Determines if primary aldosteronism is confirmed based on upright plasma aldosterone ≤6 ng/dL (170 nmol/L) on day 4 at 10 AM of fludrocortisone suppression test, provided plasma renin activity ≥1 ng/mL/h and plasma cortisol concentration lower than the 7 AM value to exclude ACTH confounding. Use when reviewing FST results to diagnose PA; triggers include "FST", "upright aldosterone ≤6 ng/dL", "PRA ≥1 ng/mL/h", and "cortisol drop".
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---
name: esa-pa-interpret-fst
description: Determines if primary aldosteronism is confirmed based on upright plasma aldosterone ≤6 ng/dL (170 nmol/L) on day 4 at 10 AM of fludrocortisone suppression test, provided plasma renin activity ≥1 ng/mL/h and plasma cortisol concentration lower than the 7 AM value to exclude ACTH confounding. Use when reviewing FST results to diagnose PA; triggers include "FST", "upright aldosterone ≤6 ng/dL", "PRA ≥1 ng/mL/h", and "cortisol drop".
---
# 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
