Baseline Plasma Aldosterone Level and Renin Activity Allowing Omission of Confirmatory Testing in Primary Aldosteronism

Abstract

Context

Previous studies have proposed cutoff value of baseline plasma aldosterone concentration (bPAC) under renin suppression that could diagnose primary aldosteronism (PA) without confirmatory testing. However, those studies are limited by selection bias due to a small number of patients and a single-center study design.

Objective

This study aimed to determine cutoff value of bPAC and baseline plasma renin activity (bPRA) for predicting positive results in confirmatory tests for PA.

Design

The multi-institutional, retrospective, cohort study was conducted using the PA registry in Japan (JPAS/JRAS). We compared bPAC in patients with PA who showed positive and negative captopril challenge test (CCT) or saline infusion test (SIT) results.

Patients

Patients with PA who underwent CCT (n = 2256) and/or SIT (n = 1184) were studied.

Main outcome measures

The main outcomes were cutoff value of bPAC (ng/dL) and bPRA (ng/mL/h) for predicting positive CCT and/or SIT results.

Results

In patients with renin suppression (bPRA ≤ 0.3), the cutoff value of bPAC that would give 100% specificity for predicting a positive SIT result was lower than that for predicting a positive CCT result (30.85 vs 56.35, respectively). Specificities of bPAC cutoff values ≥ 30.85 for predicting positive SIT and CCT results remained high (100.0% and 97.0%, respectively) in patients with bPRA ≤ 0.6. However, the specificities of bPAC cutoff values ≥ 30.85 for predicting positive SIT and CCT results decreased when patients with bPRA > 0.6 were included.

Conclusion

Confirmatory testing could be omitted in patients with bPAC ≥ 30.85 in the presence of bPRA ≤ 0.6.

Introduction

Primary aldosteronism (PA) was first described by Conn in 1955 in a patient presenting with resistant hypertension and hypokalemia and who was found to have an aldosterone-secreting adrenal adenoma (1). Classically, PA is diagnosed in the presence of hypokalemia, elevated plasma aldosterone concentration (PAC), suppressed plasma renin activity (PRA), and a unilateral adrenal tumor. However, not all patients with PA have all of these findings. Because only a minority of patients with PA (9%–37%) have hypokalemia, guidelines recommend that, regardless of serum potassium concentration, patients suspected of having PA be screened by simultaneously measuring PAC, PRA, and the PAC/PRA ratio (also known as the aldosterone-to-renin ratio [ARR]). If screening reveals an elevated ARR, confirmatory tests are recommended to exclude false-positive results (2).

In Japan, patients with an elevated ARR on screening are advised to undergo confirmatory tests, which may include the captopril challenge test (CCT), saline infusion test (SIT), or furosemide upright test (FUT) (3). Two additional testing procedures, the oral sodium loading test and the fludrocortisone suppression test, are also recommended by the Endocrine Society to diagnose PA (2). However, in patients with typical PA manifestations along with a high ARR and an elevated PAC, it may be possible to diagnose PA without further confirmatory testing. Indeed, the Endocrine Society guidelines state that confirmatory tests may be unnecessary if the patient has spontaneous hypokalemia, PRA below detection levels, and a PAC > 20 ng/dL (2). The French Endocrine Society guideline also recommends that PA is positively diagnosed without further testing in the presence of an elevated ARR and a PAC > 550 pmol/L (20 ng/dL) (4). However, those guidelines for omission of confirmatory testing are based on limited evidence from single-center cohorts (5, 6). Therefore, a cutoff value of PAC indicating that confirmatory testing is unnecessary for a definitive diagnosis of PA warrants further investigation.

The aim of this study of a multi-institutional cohort was to determine cutoff values of baseline PAC and/or ARR that would render confirmatory testing unnecessary. Therefore, we investigated patients diagnosed with PA, comparing clinical characteristics and baseline blood test results among those with or without positive results in 2 common confirmatory tests, CCT and/or SIT. The intent was to identify cutoff values for baseline PAC and/or ARR that were predictive of a positive CCT or SIT, thus obviating the need for further confirmatory testing.

Material and Methods

Design and patients

This retrospective study was conducted as a part of the Japan Primary Aldosteronism Study (JPAS) and the Japan Rare/Intractable Adrenal Diseases Study (JRAS) that are based on a nation-wide PA registry established in 41 centers, comprising 22 university hospitals and 19 city hospitals. Patients 20 to 90 years of age who were diagnosed with PA and who underwent adrenal venous sampling (AVS) from January 2006 to March 2019 were entered in the registry.

Of 4050 patients with confirmed PA in the JPAS/JRAS dataset, data from 2340 were analyzed in this study. Reasons for excluding other patients included unknown PAC or ARR on screening (n = 619); use of diuretics, aldosterone antagonists, renin-angiotensin-aldosterone system inhibitors, or β blockers at PA diagnosis (n = 336); suspected autonomous cortisol secretion (defined as serum cortisol levels > 3 μg/dL after 1 mg of dexamethasone) (n = 175); no data on AVS or failure of AVS cannulation after cosyntropin stimulation (n = 486); and noncompliance with confirmatory testing (CCT and SIT) (n = 94). We also excluded patients who lacked positive results in every confirmatory test (n = 64), although they had been diagnosed as having PA by oral salt loading test, ACTH stimulating test, and/or typical PA manifestations.

The study was conducted according to guidelines for clinical studies published by the Ministry of Health, Labour and Welfare, Japan. The study adhered to the principles of the Declaration of Helsinki. The study protocol was approved by the Ethics Committee of the National Hospital Organization Kyoto Medical Center as the project leader center and by the institutional ethics committees of the participating centers. This observational study was registered as UMIN ID 18756 and 32525.

Diagnosis of primary aldosteronism

PA was diagnosed based on the guidelines of the Japan Endocrine Society (3) and the Japan Society of Hypertension (7). Cases of PA were detected based on an ARR > 20 (calculated as the ratio of PAC in ng/dL to PRA in ng/mL/hour) and at least 1 positive confirmatory test result, including CCT, SIT, or FUT. Antihypertensive medications were usually replaced with calcium channel antagonists and/or α-adrenergic blockers, if possible, until the final diagnosis was made.

Biochemical parameters

Hypokalemia was defined as a serum potassium ≤ 3.5 mEq/L before or at PA diagnosis or if a patient was taking potassium supplement. Oral potassium was prescribed if hypokalemia was present. The estimated glomerular filtration rate (eGFR) was calculated with the following equation: eGFR (mL/min/1.73 m²) = 194 × serum creatinine (−1.094) × age (−0.287) × 0.739 (if female) (8). PAC and PRA were measured using commercial kits. PAC was determined by radioimmunoassay (SPAC-S Aldosterone kits, Fuji Rebio, Co. Ltd., Tokyo, Japan) in 38 centers and chemiluminescent enzyme immunoassay (Accuraseed Aldosterone kits; FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) in 3 centers, with a reference range in the supine position of 3.0 to 15.9 ng/dL. PRA was measured using a radioimmunoassay or an enzyme immunoassay, with reference ranges in the supine position of 0.3 to 2.9 ng/mL/h (PRA-FR RIA kits; Fuji Rebio Co. Ltd.) in 24 centers, 0.2 to 2.3 ng/mL/h (PRA EIA kits; Yamasa Co. Ltd., Choshi, Japan) in 13 centers, and 0.2 to 2.7 ng/mL/h (PRA RIA kits; Yamasa Co. Ltd.) in 3 centers. Plasma active renin concentration (ARC) was measured using an immunoradiometric assay (Renin IRMA-FR; Fuji Rebio Co. Ltd.) or chemiluminescent enzyme immunoassay (Accuraseed renin kits; FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) in 1 center, and the ARC value was converted to PRA by dividing by 5, based on Japan Endocrine Society guidelines (3).

Confirmatory tests

The CCT, SIT, or FUT were used as confirmatory tests according to guidelines of the Japan Endocrine Society (3) and Japan Society of Hypertension (7). Each confirmatory test was performed after an overnight fast, with cutoff values for each test based on Japan Endocrine Society guidelines for detecting PA (3).

For the CCT, patients received 50 mg of captopril orally after lying supine for at least 30 minutes. Blood samples were drawn to measure PAC and PRA before and again at 60 and/or 90 minutes after captopril administration. The CCT result was considered positive if the ARR was > 20 at 60 or 90 minutes after captopril administration (3).

For the SIT, patients lay supine for at least 30 minutes, after which venipuncture was performed for measurement of basal PAC. Subsequently, 2 L of 0.9% NaCl was administered intravenously over 4 hours, and the PAC was measured again. A positive SIT result was defined as a post-saline infusion PAC of > 6.0 ng/dL (3).

For the FUT, patients lay supine for at least 30 minutes before venipuncture was performed for measurement of basal PRA. Patients were then given an intravenous bolus injection of 40 mg of furosemide. After 2 hours in an upright posture, venipuncture was repeated for measurement of PRA. A positive FUT result was defined as a post-FUT PRA of < 2 ng/mL/h (3).

Adrenal gland assessment

Patients with confirmed PA underwent thin-slice adrenal computed tomography, and the findings were evaluated by radiologists at each institution. For this study, an adrenal nodule was defined as a nodule detected on imaging that was larger than 10 mm in diameter (9).

PA subtype classification was determined by AVS results. AVS was performed with adrenocorticotropic hormone (cosyntropin) stimulation according to each center’s protocol with a single bolus injection, a continuous infusion, or a bolus injection followed by continuous infusion. The selectivity index was defined as the ratio of cortisol in the adrenal vein to that in the inferior vena cava, with a selectivity index of > 5 indicating successful sampling (10). Unilateral PA was diagnosed when the lateralized ratio (calculated by dividing the aldosterone to cortisol ratio on the dominant side with that on the nondominant side) was > 4 and the contralateral ratio (calculated by dividing the aldosterone to cortisol ratio on the nondominant side by that in the inferior vena cava) was < 1 after cosyntropin stimulation (10).

Statistical analysis

Statistical analysis was performed with IBM SPSS software (version 22; IBM Corp., Armonk, NY, USA). Continuous variables are expressed as median and interquartile range and categorical variables as number and percentage. Continuous variables were analyzed using the Mann-Whitney U-test and categorical variables with the chi-square test. Statistical significance was set at P < 0.05. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic ability of baseline values of PAC and ARR. We calculated the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of PAC and ARR to indicate a positive CCT or SIT result. The Youden index was used to derive optimal cutoff values.

Results

Of 2256 patients who underwent CCT, 1912 (84.8%) had a positive result. Those with a positive result (CCTp) had a significantly lower body mass index (BMI), higher proportion of women, and greater use of oral potassium supplements than those with a negative result (CCTn) (Table 1). The serum potassium level at diagnosis was significantly lower in the CCTp group than in the CCTn group. More patients in the CCTp group had frank hypokalemia and adrenal nodules, and baseline PAC was significantly higher and baseline PRA significantly lower. Therefore, baseline ARR was significantly higher in the CCTp group than in the CCTn group, as was the proportion of patients diagnosed with unilateral PA.

Of 1184 patients who underwent SIT, 953 (80.5%) had a positive result. Those with a positive result (SITp) were significantly younger and had a higher BMI and a higher proportion of men than those with a negative result (SITn) (Table 1). They also had a higher diastolic blood pressure, and a higher proportion were taking oral potassium supplements. Compared with the SITn group, the serum potassium level at diagnosis was significantly lower and more patients had frank hypokalemia and adrenal nodules in the SITp group. Baseline PAC was significantly higher and baseline PRA significantly lower in the SITp group, so baseline ARR was significantly higher. More patients were diagnosed with unilateral PA in the SITp group than in the SITn group (Table 1).

ROC curve analysis was performed to find optimal cutoff values of baseline PAC and ARR indicating the likelihood of a positive confirmatory test result (Table 2). The optimal cutoff values predicting a positive CCT were 27.95 ng/dL for baseline PAC and 44.42 for baseline ARR, whereas those predicting a positive SIT were 18.25 ng/dL for baseline PAC and 77.15 for baseline ARR. The cutoff values of baseline PAC and ARR that would give 100% specificity for predicting a positive CCT result were 86.25 ng/dL and 564.00, respectively. The cutoff values of baseline PAC and ARR that would give a 100% specificity for predicting a positive SIT result were 33.55 ng/dL and 308.50, respectively.

Table 3 shows the number of patients according to their baseline PRA. The typical manifestations of PA are elevated PAC with PRA below detection levels. Therefore, ROC curve analysis was performed only for patients with PRA ≤ 0.3 on screening (Table 4). To predict a positive CCT result in the presence of a baseline PRA ≤ 0.3, optimal cutoff values for baseline PAC and ARR were 14.95 ng/dL and 78.75, respectively. We also calculated cutoff values that would give 100% specificity, resulting in a baseline PAC of 56.35 ng/dL and baseline ARR of 564.00. The optimal cutoff values of baseline PAC and ARR for predicting a positive SIT result were 14.55 ng/dL and 77.17, respectively. For a specificity of 100%, the cutoff for baseline PAC was 30.85 ng/dL and that for baseline ARR was 308.50.

Patients who have baseline ARR ≥ 308.50 under renin suppression (PRA ≤ 0.3) can be strongly predicted to have positive result in SIT. After investigating baseline PAC in each patient with baseline ARR > 308.50, we found that the baseline PAC of all patients were > 30.85 ng/dL (Table 5). Therefore, confirmatory testing to diagnose PA could be omitted in patients whose baseline PAC is > 30.85 ng/dL in the presence of baseline PRA ≤ 0.3 regardless of their baseline ARR.

Finally, we also investigated baseline PRA levels in patients with negative CCT and SIT results despite a baseline PAC ≥ 30.85 ng/dL (Table 6). In the presence of baseline PRA ≤ 0.6 ng/mL/h, none of the patients showed a negative SIT result despite having a baseline PAC ≥ 30.85 ng/dL. The number of patients with a negative CCT result despite baseline PAC ≥ 30.85 ng/dL was higher in patients with baseline PRA > 0.6 ng/mL/h. Table 7 shows the specificities of PAC cutoff value (≥ 30.85 ng/dL) for predicting positive CCT or SIT results according to each baseline PRA. The specificity remained 100% for a positive SIT result and high for a positive CCT result until the baseline PRA ≤ 0.6, however, the specificities of SIT and CCT decreased when patients with baseline PRA > 0.6 were included.

Discussion

In the presence of renin suppression, patients can be predicted to have positive SIT result when the baseline PAC exceeds 30.85 ng/dL and/or the ARR exceeds 308.50. As shown in Table 5, among patients with positive results in CCT and SIT, all those with baseline ARR ≥ 308.5 had baseline PAC ≥ 30.85 ng/dL. Therefore, our findings suggest that there is no need for further confirmatory testing to diagnose PA in hypertensive patients with suppressed PRA if they have a PAC ≥ 30.85 ng/dL. These findings are important because accurate and cost-effective screening tests for PA are required (11, 12).

In patients with PRA > 0.3 ng/mL/h, there were few patients who had baseline ARR ≥ 308.50 (Table 5). It means that the cutoff value of baseline ARR 308.5 is not suitable as a cutoff parameter to omit confirmatory testing in PA diagnosis regardless of PRA levels. The number of patients who had a negative CCT result increased when baseline PRA exceeded 0.6 ng/mL/h (Table 6). Furthermore, 2 patients whose baseline PRA exceeded 0.6 ng/mL/h showed a negative result in SIT despite baseline PAC ≥ 30.85 ng/dL (Table 6). Therefore, the cutoff value of baseline PAC 30.85 ng/dL would be applicable for patients who have baseline PRA ≤ 0.6 ng/mL/h.

Several studies have reported optimal cutoff values for PAC and ARR that make confirmatory tests for the diagnosis unnecessary. Nanba et al found that in patients evaluated with CCT, SIT, and FUT, positive results for all 3 confirmatory tests were more common in patients with a PAC ≥ 25 ng/dL or an ARR ≥ 100 under renin suppression than in those with lower values (6). Therefore, the authors suggested reducing the number of confirmatory tests in such patients. Maiolino et al reported that progressively increasing ARR values were associated with an exponential increase in the specificity of tests to identify an aldosterone-producing adenoma (13). They found that the optimal ARR cutoff value indicating the presence of an adenoma was approximately 33; an adenoma was uniformly found in patients with an ARR above 100. Recently, Vivien et al retrospectively investigated a PAC cutoff value eliminating the need for confirmatory testing in 173 patients with hypertension (14). In their report, approximately 90% of patients with PA and 14% with essential hypertension had a PAC above 550 pmol/L (20 ng/dL). Similarly, Umakoshi et al found that a PAC > 30 ng/dL, or in patients with spontaneous hypokalemia, a cutoff value of 20 to 30 ng/dL, rendered CCT unnecessary (15). As shown in Table 2, the cutoff values of PAC for CCT positivity were higher than those for SIT in patients without restriction of PRA, but the optimal cutoff values of ARR for CCT positivity were lower than those for SIT. We believe that the inclusion of patients without suppressed renin in our cohort study explains this result. A small number of our patients with PA did not have renin suppression (Tables 3 and 6). Patients whose renin activities were not suppressed despite having an adrenal aldosterone-producing adenoma have been reported previously (16, 17). In our study, a patient with hypokalemia, an adrenal nodule and severe baseline PAC elevation (85.2 ng/dL) but without renin suppression (PRA 3.6 ng/mL/h) had a negative CCT result but a positive SIT result (data not shown). This patient was diagnosed as having unilateral PA by AVS and underwent surgical resection. The adrenal nodule in this patient was pathologically diagnosed as an aldosterone-producing adenoma. As Oelkers had reported that hypertensive kidney damage would lead to the escape of renin from suppression in patients with PA who have severe hypertension (17); the renal function in our patient decreased before surgery, but it improved after surgery. Like this patient, other patients without renin suppression showed negative results for CCT but positive ones for SIT. Therefore, the optimal cutoff value of baseline PAC for predicting a positive CCT was higher than that for predicting a positive SIT. This is supported by ROC curve analysis in patients with renin suppression, which revealed that the optimal cutoff values of baseline PAC and ARR for CCT positivity were approximately identical to those for SIT positivity (Table 4). This indicates that cutoff values of PAC and ARR to omit confirmatory testing depend on renin activity.

Moreover, 21 patients with negative CCT result were diagnosed as having unilateral PA after positive SIT or FUT results and then underwent AVS. In addition, 6 patients with negative SIT result were diagnosed as having PA after positive CCT or FUT results. The Japan Endocrine Society had previously recommended performing 2 confirmatory tests for PA diagnosis (3). Therefore, at least 2 confirmatory tests were performed in many Japanese institutions. Individual patients with PA may present variations in PAC, PRA, and ARR results because those variables are affected by multiple factors, such as diurnal rhythm, posture, serum potassium level, and salt intake (18). Although the rate of unilateral PA among patients showing opposite results in CCT and SIT has not been reported, our results indicate that patients with unilateral PA may even show negative results in single confirmatory tests.

We considered the results of confirmatory tests as the gold standard for diagnosing PA. Many studies have reported the sensitivity and specificity of each confirmatory test, including the CCT and SIT (6, 19–21). It is reported that false negative rate in confirmatory tests may be as high as 36% to 43% (4). Of the 1164 patients who underwent both CCT and SIT in this study, 203 patients showed a positive CCT result but a negative SIT result, and 181 patients showed a negative CCT result but a positive SIT result. This means 33.0% of patients who underwent both tests showed contradictory results. We defined CCT positivity as ARR > 20 after captopril administration according to the guidelines of the Japan Endocrine Society (3) and the Japan Society of Hypertension (7). Studies have reported that PAC is a better parameter than ARR after captopril administration for diagnosing PA with a positive CCT result (21–23). SITs in our study were performed with patients in the recumbent position. Stowasser et al demonstrated that seated SIT was more sensitive than recumbent SIT for detecting both unilateral and bilateral forms of PA (24). Therefore, our patients with unilateral PA who showed negative results for recumbent SIT may show a positive result for seated SIT.

In our study, 26 patients who showed negative results in both CCT and SIT were diagnosed as having PA after a positive FUT result (Table 1). FUT is recommended as one of the confirmatory tests for PA diagnosis in Japan (3, 7), but not in other countries. In patients with baseline PRA ≤ 0.3 ng/mL/h, the cutoff value of baseline PAC that would give 100% specificity for predicting a positive result in FUT was identical to that in CCT (56.35 ng/dL) (data not shown). The AUC in the ROC curve assessing PRA at post-FUT to detect aldosterone-producing adenomas has been found to be similar to that of baseline ARR (25). Therefore, reassessments of the sensitivity and specificity of FUT for PA diagnosis are needed.

The main limitation of our study is its retrospective cross-sectional design. Therefore, a prospective study is needed to validate our results. However, our study had a multicenter design and a greater number of patients than any previously reported study. A second limitation is the inclusion only of patients with a definitive diagnosis of PA, comparing those with and without positive results on individual confirmatory tests. We did not compare patients with and without PA. This study aimed to identify the optimal cutoff values of PAC and ARR that would allow us to omit confirmatory testing. Therefore, it was important to compare patients who had positive CCT or SIT results and typical manifestations of PA (renin suppression) with patients who had a negative result on the CCT or SIT.

Conclusion

In conclusion, we found the cutoff values for PAC and ARR predicting a positive result on the CCT or SIT. In assessing patients with hypertension in whom PA is suspected, results exceeding those cutoff values may obviate the need to perform further confirmatory testing. This is particularly the case for those who present with suppressed PRA. These findings are important because they will allow patients presenting with typical PA manifestations to forego time-consuming and costly confirmatory tests.

Abbreviations

Abbreviations
 
• ARR

  aldosterone-to-renin ratio

 
• AVS

  adrenal venous sampling

 
• bPAC

  baseline plasma aldosterone concentration

 
• bPRA

  baseline plasma renin activity

 
• CCT

  captopril challenge test

 
• FUT

  furosemide upright test

 
• PA

  primary aldosteronism

 
• PAC

  plasma aldosterone concentration

 
• PRA

  plasma renin activity

 
• ROC

  receiver operating characteristic

 
• SIT

  saline infusion test

Acknowledgments

We thank the JPAS/JRAS study members for collecting the clinical data. We would like to thank Enago (www.enago.jp) for the English language review.

Financial Support: This research was supported by AMED, Japan (Grant Number JP17ek0109112 and JP19ek0109352), and the National Center for Global Health and Medicine, Japan (27–1402, 30–1008).

Additional Information

Disclosure Summary: The author reports no conflicts of interest in this work.

Data Availability: The datasets generated during and/or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.

References