these medications have been discontinued.
Don’t forget that cushingoid patients also can have very fragile skin - so we need to be careful with clipping, applying adhesive bandages, etc at venepuncture sites.
The Bile Acid Stimulation Test
Unlike our last 2 tests, the bile acid stimulation test has nothing to do with adrenal function - it’s actually all about the liver!
We commonly perform this test in patients with suspected hepatic failure/dysfunction. For example patients with end-stage liver disease, elevated bile acids on their routine biochemistry, and patients suspected to have a portosystemic shunt.
What does the test tell us?
Bile acids are a component of our bile. They function to emulsify fats, allowing nutrients to be absorbed from food.
Bile acids are formed in the liver, and are conjugated into bile, which is stored in the gall bladder. When a patient is fed, bile is released into the small intestine via the common bile duct, where it breaks down fats in the patient’s food.
The bile acids then enter the enterohepatic circulation (the hepatic portal vessels) and are ‘mopped up’ by the liver, where they are returned to the gall bladder as part of bile.
Where the liver cannot function properly, bile acids accumulate in the bloodstream.
How is the test performed?
To perform a BAST:
Fast your patient and collect a pre-prandial serum bile acid sample
Feed your patient
Collect a further post-prandial serum bile acid sample 2 hours after the patient eats
What affects our results?
A BAST can be affected by lipaemia and haemolysis - lipaemia increases your result, and haemolysis decreases it, depending on the testing method and reagents used.
Icterus does not directly affect the test itself, but the test should not be performed in patients with elevated bilirubin levels due to cholestasis (biliary tract disease), as it will not give additional information about liver dysfunction.
So that’s an overview of the three main dynamic function tests we perform in our medical patients! As you can see, they provide us with a lot of information, and there are many important considerations for us nurses when running these tests.
Do you perform these tests in practice? I’d love to hear about it. DM me on Instagram and let me know!
And don’t forget - doors to the Medicine Nurse Academy open on Tuesday, so if you want to come to the Biochemistry 101 workshop next week, I’ll see you inside!
References
Cornell University, 2020. Bile acids [Online] EClinPath. Available from: https://eclinpath.com/chemistry/liver/liver-function-tests/bile-acids/
Idexx Laboratories, undated. Canine ACTH Stimulation Testing Protocol [Online] Idexx. Available from: https://www.idexx.com/media/filer_public/07/42/074282a5-2734-444d-a3bd-68556cb8941e/acth-stimulation-testing-protocol-canine.pdf
Idexx Laboratories, undated. Dexamethasone Suppression Test - Low Dose [Online] Idexx. Available from: https://www.idexx.com/media/filer_public/07/42/074282a5-2734-444d-a3bd-68556cb8941e/acth-stimulation-testing-protocol-canine.pdf
Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians. 7th ed. Missouri: Elsevier.
