Endocrine Abstracts

Background: Patients with an elevated aldosterone renin ratio (ARR) should proceed for a confirmatory test to diagnose primary aldosteronism (PA) before undergoing further investigations such as CT imaging and adrenal venous sampling. The recent 2016 clinical guidelines have included a new recommendation: that in the setting of spontaneous hypokalaemia, undetectable plasma renin, and plasma aldosterone concentration (PAC) >20 ng/dl, patients may not require further confirmatory testing. We retrospectively evaluated the utility of this, in all patients diagnosed in our tertiary unit over the last 17 years.

Methods: All patients who had undergone both a screening test (ARR) and salt loading tests (SLT) in Changi General Hospital, Singapore, from 2001 to 2017 were included. Hypokalaemia was taken as K < 3.5 mmol/l (laboratory reference 3.5–5.0 mmol/l). Positive salt loading test was taken as post-salt loading PAC >10 ng/dl. We assessed the sensitivity and specificity of the above criteria in predicting a positive salt loading test.

Results: 90 patients, mean age 53.3±12.7 years, 51 (56.6%) males, underwent both screening ARR and confirmatory SLT from 2001–2017. 65.5% (59 of 90) patients had a positive SLT. 12.2% (11 of 90) patients fulfilled the above criteria of spontaneous hypokalaemia, undetectable plasma renin, and PAC > 20 ng/dl. Of these 11 patients, 10 (90.9%) had a positive SLT. The above criteria had a specificity of 96.8% and sensitivity of 16.9% in predicting salt loading positivity. The positive predictive value was 90.9%. In the 1 patient who fulfilled the new criteria but had a negative SLT, the lowest potassium level was 3.4 on several occasions (without other contributory factors), with baseline aldosterone 21.8 ng/dl, renin undetectable, and post-SLT aldosterone 3.46 ng/dl. The remaining 10 patients all had a potassium level of ≤3.0 mmol/l.

Conclusion: The new additional criteria (spontaneous hypokalaemia, undetectable plasma renin, and PAC >20 ng/dl) was demonstrated to be highly specific for positive salt loading test results, which would have benefitted 11% of our patients who would not have needed to undergo a confirmatory test. However, one patient with mild spontaneous hypokalaemia had a negative confirmatory test. Hence, the severity of spontaneous hypokalaemia should also be taken into account, and our data suggest that a potassium level of ≤3.0 may be included in the criteria, to help accurately identify patients who do not need further confirmatory tests.