InfusionFox
Endocrine & Metabolic

LDDST interpretation

Interpretation of the low-dose dexamethasone suppression test in dogs with a clinical pretest probability for hyperadrenocorticism. Enter the basal, 4-hour, and 8-hour cortisol values; returns a classification as suppression (normal), non-suppression (consistent with HAC), or a pattern distinguishing pituitary-dependent from adrenal-dependent disease. Not a screening test.

  • Pre-test condition. The LDDST is a confirmatory test, not a screening test. Use only when history, exam, CBC, chemistry, electrolytes, and UA are already consistent with HAC. A positive LDDST in a dog with low pre-test probability is more likely a false positive than true HAC.
About this calculator

Two-stage interpretation per the 2013 ACVIM consensus statement: Stage 1 determines whether HAC is present (using your lab's 8-hour cortisol cut-off); Stage 2 evaluates whether the pattern is consistent with PDH within HAC cases. InfusionFox identifies patterns; clinical decisions remain with the practitioner.

Drawn before dexamethasone administration.
Default 1.40 µg/dL. The 2013 ACVIM consensus statement does not specify a single value; cortisol assays differ. Verify against your lab's reference range and override above.
Interpretation
Pattern consistent with PDH
Stage 1. HAC present?

8-hour cortisol (2.00 µg/dL) exceeds the cut-off (1.40 µg/dL) → consistent with HAC.

Stage 2. PDH suppression criteria
  • [ ] 4-hour cortisol < cut-off (4-hr 1.60 ≥ cut-off 1.40 µg/dL)
  • [✓] 4-hour cortisol < 50% of baseline (4-hr is 40.0% of baseline)
  • [ ] 8-hour cortisol < 50% of baseline (8-hr is 50.0% of baseline)

The 8-hour cortisol exceeds the cut-off (consistent with HAC), and at least one suppression criterion is met (4-hour < cut-off, 4-hour < 50% of baseline, or 8-hour < 50% of baseline). Per the consensus statement, suppression on the LDDST identifies PDH within a population of dogs already known to have HAC.

Baseline
4.00 µg/dL
4-hour
1.60 µg/dL
40.0% of baseline
8-hour
2.00 µg/dL
50.0% of baseline
Cut-off used
1.40 µg/dL
Suppression calculations
$$\%\,\text{baseline}_{\,4hr} = \frac{ 1.60 }{ 4.00 } \times 100 = 40.0\%$$ $$\%\,\text{baseline}_{\,8hr} = \frac{ 2.00 }{ 4.00 } \times 100 = 50.0\%$$

cortisol at timepoint ÷ baseline cortisol × 100. Suppression is < 50%.

Using default 8-hour cut-off of 1.4 µg/dL. The consensus statement does not specify a single value, verify against your lab's reference range and override the cut-off if needed.

Sources
Behrend EN, Kooistra HS, Nelson R, Reusch CE, Scott-Moncrieff JC. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM Consensus Statement (small animal). J Vet Intern Med 2013;27:1292–1304.
Feldman EC, Nelson RW, Reusch CE, Scott-Moncrieff JCR. Canine and Feline Endocrinology, 4th ed. Elsevier, 2015. Chapter 11 (Canine Hyperadrenocorticism). LDDST protocol, interpretation of 4-hour and 8-hour cortisol patterns, and discrimination between PDH and AT.
Behrend EN. Hyperadrenocorticism. In: Ettinger SJ, Feldman EC, Côté E, eds. Textbook of Veterinary Internal Medicine, 8th ed. Elsevier, 2017.
Reference

The suppression thresholds

Stage 1. HAC present if the 8-hour cortisol exceeds the lab cut-off:

$$\text{HAC present} \iff \text{cortisol}_{8\text{h}} > \text{cut-off}$$

Stage 2. PDH supported if any of three criteria are met (Feldman 1996, cited by the 2013 ACVIM consensus statement):

$$\text{PDH}_1: \quad \text{cortisol}_{4\text{h}} < \text{cut-off}$$
$$\text{PDH}_2: \quad \text{cortisol}_{4\text{h}} < 0.50 \times \text{baseline}$$
$$\text{PDH}_3: \quad \text{cortisol}_{8\text{h}} < 0.50 \times \text{baseline}$$

Worked example with current inputs

$$50\%\;\text{of baseline} = 0.50 \times 4.00 = 2.00\;\mu g/dL$$
$$\text{cortisol}_{4\text{h}} = 1.60 \quad \text{cortisol}_{8\text{h}} = 2.00 \quad \text{cut-off} = 1.40$$
Reference

How the interpretation works

Stage 1. Is HAC present?

  • If 8-hour cortisol is at or below the cut-off → adequate suppression; pattern does not support HAC.
  • If 8-hour cortisol exceeds the cut-off → consistent with HAC; proceed to Stage 2.
  • If 8-hour is suppressed but 4-hour exceeds baseline → "inverse pattern," highly suspicious for HAC per the consensus statement; further testing indicated.

Stage 2. Is the pattern consistent with PDH?

Within a dog already shown to have HAC at the 8-hour timepoint, the consensus statement (citing Feldman 1996) identifies PDH if any of the following are met:

  • 4-hour cortisol < cut-off
  • 4-hour cortisol < 50% of basal
  • 8-hour cortisol < 50% of basal (but greater than the cut-off, already established by Stage 1)

If none are met, the dog is dexamethasone-resistant and could be either AT or one of the ~25% of PDH cases that fail to suppress. Per the consensus statement, lack of suppression does not confirm AT. Further differentiating tests (cACTH, abdominal ultrasound, or HDDST) are needed.

Sources

  • Behrend EN, Kooistra HS, Nelson R, Reusch CE, Scott-Moncrieff JC. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM Consensus Statement (Small Animal). J Vet Intern Med 2013;27:1292-1304. doi:10.1111/jvim.12192. [link]