Saline Infusion Test Protocol for Primary Aldosteronism
The proper protocol involves infusing 2 liters of normal saline intravenously over 4 hours (500 mL/hour) while the patient remains seated, with plasma aldosterone measured at baseline (t=0) and at 4 hours (t=4h), where aldosterone <162 pmol/L (5.8 ng/dL) by LC-MS/MS or <217 pmol/L (7.8 ng/dL) by immunoassay excludes primary aldosteronism. 1
Pre-Test Requirements and Contraindications
Before performing the saline infusion test, you must:
- Ensure baseline blood pressure is controlled - the test should not be performed with uncontrolled hypertension 1
- Screen for contraindications: Do not perform in patients with heart failure or advanced chronic kidney disease due to pulmonary edema risk 1
- Normalize serum potassium - hypokalemia can cause false-negative results 1
- Have the patient sit for 1 hour before starting the test 1
Step-by-Step Protocol
Patient positioning: Maintain seated position throughout the entire procedure 1
Baseline measurements (t=0): Draw blood for:
- Plasma aldosterone
- Serum potassium 1
Saline infusion: Administer 2 liters of 0.9% normal saline IV at 500 mL/hour for 4 hours 1
Post-infusion measurements (t=4h): Draw blood for:
- Plasma aldosterone
- Serum potassium 1
Interpretation of Results
The diagnostic thresholds depend on your assay method:
- LC-MS/MS (preferred): Plasma aldosterone <162 pmol/L (5.8 ng/dL) makes PA unlikely 1
- Immunoassay: Plasma aldosterone <217 pmol/L (7.8 ng/dL) makes PA unlikely 1
Critical Caveat About Immunoassays
Immunoassays can produce falsely elevated aldosterone results in up to 38% of patients, leading to misdiagnosis of PA 2. The Liaison immunoassay overestimates aldosterone by 86% and iSYS by 58% compared to mass spectrometry 2. This is particularly problematic in patients with bilateral disease, where 66% showed discordant results between immunoassay and MS 2. If using immunoassay, consider plasma purification to remove interferents or confirm positive results with LC-MS/MS 2.
Alternative Shortened Protocols (Research Context)
While the standard 4-hour protocol remains guideline-recommended, recent research suggests potential alternatives:
2-hour protocol: A 1-liter infusion over 120 minutes with aldosterone cutoff of 397 pmol/L (14 ng/dL) showed 97% specificity and 90% sensitivity 3. However, this is not yet incorporated into guidelines.
1-hour protocol: Using aldosterone >414 pmol/L (15 ng/dL) at 1 hour, or >138-414 pmol/L (5-15 ng/dL) with <60% suppression from baseline, achieved 93.7% sensitivity and 96.7% specificity 4. Again, this remains investigational.
These shortened protocols may reduce volume load and blood pressure increases (mean systolic BP rose from 140 to 148 mmHg during standard testing 3), but stick with the guideline-recommended 4-hour protocol until further validation 1.
Important Limitations
The saline infusion test has moderate quality evidence and interpretation should be considered probabilistic rather than absolute 1. The test performs better in hypokalaemic PA (91% sensitivity) than normokalaemic PA (57% sensitivity) 5, meaning a negative test does not definitively exclude PA in normokalaemic patients 5.
When Confirmatory Testing May Be Skipped
According to French guidelines, confirmatory testing is unnecessary in specific scenarios 6:
- Can confirm PA without testing: Aldosterone-to-renin ratio elevated on two occasions with plasma aldosterone >550 pmol/L (20 ng/dL)
- Can exclude PA without testing: Normal ratio on two occasions with plasma aldosterone <240 pmol/L (9 ng/dL)
Practical Considerations
The saline infusion test requires:
- In-person visit with IV access
- Space and staff for 4-hour monitoring
- Ability to maintain seated position throughout 1
If contraindicated, use the captopril challenge test instead as the next-best alternative 1, 6.