Laboratory Tests Confirming Cushing’s Syndrome in Pets 🧪

By Dr Duncan Houston, BVSc

Cushing’s syndrome (hyperadrenocorticism) in dogs and cats requires precise laboratory testing to confirm and classify the disease. With refined insights in 2025, this guide reviews Low‑Dose & High‑Dose Dexamethasone Suppression Tests, ACTH Stimulation, and Urine Cortisol–Creatinine Ratios, ensuring you know when and how each is used.

🔍 1. Recognizing When to Test

Before testing, suspicion arises from clinical signs (e.g., excessive drinking, urination, hair loss, pot‑belly in dogs; fragile skin, muscle wasting), routine bloodwork, and screening tests (e.g., elevated ALP in dogs). Only then should confirmatory tests be pursued.

🐶 2. Low‑Dose Dexamethasone Suppression Test (LDDST) in Dogs

Overview & Purpose

LDDST is the gold standard to confirm canine Cushing’s. Dog receives low-dose dexamethasone IV after a baseline cortisol draw; cortisol measured at 4 and 8 hours. In a healthy dog cortisol will suppress; with Cushing’s the pituitary/adrenals remain unresponsive.

Accuracy & Interpretation

~90 % accurate for diagnosing Cushing’s. A failure to suppress confirms disease; a partial suppression (e.g., cortisol dips then rebounds) helps differentiate pituitary (PDH) from adrenal causes.

Practical Considerations

• Timeframe: 8 hours with samples drawn morning (8–10 AM) for consistency.
• Housing: Requires hospital stay or quiet lodging.
• Side notes: Medications like glucocorticoids, phenobarbital, or mitotane can interfere—temporarily discontinue or adjust as needed.

🧬 3. ACTH Stimulation Test in Dogs

What It Measures

This test measures adrenal capacity to produce cortisol following synthetic ACTH injection. It’s less suited for diagnosing but reliable for monitoring treatment.

Procedure

• Collect baseline cortisol (morning preferred).
• Administer ACTH IV/IM.
• Repeat cortisol measurement 1–2 hours later.

Clinical Role & Limitation

Uncommonly used for initial diagnosis due to lower sensitivity. Best used to monitor dogs under meds like trilostane or mitotane. Highly specific—few false positives.

📄 4. Urine Cortisol–Creatinine Ratio (UCCR) in Dogs

Screening Test

A high UCCR alerts to possible Cushing’s, but is nonspecific. A normal UCCR effectively rules out disease. Ideal sample is first-morning urine collected at home to avoid “white-coat” stress.

Follow‑up Steps

Abnormal results require confirmatory tests like LDDST or imaging. UCCR is cost-effective for initial evaluation.

🐱 5. Testing in Cats

Cushing’s is rarer in cats but does occur. Testing adaptations are needed.

LDDST in Cats

Same protocol as dogs but with higher dexamethasone doses due to feline steroid resistance. Hospitalization for at least 8 hours is still required.

UCCR in Cats with High-Dose Dex Suppression

Collect first-morning urine on 3 days; administer dexamethasone at home after second sample; evaluate third sample suppression. May categorize disease type (pituitary vs adrenal).

Why ACTH Stimulation Is Rarely Used

Poor reliability in feline Cushing’s; not recommended.

🎯 6. Choosing the Right Test & Timing

Summary decision guide:

• If suspect Cushing’s → UCCR (dogs/cats) as initial screening.
• If UCCR is high or clinical suspicion strong → LDDST for confirmation (dogs/cats).
• If iatrogenic Cushing’s or on steroids → use ACTH stim (dogs).
• To monitor treated dogs → ACTH stim routinely every 4–6 weeks.

🏥 7. Lab Sample Handling Tips

• Fasting before morning samples is ideal.
• Samples must reach the lab promptly; plasma should be separated quickly.
• Maintain documentation: date/time, samples, medications, and clinical changes.

📊 8. Interpreting Results & Next Steps

Positive LDDST confirms Cushing’s; pattern helps distinguish PDH vs adrenal tumor (ADT). ACTH stim results guide treatment adjustments. UCCR guides further diagnostics.

🏥 9. Contextualizing Diagnosis with Imaging

Confirmation with CT/MRI adrenal glands or pituitary improves treatment planning. Adrenal tumors typically unilateral—surgical removal possible. Pituitary tumors may require trilostane or radiation.

🔧 10. Treatment Testing and Follow-up

• Trilostane/Mitotane therapy (dogs): monitor via ACTH stim every 4–6 weeks initially, then every 3–6 months.
• Surgery for adrenal tumors: pre-op LDDST and post-op cortisol monitoring to catch Addisonian crisis.
• Feline Cushing’s: rare, but monitor with LDDST.

🎗️ 11. Ask A Vet Support

These tests and results can be logistically complex—download the Ask A Vet app for:

• Real-time vet guidance on test interpretation.
• Biosample reminders, test scheduling, and uploads.
• Support when adjusting treatment or diagnosing secondary conditions.

🧭 Final Takeaway

Confirming Cushing’s syndrome requires a thoughtful sequence: screen (UCCR), confirm (LDDST or ACTH stim), classify, and monitor. In dogs, ACTH stim is indispensable for monitoring treatment. In cats, LDDST remains the cornerstone, with urine testing as a useful adjunct. Partner with your vet—and the Ask A Vet app—to ensure accurate diagnosis, stress reduction, and optimal outcomes. With expert testing and ongoing care, your pet’s Cushing’s can be managed successfully. 🐕🐈❤️