Pre-Anaesthetic Bloodwork: What It Finds, and When It Matters
In healthy young dogs, routine blood tests before surgery rarely changed the plan. In older dogs, they often found hidden disease. What the two studies show.
Written by thecanineledger
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In a German study of 1,537 dogs having surgery, blood tests were run on every dog. For the dogs whose history and examination raised no concerns, the tests would have postponed surgery in “10 dogs (0.8%)” (Alef et al., Veterinary Anaesthesia and Analgesia 2008, abstract). In a 2007 study of 101 dogs older than 7, screening made 30 new diagnoses (Joubert, Journal of the South African Veterinary Association 2007, abstract).
While those results look contradictory, they’re about different dogs. This page explains what each study found, why age and health matter, and what to ask before you say yes or no.
1. Why vets offer blood tests before an anaesthetic
To grade the risk. Before an anaesthetic, vets assign an ASA status, a grade of the dog’s health from 1 (“Normal healthy patient”) to 5. The 2020 anaesthesia guidelines from the American Animal Hospital Association (AAHA) note that a move from status 1 or 2 to 3, or from 3 to 4 or 5, “increased the odds of anesthesia-related death in dogs and cats” (AAHA, 2020 Anesthesia and Monitoring Guidelines, Phase 1).
To find problems to fix first. The guidelines say that “Disease-related risks should be corrected or minimized if possible.”
Disclosure. The AAHA guidelines “are supported by generous educational grants” from three companies, including a veterinary laboratory company.
Our note: We read the AAHA guidelines’ preanaesthesia web page, which doesn’t say which blood tests to run. We haven’t read the full guideline document.
2. In healthy dogs: little change
The German study. For 1,293 of the 1,537 dogs (84.1%), history and examination showed that the blood tests “would have been considered unnecessary under normal conditions.”
Abnormal results were common, but rarely mattered. 16.7% of those dogs had an abnormal urea level. “However, only 104 dogs (8%) would have been re-categorized at a higher physical status category had the laboratory results been available.” Extra treatment before surgery would have been given to 19 dogs (1.5%), and the anaesthetic changed for two (0.2%).
The authors’ conclusion. “In dogs, pre-anaesthetic laboratory examination is unlikely to yield additional important information if no potential problems are identified in the history and on physical examination.”
Our note: The German study was at a university hospital in 2003–2004, with a standardised history and examination. General practice today may differ.
3. In older dogs: hidden disease
The older-dog study. In 101 dogs older than 7, screening found 30 new diagnoses. The commonest were “neoplasia, chronic kidney disease and Cushing’s disease”. Neoplasia means tumours.
Some anaesthetics were called off. “Of the 30 patients with a new diagnosis, 13 did not undergo anaesthesia as result of the new diagnosis.”
The author’s view is nuanced. Sub-clinical disease “could be present in nearly 30 % of these patients”. But “The value of screening before anaesthesia is perhaps more questionable in terms of anaesthetic practice but it is an appropriate time to perform such an evaluation.”
That screening used more than blood. It also included urine tests.
4. What neither study shows
Neither reports deaths or complications. Neither abstract gives anaesthetic outcomes. The older-dog study’s author says the value of screening “still needs to be evaluated in terms of appropriate outcome variables.” That means nobody has shown, in these studies, that blood tests before an anaesthetic make dogs safer.
Both are old, and one is small. One collected data in 2003–2004; the other has 101 dogs.
Our note: We found no newer study testing whether pre-anaesthetic bloodwork reduces complications or deaths in dogs.
5. What this means for your dog
A young, healthy dog having a routine procedure, with a normal history and examination: the German evidence suggests blood tests rarely change the plan.
An older dog, or one with any illness: blood tests are more likely to find something that matters, for the anaesthetic or for the dog’s health generally.
Either way, ask what the result would change. If the vet would do something different with an abnormal result, the test has a purpose.
Our note: Section 5 is our summary of two abstracts. We found no agreed age threshold.
Questions to ask:
- Which tests are you recommending, and why for my dog?
- What would you do differently if a result was abnormal?
- Would a recent blood test from another visit be enough?
- Is the test included in the estimate, or extra?
→ How Risky Is Anaesthesia for My Dog? · → Vet Bills, Medicines and Decisions: What You’re Paying For, and What You Can Ask · → Is It Old Age, or Something Treatable? A Guide to Your Senior Dog
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “Pre-anaesthetic bloods are essential for every dog” | Not for every dog. In healthy dogs with normal exams, they rarely changed the plan. |
| “Pre-anaesthetic bloods are just an upsell” | No. In older dogs, screening found new disease in about three in ten. |
| “Blood tests make anaesthesia safer” | Not shown. Neither study measured complications or deaths. |
| “An abnormal result means the operation is off” | Rarely, in healthy dogs. Surgery would have been postponed in 0.8%. |
| “Older dogs are too risky for anaesthesia” | Not addressed here. Screening helps find who is at higher risk. |
Sources
- Alef M, von Praun F, Oechtering G. Is routine pre-anaesthetic haematological and biochemical screening justified in dogs? Vet Anaesth Analg 2008. ⚠️ abstract read. Prospective study, 1,537 dogs, one university hospital
- Joubert KE. Pre-anaesthetic screening of geriatric dogs. J S Afr Vet Assoc 2007. ⚠️ abstract read. Prospective study, 101 dogs over 7 years
- Grubb T, Sager J, Gaynor JS, et al. 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. Phase 1: Preanesthesia: expert guidelines. Web page read in the browser. Supported by grants from three companies, including a veterinary laboratory company
Always confirm with your vet before acting on anything on this page.