Not all immunoassays are equal. And in equine endocrinology, the difference is a laminitis episode away.
By the bioMérieux Editors | Reading time: 5 min
PUBLICATION DATE: July 20, 2026
A single result can shape the entire diagnosis
Every endocrine diagnosis in equine practice ultimately rests on a number. A single ACTH concentration can determine whether a horse is starting on lifelong medication. A single insulin value can trigger dietary intervention or justify a dynamic test. When that number is unreliable, the consequences are rarely trivial: a missed Pituitary Pars Intermedia Dysfunction (PPID) diagnosis, an undertreated Equine Metabolic Syndrome (EMS), or at worst, a laminitis episode that could have been prevented. Retesting adds cost and delays; inappropriate treatment adds risk.
Immunoassays are not interchangeable, even when they claim to measure the same hormone. Understanding what separates a truly high-performing test from a mediocre one is part of clinical practice, especially in PPID and EMS, where endocrine testing is not a one-off event but a life-long monitoring commitment.
What makes equine endocrine testing particularly demanding
ACTH circulates at low concentrations and is inherently unstable, making its measurement particularly sensitive to pre-analytical conditions. Time and temperature strongly influence hormone stability, with progressive degradation after sample collection, especially when samples are kept at room temperature or not processed promptly. This may lead to falsely low ACTH concentrations and affect diagnostic interpretation, particularly when results are close to decision thresholds. Therefore, rapid cooling, timely centrifugation, and minimization of delays before analysis are essential to preserve ACTH stability and ensure reliable results.
Insulin adds its own layer of complexity. Concentrations shift substantially depending on what and when the horse last ate, making sampling conditions inseparable from result interpretation. Unlike ACTH, no seasonal adjustment is needed, an elevated insulin at any time of year is a finding that warrants attention.
These biological realities mean that even a technically solid assay can produce misleading results if the pre-analytical steps are not followed rigorously. But they also mean that a poorly designed assay, one that lacks sensitivity, drifts between batches, or cross-reacts with related molecules, will amplify biological variability rather than mitigate it.
The criteria that actually matters
Running endocrine diagnostics in-house comes with real advantages: faster results, tighter control over the workflow, and the ability to act on the same day. But before a clinic commits to an in-house solution, practical questions come first: Can the team actually use the instrument? Is there space for it? Does the test volume justify the investment? These are legitimate criteria that should be answered before anything else.
Once those boxes are ticked, the question shifts to performance. Because an in-house test that is convenient but unreliable doesn't give you more control, it gives you more decisions made on uncertain ground. When it comes to endocrine testing in horses, three analytical characteristics directly shape the quality of every clinical decision you make: precision, robustness, and sensitivity.
| SENSITIVITY |
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Does the test perform reliably in the clinically relevant concentration zone? For ACTH, what matters is consistent, accurate measurement around the decision threshold. An assay that is imprecise or biased in that zone will blur the line between a horse that needs treatment and one that does not. In practice: a symptomatic horse in autumn with a result just above threshold. Is it early PPID or seasonal variation? If the assay performs poorly in that zone, the number doesn't help you decide. |
| PRECISION |
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How reproducible are results within the same run and across different runs, operators, and days? Precision reflects the consistency of repeated measurements and is typically assessed using intra-assay and inter-assay coefficients of variation (CV). While commonly accepted benchmarks define intra-assay CVs below 10% and inter-assay CVs below 15% as acceptable, true analytical robustness is demonstrated by achieving substantially lower values. In this context, CVs closer to 5% are strongly indicative of high precision and should be considered the target rather than merely meeting minimum acceptance criteria. In practice: if your ACTH result comes back at 35 pg/mL in October and 45 pg/mL two weeks later on the same untreated horse, is that biological drift or assay variability? Only a high-precision test lets you answer that question. |
| ROBUSTNESS |
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Does the assay give consistent results regardless of where, when, and how it is used? An immunoassay is robust when it delivers the same result whether the sample is processed in a busy hospital in summer or in a field clinic in winter; when minor variations in temperature, handling time, or operator technique do not translate into clinically meaningful result shifts. Fully automated workflows further reduce operator variability, an underestimated source of error in routine practice, in-house results a realistic standard of care. In practice: In treatment monitoring, testing delays may affect results. When values differ significantly, how can such delays explain the discrepancy, and how can I avoid misinterpretation and inappropriate treatment decisions? |
Fluorescent detection technologies, such as ELFA, may provide higher analytical sensitivity and better precision than conventional colorimetric ELISA. Comparative hormone assay data have reported lower variability with ELFA, indicating more reproducible results, which is clinically relevant when ACTH or insulin concentrations are close to decision thresholds.
What good development looks like
Behind every reliable result lies a rigorously structured development process. For diagnostic immunoassays, this means analytical validation covering linearity, limit of detection, limit of quantification, and inter-assay variability, the parameters that determine whether a test performs consistently across samples, operators, and time.
Clinical validation adds a further layer. Representative cohorts, correlation with clinical signs, and validation in the target species are what translate analytical performance into diagnostic relevance. An assay that has only been validated in human plasma is not a validated equine assay, regardless of how well it performs on the bench.
Chronic diseases require tools that last
PPID and EMS are not resolved after a single consultation, they are managed over years. This implies adjusting pergolide doses as PPID progresses, refining dietary restrictions and exercise programs as insulin regulation responds (or fails to) in EMS horses, and reassessing at every seasonal window where misinterpretation is most likely. Each of those decisions depends on a hormonal measurement that must be trusted.
This is where the performance characteristics described above stop being analytical abstractions and become directly clinical. A test with high inter-assay variability turns every monitoring result into a question mark. And a test that has never been validated in horses introduces a layer of uncertainty that no clinical judgement can fully compensate for.
Choosing a high-quality immunoassay at the outset is not a technical preference. In the context of chronic endocrine disease, it is one of the most consequential decisions a veterinarian makes for their patient, because every subsequent decision about treatment, diet, and follow-up interval will be built on top of it.
The test is part of the treatment plan, don’t compromise on your diagnostic tool
- Equine endocrine testing is not a commodity. The performance of the immunoassay directly shapes your diagnostic confidence and your horse's outcome.
- The right questions to ask: Has this test been validated in horses? Are seasonally adjusted reference intervals available? What is the inter-assay coefficient of variability? Is performance stable across reagent lots?
- In a disease that lasts years, choosing a consistent, well-validated test from the outset is one of the most practical decisions you can make for your patient.
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