How is the salt-loading test used to confirm primary aldosteronism?

Updated: Mar 24, 2020
  • Author: Gabriel I Uwaifo, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Answer

The salt-loading test can be done by using either an intravenous salt-loading protocol or an oral salt-loading protocol. The oral protocol calls for daily ingestion of at least 10-12 g of sodium chloride for at least 5 days before the test is performed. When the oral protocol has been met, 24-hour U-Aldo, sodium, potassium, and creatinine excretions are measured, and serum aldosterone and PRA should be determined. In normal individuals, the major U-Aldo metabolite, urinary aldosterone-18-glucuronide, should fall below a level of 17 mcg/day. Nonsuppressibility of U-Aldo-18G is highly suggestive of primary aldosteronism. Nonetheless, this test is cumbersome and rarely performed.

The 24-hour urinary creatinine measurement validates the adequacy of the urine sample collection, while a 24-hour urinary sodium value of at least 250 mEq/day confirms an adequate salt load during the days prior to the test and therefore validates the other measurements.

The alternate version of the salt loading test involves the intravenous the infusion of 500 mL/h of isotonic sodium chloride solution over 4 hours (total of 2 L of fluid volume). The serum aldosterone level and PRA are measured at baseline, 2 hours, and 4 hours. In healthy individuals, aldosterone levels are suppressed to less than 8.5 ng/dL, while the PRA is suppressed to less than 0.6 ng/mL/h. Again, S-Aldo is highly suggestive of primary aldosteronism. Rapid infusion of isotonic saline should be avoided in patients with frank volume overload due to renal or cardiac failure or other medical reasons.


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