Interpret Fludrocortisone Suppression Test for PA
STEP 1 — Gather Information
Collect day 4 10 AM upright plasma aldosterone, plasma renin activity, plasma cortisol at 7 AM and 10 AM, serum potassium; verify fludrocortisone 0.1 mg every 6 h for 4 days with KCl supplementation and medication withdrawal per protocol.
STEP 2 — Rule In / Rule Out
Is upright plasma aldosterone ≤6 ng/dL? If NO → PA excluded; if YES → proceed to assess renin and cortisol.
STEP 3 — Classify or Stratify
Is plasma renin activity ≥1 ng/mL/h AND cortisol at 10 AM < cortisol at 7 AM? If YES → PA confirmed; if NO → PA excluded (insufficient renin suppression or ACTH confounding).
STEP 4 — Decide
If PA confirmed, refer for adrenal CT to subtype disease; if excluded, consider alternative diagnoses or repeat confirmatory testing.
Clinical Guardrails / Mimics / Pitfalls
Ensure serum potassium ≥4.0 mmol/L; avoid MRAs, ACEi/ARBs, diuretics, and licorice for ≥4 weeks before test; confirm seated posture at sampling; exclude acute illness or stress that could elevate ACTH; false negatives may occur with inadequate fludrocortisone adherence or potassium supplementation.
Concrete Clinical Example
A 48‑year‑old hypertensive with spontaneous hypokalemia undergoes FST: day 4 10 AM upright aldosterone 5 ng/dL, PRA 1.3 ng/mL/h, cortisol 7 AM 20 µg/dL, cortisol 10 AM 15 µg/dL → all criteria met, PA confirmed.
Source: The Management of Primary Aldosteronism: Case Detection, Diagnosis, and Treatment: An Endocrine Society Clinical Practice Guideline, Endocrine Society, 2016, DOI:10.1210/jc.2015-4061