Diagnosing Canine Cushing’s Disease: The Dog Low Dose Dexamethasone Suppression Test

The low dose dexamethasone suppression test (LDDS) is a diagnostic tool used in veterinary medicine to identify Cushing’s disease (hyperadrenocorticism) in dogs. This test measures the body’s cortisol response to an injection of dexamethasone, a synthetic steroid. By evaluating how cortisol levels change, veterinarians can determine if a dog has Cushing’s disease and often identify whether the condition is pituitary or adrenal in origin. This test is crucial for accurate diagnosis and effective treatment planning, ensuring better health outcomes for affected dogs.

Purpose of the Test

The primary purpose of the low dose dexamethasone suppression test in dogs is to diagnose hyperadrenocorticism (Cushing’s disease). This test helps determine if a dog has Cushing’s disease by measuring how well the dog’s adrenal glands respond to the synthetic steroid dexamethasone. It involves taking blood samples before and after administering dexamethasone to see if cortisol levels are appropriately suppressed.

Procedure

Here’s the step-by-step procedure for the dog low dose dexamethasone suppression test:

1. Collect Baseline Sample: Draw a blood sample to measure the baseline cortisol level. Label this sample as the “0-hour” sample.
2. Administer Dexamethasone: Inject 0.01 mg/kg of dexamethasone intravenously (IV).
3. 4-Hour Sample: Four hours after the dexamethasone injection, draw another blood sample. Label this sample as the “4-hour” sample.
4. 8-Hour Sample: Eight hours after the dexamethasone injection, draw a final blood sample. Label this sample as the “8-hour” sample.

The collected blood samples are then analyzed to measure cortisol levels.

Interpretation of Results

The low dose dexamethasone suppression test (LDDS) is used to diagnose hyperadrenocorticism (Cushing’s disease) in dogs. Here’s how the results are interpreted:

• Normal Result: Cortisol levels should drop significantly after the administration of dexamethasone. Typically, cortisol levels fall below 40 nmol/L at 8 hours.

• Abnormal Result: If cortisol levels do not drop sufficiently, it indicates hyperadrenocorticism. Specifically, cortisol levels remain above 40 nmol/L at 8 hours.

This test helps differentiate between normal and abnormal adrenal function in dogs.

Advantages

The low dose dexamethasone suppression test (LDDST) for dogs offers several advantages:

1. Reliability: The LDDST is highly sensitive, with an overall sensitivity around 96.6%. It is considered the most reliable screening and diagnostic test for hyperadrenocorticism (Cushing’s disease) in dogs.

2. Cost-Effectiveness: Compared to other diagnostic tests, the LDDST is relatively cost-effective. It involves fewer resources and less time, making it a practical choice for many veterinary practices.

3. Differentiation Capability: The LDDST can help differentiate between pituitary-dependent and adrenal-dependent hyperadrenocorticism about 65% of the time. This is crucial for determining the appropriate treatment plan.

4. Non-Invasiveness: The test is minimally invasive, requiring only blood samples at specific intervals after the dexamethasone injection.

These factors make the LDDST a preferred choice for diagnosing Cushing’s disease in dogs.

Limitations

Here are the limitations of the low dose dexamethasone suppression test (LDDS) for dogs:

• False Positives: Stress, concurrent illnesses, and certain medications (e.g., anticonvulsants, exogenous glucocorticoids) can cause elevated cortisol levels, leading to false positives.
• False Negatives: Some dogs with pituitary-dependent hyperadrenocorticism (PDH) may show no suppression of cortisol levels due to resistance in the negative feedback loop.
• Non-Specific Results: The test may not clearly differentiate between pituitary-dependent and adrenal-dependent hyperadrenocorticism.
• Influence of Population Characteristics: The reliability of the test results can vary based on the dog’s age, clinical signs, and overall health.
• Need for Additional Testing: Due to the potential for false positives and negatives, additional tests such as the high dose dexamethasone suppression test or ACTH stimulation test may be required to confirm the diagnosis.

The Low Dose Dexamethasone Suppression Test (LDDS)

The low dose dexamethasone suppression test (LDDS) is a diagnostic tool used to identify Cushing’s disease (hyperadrenocorticism) in dogs. It measures cortisol levels before and after administering dexamethasone, a synthetic steroid.

The test involves collecting blood samples at 0, 4, and 8 hours after the injection. Cortisol levels should drop significantly after dexamethasone administration, typically below 40 nmol/L at 8 hours. Abnormal results indicate hyperadrenocorticism.

The LDDS is highly sensitive (96.6%) and cost-effective, making it a preferred choice for diagnosing Cushing’s disease in dogs. However, it has limitations, including false positives due to stress or medications, false negatives in some cases of pituitary-dependent hyperadrenocorticism, non-specific results, and influence of population characteristics.

Additional testing may be required to confirm the diagnosis.