Appropriate Laboratory Testing in the Screening and Work-up of Cushing's Syndrome

Transcription

1 Appropriate Laboratory Testing in the Screening and Work-up of Cushing's Syndrome SUSAN A. FUHRMAN, M.D. The wide array of tests available for the diagnosis of Cushing's syndrome can be daunting. This review discusses each test and its role in the diagnosis of patients with this disorder. The discussion is then followed by an algorithm that summarizes the appropriate work-up of Cushing's syndrome. (Key words: Cushing's syndrome; Cushing's disease; Adrenal hyperfunction; Hypercorticolism; Dexamethasone suppression test; Metyrapone test; Adrenal function) Am J Clin Pathol 1988;90: Author Biography: Susan A. Fuhrman, M.D. Susan A. Fuhrman divides her time between the laboratory and the university classroom in Minneapolis, Minnesota. An associate pathologist at Metropolitan- Mount Sinai Medical Center, she is also an assistant professor at the University of Minnesota in the Department of Laboratory Medicine and Pathology, co-directing a senior medical student elective in interpretation of laboratory tests. For her expertise in the clinical chemistry section of the university course, she was a co-recipient of the Minnesota Medical Foundations's Distinguished Teaching Award in 1986 and again in Dr. Fuhrman earned her S.B. from the Massachusetts Institute of Technology, Cambridge, in 1975 and her M.D. from the University of Michigan Medical School, Ann Arbor, in She completed a residency in laboratory medicine and pathology, with specialty training in chemical pathology, at the University of Minnesota in She is board-certified in AP/CP and chemical pathology. THERE ARE NUMEROUS TESTS available for the screening and evaluation of possible Cushing's syndrome. Since Cushing described this syndrome in 1932, there have been many advances in laboratory diagnostic evaluation of this disorder. A multitude of tests are currently used because there is no consensus as to which tests are superior for either the screening, detection, or definitive diagnosis of this disease. The discussion that follows will present the available tests and evaluate their diagnostic usefulness. Address reprint requests to Dr. Fuhrman: Department of Pathology, Mt. Sinai Medical Center, 2215 Park Avenue, Minneapolis, Minnesota Department of Pathology, Metropolitan-Mt. Sinai Medical Center, Minneapolis, Minnesota Screening for Cushing's Syndrome Overnight 1.0-mg Dexamethasone Suppression Screening Test The 1.0-mg overnight dexamethasone suppression screening test is very popular in the United States. It is superior to plasma Cortisol measurements, 8 A.M., 4 P.M. or 9 P.M., and to loss of diurnal variation as a method for detecting Cushing's syndrome. The test is performed by administering 1.0 mg of dexamethasone at 11 P.M. and obtaining a plasma Cortisol level at 8 A.M. the following morning. As recorded in the literature, eight cut-off values for postdexamethasone Cortisol levels have been identified, ranging from 3.5 Mg/dL to 10.0 Mg/dL. 813 Some authors have used the concept of decision levels for interpreting this test, with Cortisol levels of less than 5.0 Mg/dL indicating clear-cut nondisease (i.e., high sensitivity cut-off), values of greater than 10.0 Mg/dL indicating clear-cut Cushing's syndrome (high specificity cut-off), and values between 5.0 and 10.0 Mg/dL representing a gray area for which additional studies are necessary. 1 " 13 In practice the values frequently will be in this gray zone, and further confirmation of Cushing's syndrome will be necessary. Using a cut-off of less than 7.0 Mg/dL, this test has a sensitivity of about 98% and varying specificities, depending on the population tested. The test is 98% specific in thin, healthy adults; however, this specificity decreases to 87% in obese, healthy adults and to 77% in chronically ill patients. 813 Some of the false positive results seen have been shown to result from phenytoin treatment, depression, obesity, and estrogen use. Urinary Free Cortisol Test In contrast, the 24-hour urinary free Cortisol test is extremely sensitive and specific for the diagnosis of Cushing's syndrome. This test is extremely popular in Europe and has been the test of choice abroad for screening patients for Cushing's syndrome. It is gaining 345

2 346 FUHRMAN A.J.C.P. September 1988 significant popularity in the United States as well, and, because of its superior sensitivity and specificity compared with the overnight dexamethasone suppression test, it should be the test of choice for screening for Cushing's syndrome. The reference range for this test is method dependent; however, when reference ranges are appropriately determined for a given method and laboratory, the sensitivities and specificitie are close to 100%. 9 ' 10 Thus, further confirmatory testing to diagnose Cushing's syndrome is not needed when the 24-hour urinary free Cortisol level is elevated. The only drawback to the 24-hour urinary free Cortisol is that it is a timed 24-hour urinary collection and requires precise patient compliance. Both of the aforementioned screening tests (the overnight dexamethasone test and the 24-hour urinary free Cortisol) are relatively inexpensive outpatient procedures. Low-Dose Dexamethasone Suppression Test Because of the relative nonspecificity of the overnight 1.0-mg dexamethasone screening test, which requires confirmation in obese patients and others, the low-dose dexamethasone suppression test has been used as a confirmatory test for Cushing's syndrome. It is important to note that confirmation of Cushing's syndrome by means of the low-dose dexamethasone suppression test is not necessary if there is a clear-cut elevation in urinary free Cortisol because of the high specificity of the urinary free Cortisol in the diagnosis of Cushing's syndrome. The low-dose test is more complex than the overnight test and usually requires the patient to be hospitalized. The low-dose dexamethasone suppression test is performed by administration of 0.5 mg of dexamethasone every six hours for 48 hours, accompanied by baseline 24-hour and 48-hour plasma samples for Cortisol and/or baseline and two 24-hour urine collections for 17-hydroxysteroids. A normal response to this test is a plasma Cortisol level at 48 hours of less than 5.0 /tg/dl and urinary 17-hydroxy-steroid level of less than 5.0 mg/24 hours. Using these values, sensitivity is 97% with specificity of 99%. 3I3 False positive results have been seen with phenytoin and in cases of clinical depression. The difficulty encountered with false positive results resulting from phenytoin seen with both the overnight screening test and the standard low-dose test is related to increased dexamethasone clearance secondary to increased liver metabolism induced by the anticonvulsant. This activity results in suboptimal dexamethasone levels and can be a problem in all the dexamethasone suppression tests. At this point in the evaluation, a firm diagnosis of Cushing's syndrome can be made. After a diagnosis of Cushing's syndrome is confirmed, the next step is to determine a cause. Table. 1. Cushing's Syndrome: Causes I. ACTH mediated A. Pituitary (68%) 1. Adenoma (usually microadenoma) (40%) 2. Nonadenoma (hyperplasia)/hypothalamic (28%) B. Ectopic Cushing's syndrome (ACTH) II. Pituitary adrenocortical A. Adrenal (17%) 1. Adenoma / (9%) 2. Carcinoma / (8%) 3: Nodular hyperplasia / (?) Reprinted from Howanitz PJ, Howanitz JH. Hypercortixolism. Clin Lab Med I984;4: , with permission of the publisher. Determining a Cause The relative frequencies of causes for Cushing's syndrome are summarized in Table 1. As can be seen from this table, there are numerous causes for Cushing's syndrome. The most common cause is a pituitary adenoma or hyperplasia. Adrenal causes such as adenoma or carcinoma are much less common, as is ectopic ACTH syndrome. Neoplasms that result in Cushing's syndrome generally produce either ACTH (pituitary adenoma, or ectopic ACTH-producing tumors) or Cortisol; however, a recent case of a metastatic small-cell carcinoma of the prostate producing corticotrophin-releasing factor (CRf) and Cushing's syndrome has been reported. 5 There are numerous tests that have been used to determine the causes of Cushing's syndrome in a given patient. The most commonly used tests are plasma ACTH, high-dose dexamethasone suppression test, metyrapone test, computed tomography (CT) scan of the sella turcica, and CT scan of the abdomen. The number and diversity of these tests bear witness to the complexity and difficulty of determining the cause of this syndrome. Plasma ACTH The plasma ACTH concentration can be extremely helpful as an initial test in determining the cause of Cushing's syndrome. Plasma ACTH levels are, with few exceptions, undetectable (<20 pg/ml) in patients with adrenal adenomas or carcinomas. Several rare cases of adrenocortical carcinoma with normal or increased plasma ACTH levels and demonstrable ACTH production by tumor have been reported, 814 but these lesions are rare exceptions. Values within to slightly above the reference range are usually seen in patients with pituitary adenomas. Approximately one-third of patients with ectopic ACTH syndrome have ACTH values that are within the reference range, thus overlapping those with pituitary adenomas. The remainder have higher

3 Vol. 90 No. 3 PATHOLOGY PATTERNS 347 values that frequently are significantly elevated. Values in excess of pg/ml are very suggestive of an ectopic source. 4 " 17 Therefore, plasma ACTH levels are an appropriate initial method for quick identification of those patients with an adrenal tumor or with probable ectopic ACTH syndrome associated with massive secretion of plasma ACTH. Localization of adrenal and ectopic tumors that are suggested by abnormal plasma ACTH levels is accomplished by a combination of radiologic and other techniques. CT scanning of the abdomen is an excellent method of confirming and locating an adrenal tumor. 18 Radiologic and bronchoscopic examinations of the chest will usually reveal the source of ectopic ACTH production if that diagnosis is under consideration. Plasma ACTH levels are subject to diurnal variation in a similar fashion to Cortisol levels. Values drawn at 9 A.M. appear to afford the best discrimination between normals and those with various causes for their Cushing's syndrome. 12 ACTH is unstable and adheres to glass; therefore, care must be taken to use edetate (EDTA) anticoagulated plasma collected on ice in plastic syringes, and spun and stored frozen in plastic tubes. Discriminating Between Pituitary Adenoma and Ectopic ACTH Syndrome Because the vast majority of patients with Cushing's syndrome have pituitary adenomas, further evaluation is necessary to definitively separate pituitary adenoma from ectopic ACTH syndrome. Unfortunately, it is this differential diagnosis that can be difficult. Most patients with Cushing's syndrome will have a pituitary adenoma and a smaller proportion pituitary hyperplasia. These pituitary lesions often cannot be visualized accurately by current radiologic techniques. With newer surgical and radiologic modalities, the number of documentable adenomas may increase as these more sensitive methods are used to locate them. However, the diagnosis is currently made primarily by careful dynamic biochemical testing and interpretation of the results. This testing may not be definitive. In a recent review of Cushing's syndrome from the Mayo Clinic, the authors noted that in approximately one-third of their more than 250 cases seen since 1972, misleading data in either laboratory tests or isotopic or x-ray imaging were obtained. 6 This discordance in test results has been reported by others as well. 8 ' 11 ' 13 ' 19 The two dynamic biochemical tests most widely used for the differential diagnosis of Cushing's syndrome are the high-dose dexamethasone test and the metyrapone test. High-Dose Dexamethasone Test The high-dose dexamethasone test consists of 2.0 mg of dexamethasone every 6 hours for 48 hours, accompanied by baseline 24- and 48-hour plasma samples for Cortisol and/or baseline and two 24-hour urine samples for free Cortisol and 17-hydroxysteroids. Suppression of plasma or urinary 17-hydroxysteroids to values less than 50% of baseline indicates a pituitary origin of Cushing's syndrome. No suppression or suppression of less than 40% indicates ectopic ACTH or adrenal tumor. In a recent study by Ashcraft and associates, 3 an absolute cut-off point for serum Cortisol of 10 /ug/dl at 1600 hours after high-dose dexamethasone was suggested. This value gave a reported sensitivity for the diagnosis of pituitary adenoma of 100%, with a specificity of 92%. However, it should be noted that this cut-off point has been less well studied than the 50% suppression of baseline stated previously. This test generally has a sensitivity and specificity in the 90% range, although lower sensitivities have been reported. 18 Researchers at the Mayo Clinic 6 found, in analyzing their series of more than 250 cases, that approximately 12% of their patients with pituitary adenomas failed to suppress urinary metabolites or plasma Cortisol to less than 50% of baseline after administration of dexamethasone (sensitivity 88%). Crapo, 8 in a review of Cushing's syndrome, made several points regarding the high-dose dexamethasone suppression tests tha^t are not generally discussed. If the urine excretion of Cortisol or 17-hydroxysteroids is suppressed by 50% or greater of baseline during a 2.0-mg low-dose dexamethasone test, it is not necessary to do the high-dose dexamethasone test to demonstrate the suppression. On the other hand, it is occasionally necessary to extend the 8.0-mg high-dose test from two to three days in order to obtain 50% or greater suppression in patients with pituitary adenoma. He cautioned that the 50% suppression should not be applied rigidly because some patients with bona fide pituitary adenomas may suppress only 40% or 45% as compared with baseline. The high-dose dexamethasone test discriminates well between ACTH-dependent and ACTH-independent (adrenal tumor) Cushing's syndrome but can also have positive results in ectopic ACTH syndrome. In a study of 100 patients with Cushing's syndrome, Little and coworkers 15 found that 6% of patients with the ectopic ACTH syndrome and none of the patients with adrenal tumors had reproducible suppression of 17-hydroxysteroids of 40% or more of baseline after administration of high-dose dexamethasone. In Crapo's review of the literature, 8 no patient with an adrenal tumor was found to

4 348 have reproducibly suppressed 17-hydroxysteroids after administration of high-dose dexamethasone. Patients with nodular adrenal hyperplasia and ectopic ACTH syndrome appear to show suppression after high-dose dexamethasone up to 25% of the time (specificity 75%). One explanation of discrepancies that arise in the high-dose dexamethasone test is spontaneous cyclic changes in Cortisol production and ACTH secretion that have been demonstrated in patients with Cushing's syndrome of various causes. Patients with ACTH-secreting bronchial adenomas have been shown to have cyclic secretion of ACTH and thus may appear to show suppression when, in fact, ACTH production is independent of the dexamethasone and varying because of the inherent cyclic nature of secretion. This cyclic secretion has also been shown to occur in patients with pituitary adenomas, suggesting that a causal relationship between dexamethasone administration and suppression of plasma or urinary corticosteroids may not always exist in these patients with Cushing's syndrome. Thus, the reproducibility of dexamethasone suppression testing may be important in accurate differential diagnosis in some patients with Cushing's syndrome. 813 Metyrapone Test The other widely used test for the differential diagnosis of Cushing's syndrome is the metyrapone test. The metyrapone test can be a confusing test as described for the differential diagnosis of Cushing's syndrome. Metyrapone inhibits the 1 i-beta hydroxylase enzyme in the adrenal cortex, thereby blocking the conversion of 11-deoxycortisol to Cortisol. The decrease in plasma Cortisol results in a feedback signal to the pituitary that increases ACTH production and secretion. This ACTH then stimulates the adrenal cortex and induces an increase in the Cortisol precursor, 11-deoxycortisol. In patients with Cushing's syndrome resulting from pituitary adenoma, ACTH secretion is increased further because of intact feedback and pituitary corticotrophe function. In contrast, in patients with either adrenal tumors or ectopic ACTH secretion, ACTH secretion cannot be acutely increased because these patients have chronic pituitary corticotrophe suppression resulting from high Cortisol levels that are independent of the pituitary gland; thus, their pituitaries are essentially "turned off." Patients with pituitary neoplasms do not have "turnedoff" pituitaries and are expected to have a normal postmetyrapone 8 A.M. serum 11-deoxycortisol concentration (>10 Mg/dL), whereas patients with either adrenocortical neoplasm or ectopic ACTH syndrome will have suppressed 11-deoxycortisol concentrations (<7-10 Mg/dL), again, resulting from the pituitary's inability to FUHRMAN A.J.C.P. September 1988 increase ACTH production As part of this test, adequate suppression of Cortisol production must be documented by an 8 A.M. Cortisol of less than 7 Mg/dL. Carpenter, 6 describing the experience at the Mayo Clinic, found the overnight metyrapone test to be exquisitely sensitive (sensitivity 100%) in detecting pituitary adenomas in his population. He did not address the issue of specificity and ectopic ACTH syndrome. In a recent study by Sindler and associates, 19 the metyrapone test was found to be more accurate in distinguishing between adrenocortical neoplasm and pituitary adenoma than the high-dose dexamethasone suppression test. In this study, the dexamethasone suppression test had a sensitivity of 78% and a specificity of 86%, for an overall diagnostic accuracy of 81%. The metyrapone test had a sensitivity, specificity, and diagnostic accuracy of 100% in the same group of patients. As has been previously stated, the adrenocortical neoplasms fall out of the diagnostic scheme relatively early because of their very low basal ACTH values and the subsequent finding of an adrenal mass by CT scan. It must be remembered, however, that in the Sindler study the test analysis was based on distinguishing between pituitary adenomas and adrenocortical neoplasms. However, the diagnostic problem is more frequently in distinguishing between pituitary adenomas and ectopic ACTH syndrome. Unfortunately, in this study, as in others, there is very limited information on the response to metyrapone in ectopic ACTH syndrome. One patient in the series had ectopic ACTH syndrome, and this patient had discordant responses to metyrapone and high-dose dexamethasone testing, with 90% suppression after dexamethasone and a postmetyrapone 11-deoxycortisol level of less than 10 Mg/dL. This inconsistency in response to dexamethasone and metyrapone in ectopic ACTH syndrome has been described by several authors. 6,818 It is this inconsistency that has led many investigators to conclude that neither the dexamethasone nor metyrapone test is uniformly useful in differentiating ectopic ACTH syndrome from pituitary adenoma. Corticotrophin-Releasing-Factor Stimulation Test Another dynamic test that has received attention in the last few years is the corticotropin-releasing-factor (CRF) stimulation test. This test has been specifically cited as having possible usefulness in distinguishing pituitary adenoma from ectopic ACTH production. Patients with pituitary adenoma have increased blood levels of ACTH after injection of CRF, whereas those with ectopic forms of ACTH production do not increase their ACTH level in response to CRF stimulation. Initially, it was hoped that this test would be able to differ-

5 Vol. 90 No. 3 PATHOLOGY PATTERNS 349 CLINICAL FEATURES Step 1: ( + ) SCREENING TEST Urinary Free Cortisol (preferable test) OR l.tprng Dexamethasone H FIG. 1. Cushing's syndrome workup. Note: If you performed the 24- hour urinary free Cortisol as Step 1, you can skip Step 2 and confirm or exclude Cushing's syndrome on the basis of that test alone. *If there is discordance among these results, radiographic studies to clarify the location of the lesion will be necessary. LOU <20 pg/ml Adrenal Tumor Abd omen CT entiate completely between pituitary adenoma and the ectopic ACTH syndrome. 7 However, more recent reports have found a lack of sensitivity for the diagnosis of pituitary adenoma. In a recent article by Nieman and colleagues, 16 the CRF stimulation test had a diagnostic sensitivity, specificity, and accuracy of 88%, 100%, and 90%, respectively, for the diagnosis of Cushing's disease (pituitary adenoma). In this study, high-dose dexamethasone testing was done on these patients as well. By combining the results of both the CRF and the high-dose dexamethasone test, requiring negative results (no suppression or stimulation) from both, there was a diagnostic sensitivity of 100%. This outcome was at the expense of specificity, which decreased to 86%. This study concluded that Step 3: (+) P1tu 1tary Tumor Head CT Step 2: ( + ) Cushing's Syndrome 1 Plasma ACTH I pg/ml Metyrapone Test* H1gh-dose Dexamethasone Test* CRF Stimulation Test* \ 2.0-mg Dexamethasone (Low-Dose Test) OR Urinary Free Cortisol (if not already performed) i > Ecto p1c ACTH (-) Not Cushing's Syndrome HIGH >2QO pg/m Ectopic ACTH Chest Tomograms Chest CT Not Cushing's Syndrome the CRF test was as good as the high-dose dexamethasone test in discriminating between Cushing's disease and ectopic ACTH and, when taken in combination with the dexamethasone suppression test, the diagnostic utility of the CRF stimulation test could be improved significantly. However, as studied, it is not sensitive enough by itself to replace any of the tests in current use. Summary There is a wide array of tests available for the diagnosis of Cushing's syndrome. Despite this diagnostic armamentarium, the diagnosis can be difficult, and multiple tests frequently are needed to determine a specific cause. In most cases the combination of dynamic ma-

6 350 FUHRMAN AJ.C.P. September 1988 nipulations with biochemical testing and radiologic imaging will eventually disclose the specific cause. The algorithm in Figure 1 outlines a straightforward diagnostic approach that can be applied to most patients with Cushing's syndrome. References 1. Aron DC, Findling JW, Fitzgerald PA, Forsham PH, Wilson CB, Tyrrell JB: Cushing's syndrome: problems in management. Endocrine Rev 1982;3: Aron DC, Tyrrell JB, Fitzgerald PA, Findling JW, Forsham PH: Cushing's syndrome: problems in diagnosis. Medicine 1981;60: Ashcraft MW, VanHerle AJ, Vener SL, Geffner DL: Serum Cortisol levels in Cushing's syndrome after low- and high-dose dexamethasone suppression. Ann Intern Med 1982;97: Besser GM, Edwards CRW: Cushing's syndrome. Clin Endocrinol Metabol 1972;1: Carey RM, Varma SK, Drake CR Jr, et al: Ectopic secretion of corticotropin-releasing factor as a cause of Cushing's syndrome. N Engl J Med 1984;311: Carpenter PC: Cushing's syndrome: update of diagnosis and management. Mayo Clin Proc 1986;61: Chrousos GP, Schulte HM, Oldfield EH, Gold PW, Cutler GB Jr, Loriaux DL: The corticotropin-releasing factor stimulation test: an aid in the evaluation of patients with Cushing's syndrome. N Engl J Med 1984;310: Crapo L: Cushing's syndrome: a review of diagnostic tests. Metabolism 1979;28: Dunlap NE, Grizzle WE, Siegel AL: Cushing's syndrome: screening methods in hospitalized patients. Arch Pathol Lab Med 1985;109: Eddy RL, Jones AL, Gilliland PF, Ibarra JD Jr, Thompson JQ, McMurry JF Jr: Cushing's syndrome: a prospective study of diagnostic methods. Am J Med 1973;55: Gold EM: The Cushing's syndromes: changing views of diagnosis and treatment. Ann Intern Med 1979;90: Horrocks PM, London DR: Diagnostic value of 9 am plasma adrenocorticotrophic hormone concentrations in Cushing's disease. Br Med J 1982;285: Howanitz PJ, Howanitz JH: Hypercortisolism. Clin Lab Med 1984;4: Komonicky P, Spark RF, Melby JC: Treatment of Cushing's syndrome with trilostane, an inhibitor of adrenal steroid biosynthesis. J Clin Endocrinol Metabol 1978;47: Little GW, Nicholson WE, Island DP, et al: Clinical and laboratory studies of ectopic humoral syndromes. In: Pincus G, ed. Recent progress in hormone research. New York: Academic Press, 1969;26: Nieman LK, Chrousos GP, Oldfield EH, Avgerinos PC, Cutler GB Jr, Loriaux DL: The ovine corticotropin-releasing hormone stimulation test and the dexamethasone suppression test in the differential diagnosis of Cushing's syndrome. Ann Intern Med 1986;105: RatclirFe JG, Knight RA, Besser GM, Landon J, Stansfeld AG: Tumour and plasma ACTH concentrations in patients with and without the ectopic ACTH syndrome. Clin Endocrinol 1972;1: Scully RE, Mark EJ, McNeely WF, McNeely BU: Case study. Case Records of the Massachusetts General Hospital 1986;315: Sindler BH, Griffing GT, Melby JC: The superiority of the metyrapone test versus the high-dose dexamethasone test in the differential diagnosis of Cushing's syndrome. Am J Med 1983;74: