The saline suppression test (SST) is conducted to confirm the diagnosis of primary aldosteronism (PA) in patients with an elevated aldosterone:renin ratio. Studies have speculated that SST can predict PA subtype as either unilateral (predominantly an aldosterone-producing adenoma) or bilateral (adrenal hyperplasia) . An accurate prediction of bilateral disease may reduce the need for adrenal vein sampling (AVS).
To identify SST parameters that distinguish bilateral from unilateral PA.
A retrospective analysis was performed on 89 patients who underwent the SST at Monash Health (February 2011 - May 2017). Clinical information collected included patient demographics, SST, AVS and histology results. A positive SST was defined as plasma aldosterone concentration (PAC) >140pmol/L at 4 hours post-infusion of 2L normal saline in the recumbent position . Patients with positive SST results were categorized into three PA subtypes: unilateral, bilateral and undetermined (unsuccessful AVS or no AVS). Results were expressed as median (lower and upper quartiles).
84 patients had a positive SST: 25 unilateral, 25 bilateral and 34 undetermined. The unilateral group had significantly higher PAC compared to the bilateral group both at 0 hours, 538 pmol/L (441-748) vs 323 pmol/L (250-429) (p=0.004), and at 4 hours, 462 pmol/L (280-764) vs 230 pmol/L (195-298) (p=0.05).
Compared to the bilateral group, the PAC in the unilateral group demonstrated a lower absolute reduction at 4 hours, -69 pmol/L (-178-30) vs -87 pmol/L (-142--44) and a smaller percentage decrease at 4 hours, -17% vs -27%, however these were not statistically significant.
Unilateral causes of PA had a higher PAC during the SST both at 0 and 4 hours. However, we did not identify a clear SST parameter which differentiated unilateral from bilateral PA. A seated SST which is more sensitive for bilateral PA  may be better for predicting PA subtypes.