C Springer-Verlag 1992
Urinary free cortisol versus 17-hydroxycorticosteroids : a comparative study of their diagnostic value in Cushing’s syndrome
T. Mengden, P. Hubmann, J. Müller, P. Greminger, and W. Vetter Departement für Innere Medizin, Universitätsspital Zürich
Summary. We evaluated the usefulness of the basal urinary 24-h excretion rates of free cortisol versus 17-hydroxycorticosteroids in the diagnosis of Cushing’s syndrome. On an outpatient basis, both urinary free cortisol and 17-hydroxycorticosteroids levels were determined in 48 patients with Cush- ing’s syndrome, as well as in 95 obese and 94 healthy control persons of normal weight. Deter- mination of the urinary free cortisol content al- lowed a clear-cut distinction between the patients with hypercortisolism and the controls, resulting in a sensitivity of 100% and specificity of 98% for the diagnosis of Cushing’s syndrome. The diag- nostic accuracy of urinary free cortisol was dis- tinctly superior to that of 17-hydroxycorticoster- oids, which showed a wide overlap of values be- tween the groups, with a sensitivity of 73% and a specificity of 94%. In conclusion, the measure- ment of basal urinary free cortisol provided an ex- cellent diagnostic sensitivity and specificity in the assessment of adrenocortical function. This simple and accurate test thus seems to be particularly use- ful in the outpatient evaluation of patients with suspected Cushing’s syndrome.
Key words: Cushing’s syndrome - Urinary cortico- steroids - Diagnosis
Although diseases of the adrenal cortex are rela- tively uncommon, many of the signs and symptoms of adrenal dysfunction are present in patients with nonadrenal diseases commonly seen in clinical practice. Thus, Cushing’s syndrome is suspected in a large number of patients with obesity and hy- pertension. However, in the absence of other spe- cific signs like proximal muscle weakness, osteopo- rosis, or spontaneous ecchymoses, a reliable diag- nosis cannot be made from clinical criteria alone [1,12,15].
Abbreviations : 17-OHCS=17-hydroxycorticosteroids; ACTH = adrenocorticotropic hormone; BMI=body-mass index; UFC=urinary free cortisol
As the diagnosis of Cushing’s syndrome has significant prognostic and therapeutic implica- tions, laboratory tests for the evaluation of hyper- cortisolism should be sensitive and specific. Other factors to be considered in selecting an appropriate test are simplicity, convenience of performance, and safety to the patient.
Two of the most commonly used laboratory parameters of adrenal cortical activity are the 24-h urinary excretion rates of 17-hydroxycorticoster- oids (17-OHCS) and free cortisol. They provide an integrated assessment of the amount of cortisol produced over a 24-h period and avoid the prob- lems of episodic secretion inherent in plasma cor- tisol assays [5, 20].
There are, however, multiple extraadrenal fac- tors that may alter adrenal function tests and cause errors in the diagnosis of hypercortisolism [1, 8]. In obese patients, the cortisol production rate may be increased, resulting in elevated urinary 17- OHCS, 17-ketosteroids, and, rarely, free cortisol levels and a possible erroneous diagnosis of Cush- ing’s syndrome [9, 16, 17].
The aim of the present study was to evaluate on an outpatient basis the diagnostic value of the basal 24-h urinary excretion rates of free cortisol versus 17-OHCS in patients with Cushing’s syn- drome as compared with normal-weight and obese controls.
Methods
From 1976 to 1991, patients with Cushing’s syn- drome as well as healthy lean and obese controls were consecutively included in this prospective study and on an outpatient basis evaluated for the presence or absence of hypercortisolism. Seventy- one patients had Cushing’s syndrome (48 women, 23 men, mean age 40, range 25-72 years) as diag- nosed by clinical findings, elevated levels of plasma and urinary steroids, and pathological low-dose dexamethasone suppression tests. The differential diagnosis was performed using the adrenocortico-
tropic hormone determination of plasma concen- tration (ACTH), high-dose dexamethasone sup- pression test result, and invasive and noninvasive localization procedures, including computed to- mography (CT), magnetic resonance imaging (MRI), and isotope scan. Of the 71 patients with Cushing’s syndrome, 42 (60%) had Cushing’s dis- ease, 10 (14%) adrenal adenoma, 9 (13%) ectopic ACTH syndrome, 7 (9%) adrenal carcinoma, and 3 (4%) primary nodular adrenocortical dysplasia.
Obesity was defined as a body-mass index above 30 kg/m2 [BMI = weight (in kg) over height squared (in m2)] and diagnosed in 143 subjects (106 women, 37 men, mean age 42, range 18-69 years, BMI 35±3.8 kg/m2). The normal control group consisted of 119 subjects with a body-mass index below 25 kg/m2 (71 women, 48 men, mean age 36, range 18-71 years, BMI 22.6±3.7 kg/m2).
Laboratory methods
The 24-h urine collections were performed on an outpatient basis and aliquots used for the determi- nation of urinary free cortisol (UFC) and 17- OHCS. UFC was determined by solvent extraction of the steroid from the urine and assay of the ex- tract by the competitive protein binding assay ac- cording to Murphy [10]. Urinary 17-OHCS (Porter-Silber chromogens extracted from urine specimens after hydrolysis with ß-glucuronidase) were assayed by the method of Peterson et al. [14]. The upper and lower limits of urinary corticoste- roids in our laboratories were defined as 20- 120 µg/24 h for free cortisol and 3-13 mg/24 h for 17-OHCS. For UFC, the interassay variability was 8.0% and intraassay variability 8.2%. Interassay variability for 17-OHCS was 5%.
Statistical analysis
For statistical analysis only those patients were considered in whom both free cortisol and 17- OHCS were determined from one 24-h urine col- lection. Results are expressed as means±SD. The nonparametric Mann-Whitney U-test was used for interindividual comparison of UFC and 17-OHCS, and P<0.05 was considered statistically signifi- cant.
Results
UFC and 17-OHCS concentrations could be deter- mined in 48 patients with Cushing’s syndrome, in 95 obese and 94 normal controls. Results are given in Figs. 1 and 2 and Table 1.
log UFC
10000
0
1000
100
g
0
8
8
0
0
Controls
Obesity
Cushing’s Syndrome
log 17-OHCS
100
·
8
8
10
8
0
D
8
Controls
Obesity
Cushing’s Syndrome
Urinary free cortisol values
All patients with Cushing’s syndrome excreted more than 120 µg UFC/24 h (mean 954.0±1424.0; range 140-7918 µg/24 h), whereas all of the normal control values were below that level (mean 49.6±
| Sensitivity (%) | Specificity (%) | Positive predictive value (%) | Negative predictive value (%) | |
|---|---|---|---|---|
| UFC | 100 | 98 | 92 | 98 |
| 17-OHCS | 73 | 94 | 75 | 93 |
22.6; range 12.4-119 µg/24 h; Fig. 1). In 4% of the obese controls, the values were higher than 120 µg/ 24 h (mean 50.87±28.1; range 15.7-155.3 µg/ 24 h), but their mean excretion rate was not differ- ent from that of the normal controls (49.3 vs. 50 µg/24 h; n.s.).
17-OHCS values
In contrast to the sharp distinction provided by the measurement of UFC, 17-OHCS values exhib- ited a considerable overlap between patients with adrenal dysfunction and control subjects (Fig. 2). Only 73% of the patients with confirmed Cush- ing’s syndrome exhibited elevated 17-OHCS values above 13 mg/24 h (mean 25.8+22.6; range 4.2- 113.9 mg/24 h). There was a considerable overlap with the control group, in which 2% of the normal and 11% of the obese subjects showed higher values than 13 mg/24 h.
Although the mean 17-OHCS value of the obese subjects (7.5±4.1; range 1.7-22 mg/24 h) was higher than that of the normal controls (6.9± 3.2; range 1.6-16.5 mg/24 h), this difference did not reach statistical significance (P=0.052).
Sensitivity and specificity
UFC was more sensitive and specific than 17- OHCS in the diagnosis of Cushing’s syndrome. The respective values for the sensitivity, specificity, positive and negative predictive value are given in Table 1.
Discussion
Cushing’s syndrome is an uncommon disease, but in clinical practice it is often considered in the dif- ferential diagnosis of hypertensive obese patients [13, 15]. Because of the severe prognostic implica- tions, outpatient screening tests for the assessment of hypercortisolism should have a high sensitivity. A variety of diagnostic methods for the assessment of adrenal function has been proposed [1, 8], but
many of these are either hampered by a lack of diagnostic accuracy or the need for hospitalization and inconvenience for the patient.
Provided that complete 24-h urine specimens are collected, the measurement of urinary cortico- steroids offers a reasonable index of the cortisol production rate over a 24-h period with the advan- tage that it can be performed on an outpatient basis.
For the diagnosis of Cushing’s syndrome the determination of urinary 17-OHCS proved of lim- ited value as a diagnostic parameter. Some 27% of the patients with Cushing’s syndrome had nor- mal excretion rates of 17-OHCS. In addition, in 11% of the obese controls, the 17-OHCS levels ranged above the normal limit. In contrast, the measurement of UFC provided a reliable index of adrenocortical hyperfunction and was elevated in all patients with Cushing’s syndrome. Its excellent sensitivity of 100% and specificity of 98% were clearly superior to those of 17-OHCS, which is in agreement with previous reports [2, 4, 10, 19]. The UFC seems to be particularly sensitive in the diag- nosis of hypercortisolism, as with increasing cor- tisol production rate, there is a disproportionate increase in UFC, exceeding the excretion of other urinary metabolites of cortisol [2].
According to the review of Crapo [3], the mea- surement of UFC has a sensitivity of 97% and specificity of 94%. The overnight dexamethasone suppression test is an excellent screening test for subjects suspected of having Cushing’s syndrome, but approximately 13% of obese controls give false-positive results [3].
Only two studies reported a higher specificity of 100% by the use of a modified 3 mg/d dexa- methasone-suppression test over 5 days [19] or the determination of the integrated concentration of plasma cortisol [21]. These procedures are, how- ever, rather laborious and not suitable for routine clinical use.
In obese patients, the cortisol production rate is often increased, and elevated levels of urinary corticosteroids may lead to the erroneous diagnosis
of Cushing’s syndrome. In our obese subjects, 4% of the free cortisol values but 11% of the 17-OHCS were elevated. Thus, there was a three times higher chance of a false-positive result in obese patients when using the 17-OHCS test. However, even the determination of UFC is not absolutely specific in massively obese patients.
The basal excretion rates of either free cortisol or 17-OHCS did not allow one to determine the specific etiology of the hypercortisolism. Very high levels are, however, often observed in patients with ectopic ACTH syndrome or adrenal carcinoma [18]. The differential diagnosis must be based on the determination of plasma ACTH, on functional tests (e.g., high dose dexamethasone suppression, corticotropin-releasing factor stimulation) and on noninvasive and invasive localization procedures [7,15].
The incidence of the different etiologies of Cushing’s syndrome in our patients was similar to that in a recent series [13]. Cushing’s disease accounts for approximately 60%-70%, primary adrenal tumors for 20%, and ectopic ACTH secre- tion for 15% of patients with hypercortisolism. Pri- mary nodular adrenocortical dysplasia probably represents a distinct, but rare subtype of Cushing’s syndrome [6, 18].
In conclusion, our results document the excel- lent diagnostic accuracy of UFC, which was remarkably superior to the 17-OHCS. If extraad- renal factors like nutrition, pregnancy, severe liver or renal disease, thyroid dysfunction, stress, or drugs are considered, the determination of UFC is a simple and reliable method for screening and definitively establishing the diagnosis of patients with suspected Cushing’s syndrome. Since the lower and upper limit of normally varied signifi- cantly in different studies, each laboratory must determine its own normal values.
Acknowledgements. We wish to thank Mrs. B. Küffer, Mrs. H. Seiler, Mrs. D. Schmid, and Mrs. I. Glesti for excellent technical assistance.
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Received: August 1, 1991 Returned for revision: October 23, 1991 Accepted: April 16, 1992
Prof. Dr. med. W. Vetter Departement für Innere Medizin Universitätsspital Zürich Rämistrasse 100 CH-8006 Zürich, Switzerland