Canine Cushing's Disease Test Interpretation (LDDST, ACTH Stim, UCCR)
Interpreting LDDST, ACTH stim test, and UCCR for canine Cushing's disease, plus differentiating pituitary vs adrenal-dependent disease.
When to test
| Indication | Notes |
|---|---|
| Classic clinical signs (PU/PD, polyphagia, pot-belly, alopecia, panting) | Pretest probability matters - don't screen low-suspicion patients. |
| Elevated ALP (especially > 5× upper limit) | Most common biochemistry sign; not specific but supportive. |
| USG persistently < 1.020 + classic signs | Useful supporting finding. |
Screening tests
| Test | Interpretation |
|---|---|
| LDDST (low-dose dex suppression) | Gold standard for screening. Cortisol > 1.4 μg/dL at 8h = consistent with HAC. |
| ACTH stimulation test | Less sensitive than LDDST (~60-80%) but more specific. Post-ACTH > 22 μg/dL diagnoses HAC. |
| UCCR (urine cortisol:creatinine) | Sensitive screen (~75-100%) but not specific. NEGATIVE result rules HAC out. |
Differentiating PDH vs ADH
| Test | PDH (pituitary) | ADH (adrenal) |
|---|---|---|
| LDDST 4h cortisol | Suppresses to < 50% baseline | Does NOT suppress |
| LDDST 8h cortisol | Rebounds to > 1.4 | Stays elevated |
| HDDST (high-dose) 4h cortisol | Suppresses | Does NOT suppress (ADH) |
| Endogenous ACTH | Normal-high | Low / undetectable |
| Abdominal ultrasound | Bilateral symmetric adrenomegaly | Unilateral adrenal mass + contralateral atrophy |
Common pitfalls
| Pitfall | Why it matters |
|---|---|
| Testing stressed or systemically ill patients | False positives common - stabilise non-adrenal illness first. |
| Recent corticosteroid use (including topical) | Suppresses HPA axis - false NEGATIVE on ACTH stim, false positive on LDDST |
| Phenobarbital therapy | Increases liver enzymes (mimics HAC ALP elevation) |
| Diagnosing HAC from elevated ALP alone | ALP is sensitive but very non-specific - must have clinical signs |
Both LDDST and ACTH stim test have false positives and negatives - always interpret results alongside clinical signs and ultrasound findings. Iatrogenic Cushing's (from recent steroid use) is more common than spontaneous.
Frequently asked questions
LDDST or ACTH stim - which should I do first?
LDDST is the more sensitive screen (~95%) for spontaneous HAC and also helps differentiate PDH from ADH. ACTH stim is the only test for iatrogenic HAC and is preferred for monitoring trilostane therapy. For a first-time work-up of clinical signs, LDDST.
A negative UCCR means no Cushing's?
Effectively yes - the UCCR is very sensitive (~75-100%) but very non-specific. Many non-cushingoid sick dogs have an elevated UCCR. Use it to rule OUT, not to rule IN.
How do I distinguish iatrogenic from spontaneous HAC?
ACTH stim test. In iatrogenic HAC (from corticosteroid administration), the adrenal glands are atrophied - baseline AND post-ACTH cortisol are LOW. Spontaneous HAC shows an exaggerated response. Always ask about steroid sources (ear drops, eye drops, topicals).