Papers in Press. First published July 9, 2004 as doi:10.1373/clinchem.2004.033159 Clinical Chemistry 50:9 000 000 (2004) Endocrinology and Metabolism Rapid Screening Test for Primary Hyperaldosteronism: Ratio of Plasma Aldosterone to Renin Concentration Determined by Fully Automated Chemiluminescence Immunoassays Frank Holger Perschel, 1* Rudolf Schemer, 3 Lysann Seiler, 4 Martin Reincke, 4 Jaap Deinum, 5 Christiane Maser-Gluth, 6 David Mechelhoff, 1 Rudolf Tauber, 1 and Sven Diederich 2 Background: The ratio of plasma aldosterone concentration to plasma renin activity (PAC/PRA) is the most common screening test for primary hyperaldosteronism (PHA), but it is not standardized among laboratories. We evaluated new automated assays for the simultaneous measurement of PAC and plasma renin concentration (PRC). Methods: We studied 76 healthy normotensive volunteers and 28 patients with confirmed PHA. PAC and PRC were measured immunochemically in EDTA plasma on the Nichols Advantage chemiluminescence analyzer, and PRA was determined by an activity assay. Results: In volunteers, PAC varied from 33.3 to 1930 pmol/l, PRA from 1.13 to 19.7 ng ml 1 h 1, and PRC from 5.70 to 116 mu/l. PAC/PRA ratios ranged from 4.35 to 494 (pmol/l)/(ng ml 1 h 1 ) and PAC/PRC ratios from 0.69 to 71.0 pmol/mu. In PHA patients, PAC ranged from 158 to 5012 pmol/l, PRA from 0.40 to 1.70 ng ml 1 h 1, and PRC from 0.80 to 11.7 mu/l. PAC/ PRA ratios were between 298 and 6756 (pmol/l)/ 1 Clinical Chemistry and Pathobiochemistry and 2 Endocrinology, Diabetes, and Nutritional Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Berlin, Germany. 3 Nichols Institute Diagnostics, Bad Vilbel, Germany. 4 Department of Internal Medicine II, University of Freiburg, Freiburg, Germany. 5 Department of Medicine, University Medical Center Nijmegen, The Netherlands. 6 Department of Pharmacology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany. *Address correspondence to this author at: Institut für Klinische Chemie und Pathobiochemie, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, D-12200 Berlin, Germany. Fax 49-30-8445-4152; e-mail frank.perschel@charite.de. Received February 23, 2004; accepted June 18, 2004. Previously published online at DOI: 10.1373/clinchem.2004.033159 (ng ml 1 h 1 ) and PAC/PRC ratios between 105 and 2328 pmol/mu. Whereas PAC or PRC showed broad overlap between PHA patients and volunteers, the PAC/ PRC ratio indicated distinct discrimination of these two groups at a cutoff of 71 pmol/mu. Conclusion: The PAC/PRC ratio offers several practical advantages compared with the PAC/PRA screening method. The present study offers preliminary evidence that it may be a useful screening test for PHA. Further studies are required to validate these results, especially in hypertensive cohorts. 2004 American Association for Clinical Chemistry Primary hyperaldosteronism (PHA) 7 is recognized as the most common endocrine form of secondary hypertension [see, e.g., Refs. (1 6)] with an estimated prevalence between 5% and 15% in the hypertensive population (7). Because diagnosis of PHA facilitates effective therapy, extended screening including normokalemic patients is widely accepted (7 9). The most common screening test for PHA is the ratio of plasma aldosterone concentration to plasma renin activity (PAC/PRA) (7, 9). Because the measurement of PRA requires special preanalytical prerequisites, is timeconsuming, and shows poor interlaboratory reproducibility (10 12), this strategy is not well recommended for screening of hypertensive patients in primary care centers. The lower limit of detection varies among the different PRA assays; therefore, the effect on the PAC/PRA 7 Nonstandard abbreviations: PHA, primary hyperaldosteronism; PAC, plasma aldosterone concentration; PRA, plasma renin activity; PRC, plasma renin concentration; RLU, relative light unit(s); APA, aldosterone-producing adenoma; and IHA, idiopathic hyperaldosteronism. 1 Copyright 2004 by The American Association for Clinical Chemistry
2 Perschel et al.: Rapid Screening Test for PHA ratio and the resulting cutoff values for PHA can be dramatic (12). The validity and usefulness of the PAC/ PRA ratio for screening for PHA has therefore been challenged (13 15). Recently developed immunoassays for measurement of circulating renin in plasma (PRC) may overcome this limitation. However, studies evaluating these assays with regard to the diagnosis of PHA are rare (16, 17). An additional problem is that the proposed cutoff values published to date are derived from measurements of aldosterone in either plasma (2, 3, 6, 11, 13, 15, 17) or serum (1, 4, 16) and are based on different assays. Because both PRA and PRC are measured in plasma and because a simple screening test should be done from only a single specimen, aldosterone should also be measured in plasma. We therefore compared the new ratio between plasma aldosterone concentration and plasma renin concentration (PAC/PRC) with the established PAC/PRA screening test in patients with PHA and in healthy volunteers to evaluate its clinical and diagnostic significance. Materials and Methods immunoassays and instrumentation The PAC and PRC were measured on a fully automated chemiluminescence analyzer (Nichols Advantage ; Nichols Institute Diagnostics). This system incorporates sensitive acridinium ester detection technology in combination with magnetic particles as solid phase. Samples, reagents, and magnetic particles are pipetted into disposable cuvettes and incubated at 37 C until the reaction is stopped by a washing step, and the emitted light is measured in relative light units (RLU). The system is calibrated by a two-point recalibration against a stored master curve. PRC was measured with the Nichols Advantage Direct Renin TM assay. This two-site immunometric assay uses an acridinium-ester-labeled monoclonal antibody, a second biotinylated monoclonal antibody, and streptavidincoated magnetic particles. To avoid prorenin activation during the assay, the incubation time is limited to 30 min at 37 C. This assay is calibrated to the WHO reference material (National Institute for Biological Standards and Control code 68/356). In an evaluation study (18) this PRC assay showed the following performance: Precision. The intraassay variation for three samples with various concentrations of renin was 1.7 5.3%. Interassay variation in seven samples with renin concentrations from 10 to 466.5 mu/l was between 2.7 8.2%. Accuracy. Dilution of four plasma samples with the sample diluent gave parallel lines whose slopes did not differ. Renin measurements in two series of mixtures prepared from two plasma samples with low and a high renin concentrations, respectively, yielded renin values between 86% and 111%. Limits of detection and functional sensitivity. The limits of detection, defined as the means plus 3 SD in two series of 20 runs of the sample diluent, were 0.013 and 0.094 mu/l, respectively. The functional sensitivity, defined as the sample renin concentration at which the CV for four series of five assay runs is 20%, was 2.65 mu/l; 1 mu/l is equivalent to 0.6 ng/l (18, 19). The aldosterone assay on Nichols Advantage is a competitive one-site immunometric assay, using a biotinylated monoclonal antibody bound to streptavidin-coated magnetic particles. Acridinium-ester-labeled aldosterone competes with sample aldosterone for the limited amount of biotinylated antibody. The cross-reactivity to any steroid hormones is negligible. According to the manufacturer s package insert, the assay has an analytical sensitivity of 33.3 pmol/l, and the dynamic range is 0 3330 pmol/l. The within-run imprecision (CV) at 119, 228, 547, 835, and 1990 pmol/l is 14.0%, 5.4%, 4.1%, 4.4%, and 2.9%, respectively. The total CV at 119, 228, 547, 835, and 1990 pmol/l is 18.6%, 8.5%, 5.2%, 6.3%, and 4.9%, respectively. Addition and dilution study results are between 88% and 110%. Parallelism results are between 92% and 102%. A method comparison with a commercially available RIA gave a correlation coefficient of 0.96 and a slope of 1.04; 1 pmol/l is equivalent to 0.36 ng/l. PRA was measured as described previously (10, 11). The intraassay variation of this assay was 3.9% at 11.7 ng ml 1 h 1 ; the interassay variation was 3.6% at 11.0 ng ml 1 h 1 and 6.2% at 3.29 ng ml 1 h 1, respectively. The detection limit was 0.40 ng ml 1 h 1. All laboratory testing was done by qualified staff of the Charité. The study was not performed as a blind study. Data analysis and estimation of cutoff values by use of ROC curve analysis were done with the Analyze-It software package (Analyze-It Software Ltd.). volunteers and patients We tested 76 healthy, normotensive volunteers [employees and students of the Charité; 23 males (age range, 22 75 years) and 53 females (age range, 16 69 years], who were not taking any relevant medications. Smoking and use of oral contraceptives were not exclusion criteria. Blood samples were drawn in an upright sitting posture. The patients studied included 28 patients with confirmed PHA [16 males and 12 females; mean (SD) age, 51 (13.1) years; age range, 26 72 years]: 9 patients had aldosterone-producing adenomas (APAs), 18 had idiopathic hyperaldosteronism (IHA), and 1 patient had glucocorticoid-suppressible hyperaldosteronism. The characteristics of these PHA patients are summarized in Table 1. The screening of our patients was done according to a generally accepted protocol (7 9), in which plasma aldosterone was measured with a commercially available RIA (not with the new immunometric assay) and had to be 416 pmol/l (150 ng/l). The ratio between PAC and PRA had to be 555 (pmol/l)/(ng ml 1 h 1 )[ (200 ng/l)/(ng ml 1 h 1 )].
Clinical Chemistry 50, No. 9, 2004 3 Patient Sex Age, years Diagnosis Table 1. Characterization of 28 patients with confirmed PHA. a Confirmatory tests Differential diagnostic tests Saline infusion test b 24-h Urinary aldosterone c Posture Adrenal vein test d CT e or MRT f sampling g 1 F 46 APA left 2 ND Surgery 2 F 52 APA right ND 1 Lateral. Surgery 3 M 38 APA left 2 Lateral. Surgery 4 M 49 APA right 2 ND Surgery 5 F 50 APA right 2 ND Surgery 6 F 52 APA left ND 2 Lateral. Surgery 7 F 41 APA right ND 1 Lateral. Surgery 8 F 50 APA left 2 NU Surgery 9 M 38 APA left ND 2 ND Surgery 10 F 34 IHA 1 NU Spiro. 11 F 62 IHA ND NU No lateral. Spiro. 12 F 63 IHA 1 ND Spiro. 13 M 53 IHA ND 2 No Lateral. Spiro. 14 M 72 IHA 1 ND Amiloride 15 M 31 IHA ND NU No lateral. Spiro. 16 M 54 IHA 1 ND Eplerenone 17 F 31 IHA 1 ND Spiro. 18 M 59 IHA NU 1 ND Spiro. 19 M 60 IHA ND 1 No lateral. Amiloride 20 F 54 IHA ND 1 ND Spiro. 21 M 63 IHA 1 ND Spiro. 22 M 72 IHA 1 ND Others 23 M 52 IHA ND 1 ND Spiro. 24 M 68 IHA NU 1 ND Spiro. 25 M 59 IHA 1 No lateral. Spiro. 26 F 55 IHA ND 1 ND Spiro. 27 M 65 IHA ND 1 ND Amiloride 28 M 26 GSH h ND 2 ND Dexa., spiro. a For details on test protocols, see Quinkler et al. (8) and Young (12). Only specific medications, which were used after confirmatory and differential diagnostic tests, were given. Most patients with IHA needed additional antihypertensive drugs. b Saline infusion test:, plasma aldosterone 230 pmol/l ( 85 ng/l) after 2Lofisotonic saline; ND, not done. c 24-h Urinary aldosterone:, urinary aldosterone 39 nmol/day and urinary sodium 200 mmol/day; ND, not done; NU, not usable (urinary sodium 200 mmol/day). d Posture test: 2, decrease in plasma aldosterone; 1, increase in plasma aldosterone; NU, not useable because of concomitant increase in serum cortisol. e CT, computed tomography; MRT, magnetic resonance tomography; spiro., spironolactone; GSH, glucocorticoid-suppressible hyperaldosteronism; Dexa., dexamethasone. f CT or MRT:, unilateral tumor;, no tumor. g Adrenal vein sampling: lateral., lateralization; ND, not done; NU, not usable because of unsuccessful catheterization. h Glucocorticoid-suppressible hyperaldosteronism was confirmed by genetic testing. Therapy Diagnosis was confirmed by successful surgery (all APAs), pathologic saline infusion test (7 9), and increased 24-h urinary aldosterone excretion on a high sodium diet (12) (for details see Table 1). Seven of the 28 patients were normokalemic [mean (SD) serum potassium, 3.4 (0.5) nmol/l; range, 2.0 4.3 mmol/l]. Systolic blood pressure was between 140 and 220 mmhg [mean (SD), 179 (21) mmhg], diastolic blood pressure was between 80 and 140 mmhg [mean (SD), 104 (12.5) mmhg]. The described characterization of the PHA patients was done routinely in the outpatient clinics of the departments of endocrinology at the Universities of Freiburg and Berlin. For this evaluation study, separate blood samples were drawn from all PHA patients in an upright sitting posture at 0800. The results presented in Tables 2 4 and Fig. 1 are from this standardized blood sampling. Although several authors have recommended PAC/PRA screening without discontinuing hypertensive medications (20 22), our study protocol for the evaluation of PAC/PRC was the following: Most of the patients were on antihypertensive therapy, but -blockers, angiotensin-converting enzyme inhibitors, angiotensin antagonists, and diuretics were stopped for at least 3 days and spironolactone for at least 4 weeks before blood sampling. Results evaluation of pac assay on nichols advantage The intraassay imprecision (CV) at concentrations of 114, 566, and 1906 pmol/l was 8.2%, 4.1%, and 4.4%, respec-
4 Perschel et al.: Rapid Screening Test for PHA Fig. 1. PAC, PRA, PRC, and the resulting PAC/PRA and PAC/PRC ratios for 76 healthy volunteers and 28 patients with PHA. The data points for PAC, PRA, PRC, and the ratios are plotted logarithmically;, healthy volunteers; F, patients with PHA. tively. The interassay CV at 128, 611, and 1956 pmol/l was 20%, 6.6% and 4.2%, respectively. The linearity on dilution was evaluated in two samples with concentrations of 525 and 1141 pmol/l. The measured concentrations were 76 100% of the expected. Parallelism in two samples of different concentrations was 89.5 118.3%. Calibration stability was evaluated with eight different calibrations. The CV was 3.0% for calibrator A (mean of 60 127 RLU) and 3.2% for calibrator B (mean of 23 062 RLU). The analytical sensitivity was 25 pmol/l. clinical results All methods tested (Fig. 1 and Table 2) differed significantly between PHA patients and healthy volunteers. Whereas single measurements of PAC, PRA or PRC concentrations showed broad overlap between both groups, the ratios PAC/PRA and PAC/PRC provided distinct discrimination between these two groups. PRC and PRA showed good correlation [Pearson regression coefficient (r) 0.72]. Because the same PAC values served for calculation of the ratios, PAC/PRC and PAC/PRA correlated similarly (r 0.73). As expected, differences occurred more frequently at the ends of lower renin concentration and activity ranges near the detection limits of both methods. ROC analysis (Table 3) showed the superiority of the ratios compared with PAC, PRA, and PRC alone. The PAC/PRC ratio performed as least as well as the PAC/ PRA ratio for differentiating PHA patients from healthy volunteers. With respect to the requirements of a screening test, we evaluated the cutoff values at a sensitivity of 100% (Tables 3 and 4). Discussion Because PHA is a common cause of secondary hypertension and its diagnosis can lead to cure or improvement of hypertension (APA) or targeted pharmacotherapy (IHA), validated and cost-effective routine screening protocols need to be established. At present, the most common screening test is the ratio Table 2. PAC, PRA, PRC, and the resulting PAC/PRA and PAC/PRC ratios for 76 healthy volunteers and 28 patients with PAH. Median (range) Method Units Healthy volunteers PHA patients PAC pmol/l 182 (33.3 1930) 751 (158 5012) PRA ng ml 1 h 1 4.64 (1.13 19.7) 0.57 (0.40 1.70) PRC mu/l 27.8 (5.70 116) 1.63 (0.80 11.7) PAC/PRA (pmol/l)/ 46.9 (4.35 494) 1393 (298 6756) (ng ml 1 h 1 ) PAC/PRC pmol/u 6.24 (0.69 71.0) 388 (105 2328) PAC ng/l 65.8 (12.0 69.6) 271 (56.9 1806) PRA ng ml 1 h 1 4.64 (1.13 19.7) 0.57 (0.40 1.70) PRC ng/l 16.7 (3.42 69.4) 0.98 (0.48 7.02) PAC/PRA (ng/l)/ 16.9 (1.57 178) 502 (107 2435) (ng ml 1 h 1 ) PAC/PRC ng/ng 3.75 (0.41 42.6) 233 (63.2 1398)
Clinical Chemistry 50, No. 9, 2004 5 Table 3. Calculated cutoff values and resulting specificity with respect to 100% sensitivity to differentiate patients with PHA from healthy volunteers. Method Units Optimum value for cutoff Sensitivity, % Specificity, % PAC pmol/l 155 100 43.4 PRA ng ml 1 h 1 1.7 100 94.7 PRC mu/l 5.2 100 89.5 PAC/PRA (pmol/l)/ 185 100 98.7 ratio (ng ml 1 h 1 ) PAC/PRC ratio pmol/mu 71 100 100 between serum or plasma aldosterone (PAC) and PRA. Because several authors have determined PAC/PRA cutoffs to screen for PHA (1 9), this procedure is widely accepted as the best validated screening protocol. However, simultaneous measurement of PAC and PRA has some disadvantages: The method for measuring PRA requires cooling of the specimen during transport and storage, is time-consuming, and shows weak interlaboratory reproducibility (10 12). Because aldosterone is usually measured in serum, determination of the PAC/PRA ratio requires an additional, simultaneously drawn, blood tube. In addition, PAC, PRA, and the PAC/PRA ratio show large intra- and interpatient variations in patients with PHA (15); therefore, several authors have cast doubt on the validity and sensitivity of the PAC/PRA ratio (13 15). Alternative screening procedures that overcome these disadvantages seem to be necessary. Measurement of PRC instead of PRA may reduce some preanalytical and analytical problems and, therefore, improve intra- and interlaboratory reproducibility (23, 24). Major advantages of the new PAC and PRC assays evaluated in our study include the ease and performance of the test (no cooling, a single plasma sample, and good interlaboratory reproducibility because the assays are automated) and the rapid availability of results (automated immunochemiluminometric assays), which is a prerequisite for extensive screening of hypertensive patients for PHA. The performance of the new PAC assay was evaluated with a preliminary reagent lot. However, our results confirmed the assay specification given by the manufacturer. Because this is the first study with a fully automated system and only two studies with manual PRC assays for Table 4. Proposed cutoff values for the PAC/PRC ratio with different units used in the literature. Cutoff value PAC unit PRC unit for PAC/PRC pmol/l mu/l 71 pmol/l ng/l 118 ng/l mu/l 26 ng/l ng/l 43 screening for PHA have been published to date (16, 17), the comparison of our data with data from the literature is limited. Trenkel et al. (16) suggested a cutoff value of 50 (aldosterone measured in serum by RIA, expressed in ng/l; PRC measured by IRMA, expressed in ng/l), which corresponds to 83 when using the units in Table 2 (PAC expressed in pmol/l, PRC in mu/l). Ferrari et al. (17) recommended a cutoff of 150 (aldosterone measured in plasma by RIA, expressed in ng/l; PRC measured by IRMA, expressed in ng/l) corresponding to 90 when using the units in Table 2. Our data analysis (Tables 2 and 3; Fig. 1) produced a tentatively proposed cutoff value of 71. Subsequent studies including essential hypertensive cohorts may necessitate readjustment, although there is good concordance with the recommendations made by Trenkel et al. (16) and Ferrari et al. (17). Although the simultaneous measurement of PAC/PRC in plasma samples from 28 patients with known PHA showed no overlap with healthy volunteers (Fig. 1), there are certain limitations of our study: The PAC/PRC ratio seems to be superior to the PAC/PRA ratio. Although the PAC/PRA ratio of our PHA patients in the initial outpatient characterization was clearly above the cutoff value of 555 (pmol/l)/(ng ml 1 h 1 ) [200 (ng/l)/ (ng ml 1 h 1 )], the standardized reexamination of these patients by use of a second blood sample showed that some patients would have failed this diagnostic criterion. Our proposed cutoff value (Table 3), which was chosen to achieve a 100% sensitivity with the new automated assay, was therefore somewhat lower than the values recommended in the literature. In addition, some patients with confirmed PHA had PACs in the standardized reexamination that were 414 pmol/l (150 ng/l) and therefore would also fail the second diagnostic criterion for positive screening of PHA. Because there is no doubt in the correct characterization of our patients as having PHA, we have two explanations for these findings: (a) a recent study by Tanabe et al. (15) has demonstrated high intraindividual variability of PAC, PRA, and the PAC/PRA ratio in patients with PHA; and (b) because our plasma aldosterone was measured with the new immunochemiluminometric assay, these data show that each PAC and PRC assay requires separate validation of cutoff values. Evaluating diagnostic tests in a group of patients already known to have the disease and in a group of healthy volunteers can lead to overestimation of diagnostic accuracy (25). Thus, our data necessarily need to be complemented by further studies in hypertensive cohorts, e.g., in groups of patients with essential hypertension and groups with renovascular disease. The clinical conditions necessary for testing of the PAC/PRC or PAC/PRA ratios, such as discontinuing drug therapy, recording of dietary sodium intake, or time of day for sampling, are not yet sufficiently standardized, which contributes to the different published cutoff values and complicates the use of the screening test in clinical
6 Perschel et al.: Rapid Screening Test for PHA routine. An adequate standardization that meets general acceptance is required. In summary, our results suggest that the simultaneous measurement of PAC and PRC with automated immunochemiluminometric assays may be useful in the screening for PHA. Nevertheless, subsequent studies are required for further comparison of the PAC/PRC ratio vs the PAC/PRA ratio used for screening at present. These studies will have to deal with the following questions: (a) Because we tested only for the differentiation of patients from normotensive volunteers, has the PAC/PRC ratio similar sensitivities for screening of large hypertensive populations? Prospective studies are needed. (b) Is the newly developed assay for PAC really a method that offers reliable results if performed by different laboratories in clinical routine? (c) How is the PAC/PRC ratio influenced by clinical conditions, especially drug effects? Regardless of the answers to these questions, we expect that the new ratio method will evolve as a rapid and easily performed screening test for PHA. We would like to acknowledge J. Wilde for statistical calculations and P. Exner, R. Göber, K. Hanusa, and M. Schrödter for excellent technical assistance. References 1. Fardella CE, Mosso L, Gomez-Sanchez C, Cortes P, Soto J, Gomez L, et al. Primary hyperaldosteronism in essential hypertensives: prevalence, biochemical profile, and molecular biology. J Clin Endocrinol Metab 2000;85:1863 7. 2. Loh KC, Koay ES, Khaw MC, Emmanuel SC, Young WF Jr. Prevalence of primary aldosteronism among Asian hypertensive patients in Singapore. J Clin Endocrinol Metab 2000;85:2854 9. 3. Calhoun DA, Nishizaka MK, Zaman MA, Thakkar RB, Weissmann P. Hyperaldosteronism among black and white subjects with resistant hypertension. Hypertension 2002;40:892 6. 4. Mosso L, Carvajal C, Gonzalez A, Barraza A, Avila F, Montero J, et al. Primary aldosteronism and hypertensive disease. Hypertension 2003;42:161 5. 5. Lim PO, MacDonald TM. Primary aldosteronism, diagnosed by the aldosterone to renin ratio, is a common cause of hypertension. Clin Endocrinol (Oxf) 2003;59:427 30. 6. Stowasser M, Gordon RD, Gunasekera TG, Cowley DC, Ward G, Archibald C, et al. High rate of detection of primary aldosteronism, including surgically treatable forms, after non-selective screening of hypertensive patients. J Hypertens 2003;21:2149 57. 7. Young WF Jr. Minireview: primary aldosteronism-changing concepts in diagnosis and treatment. Endocrinology 2003;144: 2208 13. 8. Quinkler M, Lepenies J, Diederich S. Primary hyperaldosteronism. Exp Clin Endocrinol Diabetes 2002;110:263 71. 9. Montori VM, Young WF Jr. Use of plasma aldosterone concentration-to-plasma renin activity ratio as a screening test for primary aldosteronism. A systematic review of the literature. Endocrinol Metab Clin North Am 2002;31:619 32. 10. Oelkers W, Diederich S, Bahr V. Diagnosis and therapy surveillance in Addison s disease: rapid adrenocorticotropin (ACTH) test and measurement of plasma ACTH, renin activity, and aldosterone. J Clin Endocrinol Metab 1992;75:259 64. 11. Oelkers W, Diederich S, Bahr V. Primary hyperaldosteronism without suppressed renin due to secondary hypertensive kidney damage. J Clin Endocrinol Metab 2000;85:3266 70. 12. Young WF Jr. Primary aldosteronism: management issues. Ann N Y Acad Sci 2002;970:61 76. 13. Montori VM, Schwartz GL, Chapman AB, Boerwinkle E, Turner ST. Validity of the aldosterone-renin ratio used to screen for primary aldosteronism. Mayo Clin Proc 2001;76:877 82. 14. Schwartz GL, Chapman AB, Boerwinkle E, Kisabeth RM, Turner ST. Screening for primary aldosteronism: implications of an increased plasma aldosterone/renin ratio. Clin Chem 2002;48:1919 23. 15. Tanabe A, Naruse M, Takagi S, Tsuchiya K, Imaki T, Takano K. Variability in the renin/aldosterone profile under random and standardized sampling conditions in primary aldosteronism. J Clin Endocrinol Metab 2003;88:2489 94. 16. Trenkel S, Seifarth C, Schobel H, Hahn EG, Hensen J. Ratio of serum aldosterone to plasma renin concentration in essential hypertension and primary aldosteronism. Exp Clin Endocrinol Diabetes 2002;110:80 5. 17. Ferrari P, Shaw SG, Nicod J, Saner E, Nussberger J. Active renin versus plasma renin activity to define aldosterone-to-renin ratio for primary aldosteronism. J Hypertens 2004;22:377 81. 18. De Bruin RA, Bouihuizen A, Diederich S, Perschel FH, Boomsma F, Deinum J. Validation of a new rapid renin assay. Clin Chem; in press. 19. Simon D, Hartmann DJ, Badouaille G, Caillot G, Guyenne TT, Corvol P, et al. Two-site direct immunoassay specific for active renin. Clin Chem 1992;38:1959 62. 20. Gallay BJ, Ahmad S, Xu L, Toivola B, Davidson RC. Screening for primary aldosteronism without discontinuing hypertensive medications: plasma aldosterone-renin ratio. Am J Kidney Dis 2001; 37:699 705. 21. Seifarth C, Trenkel S, Schobel H, Hahn EG, Hensen J. Influence of antihypertensive medication on aldosterone and renin concentration in the differential diagnosis of essential hypertension and primary aldosteronism. Clin Endocrinol (Oxf) 2002;57:457 65. 22. Mulatero P, Rabbia F, Milan A, Paglieri C, Morello F, Chiandussi L, et al. Drug effects on aldosterone/plasma renin activity ratio in primary aldosteronism. Hypertension 2002;40:897 902. 23. Cartledge S, Lawson N. Aldosterone and renin measurements. Ann Clin Biochem 2000;37:262 78. 24. Morganti A, Pelizzola D, Mantero F, Gazzano G, Opocher G, Piffanelli A, on behalf of the Italian Multicenter Study for Standardization of Renin Measurement. Immunoradiometric versus enzymatic renin assay: results of the Italian Multicenter Comparative Study. J Hypertens 1995;13:19 26. 25. Lijmer JG, Mol BW, Heisterkamp S, Bonsel GJ, Prins MH, van der Meulen JHP, et al. Empirical evidence of design-related bias in studies of diagnostic tests. JAMA 1999;282:1061 6.