68 nature clinical practice endocrinology & metabolism february 2009 vol 5 no 2

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cOMMentarY

The function of the hypothalamo–pituitary–adrenal (HPA) axis is routinely assessed by the standard-dose short synacthen test (SST), in which 250 µg of synthetic corticotropin is used to stimu-late cortisol secretion by the adrenal glands. In patients with secondary adrenal failure (e.g. caused by pituitary tumors or recovery from prolonged corticosteroid therapy), the lack of endogenous adrenocorticotropic hormone (ACTH) manifests as a reduced-peak cortisol response following exogenous corticotropin administration.

Many studies reported that the SST has advan-tages over the ‘gold standard’ methods used for assessing the HPA axis, such as the insulin-induced hypoglycemia test and the metyrapone test.1 The SST generates few false-positive and false-negative results, it can be performed at any time of the day and in outpatient settings and it is safer and cheaper than alternative tests; however, it does have potential pitfalls. In situa-tions in which the trophic drive from the pituitary gland to the adrenal glands is changing acutely (e.g. after pituitary surgery or pituitary apoplexy), the adrenal glands need some time to ‘reset’ to the new ACTH levels, during which time the SST might produce false-positive results. According to Dickstein et al.,2 false-positive results are due to

SUMMARYIn the standard corticotropin-stimulation test, which is used to assess the function of the hypothalamo–pituitary–adrenal axis, synthetic corticotropin is administered at a dose of 250 µg. Previous studies suggested that tests that use lower corticotropin doses had better discrimination than standard-dose tests possibly by avoiding supraphysiological concentrations of corticotropin. Here, we discuss a meta-analysis by Kazluaskaite et al. that compared standard-dose and low-dose corticotropin-stimulation tests and concluded that the low-dose test has a superior sensitivity and specificity. Superiority over the standard-dose test, however, was only found following a series of internal corrections and adjustments conducted by the investigators that might be inappropriate. Unadjusted meta-analyses demonstrated no advantage of the low-dose test over the standard-dose test. As data on the long-term follow-up of patients with borderline results are only available for the standard 250 µg dose test, this test should continue to be advocated, with assay-specific cut-off levels.

Keywords corticotropin, corticotropin-stimulation test, cortisol, hypothalamo–pituitary–adrenal insufficiency, short synacthen test

the saturation of the normal ACTH–cortisol dose response. As circulating ACTH concentrations can reach 60–70 ng/l after the administration of corticotropin doses as low as 5–10 µg,3 the 250 µg dose used in the conventional SST might be too high. To overcome this difficulty, Dickstein and colleagues proposed the use of a low-dose SST with 1 µg corticotropin. The performances of the low-dose SST and standard-dose SST were subsequently compared in several studies, but none reported convincing evidence in support of the superiority of the low-dose SST. Studies showed almost identical cortisol profiles follow-ing either test,3 and an extensive meta-analysis by Dorin et al.4 demonstrated that low-dose SST and standard-dose SST perform very similarly in the diagnosis of HPA insufficiency. These observa-tions suggest that saturation of the ACTH–cortisol dose response might not be avoided, even with the 1 µg dose.

The meta-analysis by Kazlauskaite et  al.5 reopens the debate on the use of low-dose SST versus standard-dose SST. Kazlauskaite et  al. pooled data of 679 individuals from 23 studies (many of which had been also analyzed by Dorin et al.4) and reported that the low-dose SST was associated with a higher area under the receiver–operator characteristic curve than the standard-dose SST (0.92, 95% CI 0.89–0.94 versus 0.79,

PM Stewart is a Professor of Medicine and Endocrinology at the Institute of Biomedical Research, University of Birmingham, Queen Elizabeth Hospital and PM Clark is a Consultant Clinical Scientist and Head of the Regional Endocrine Laboratory at University Hospital Birmingham National Health Service Foundation Trust, Birmingham, UK. 

Correspondence*Department of Endocrinology University of Birmingham and Queen Elizabeth Hospital Edgbaston Birmingham B15 2TT UK p.m.stewart@bham.ac.uk

Received 17 September 2008 Accepted 29 October 2008 Published online 2 December 2008

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The low-dose corticotropin-stimulation test revisited: the less, the better? Paul M Stewart* and Penelope M Clark

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Competing interestsThe authors declared no competing interests.

95% CI 0.74–0.84, respectively). The investigators, who applied the same cortisol cut-off values for all studies, concluded that the low-dose SST is superior for diagnosing chronic HPA insufficiency. Importantly, the superiority of the low-dose SST was only found after internal corrections were made to adjust for variation in the cortisol assays. Kazlauskaite et al. excluded five studies that used fluorescence polarization immunoassay (FPIA), on the basis of the reduced specificity of this assay compared with others. Furthermore, for the two studies in which cortisol levels were measured in plasma samples, the investigators used a published correction factor to convert plasma cortisol levels to their expected serum value. In an unadjusted analysis, Kazlauskaite et al. did not find any signifi-cant differences in the performance between the low-dose SST and standard-dose SST—similar to the findings of Dorin et al. 

In our opinion, the approaches taken by Kazluaskaite et al. to harmonize the cortisol results obtained in different studies are inappropriate. First, we believe that the exclusion of the studies that used FPIA is unjustified. Currently used cortisol immunoassays use a variety of markers to label the anticortisol antibody, and the specifi-city of these immunoassays is determined mainly by the specificity of the antibody, rather than by the label. Differences in antibody specifity might be a valid reason for excluding some studies, but Kazlauskaite et al. have not applied this criterion consistently, only for FPIA. Moreover, the authors did actually include one study that used FPIA.6 Second, we also disagree with the correction of the cortisol results according to sample type. The differences between serum and plasma concentra-tions of any component are assay-specific and depend on how the plasma sample was prepared. The correction by Kazlauskaite et al. was based on a single study that compared serum cortisol levels with cortisol levels in plasma treated with ethylenediaminetetraacetic acid (EDTA), as measured by radioimmunoassay. Importantly, the manufacturers indicate that this radio-immunoassay can only be used with heparin-treated plasma samples because the assay results differ significantly when EDTA-treated samples are used. The concept of a mathematical correc-tion of results to allow for the effects of sample variety on assay specificity and calibration is likely to be flawed. Third, we believe that the use of the

same cortisol cut-off levels for all studies might have biased the findings of Kazlauskaite et al. As the various cortisol assays are not harmonized, cut off-levels for any cortisol assay should be assay-specific.

After considering these reservations, a peak cortisol level <440 nmol/l is a very strong predic-tor of HPA insufficiency, irrespective of the corti-sol assay used. Values close to the cut-off levels of any test might be difficult to interpret, but far more data are available on the follow-up of patients with ‘borderline’ values obtained by the standard-dose SST than by low-dose SST.7 This advantage of the conventional SST needs to be considered alongside the inherent difficulty of its reproducibility and the additional costs of pre-paring and administering a 1 µg low-dose SST from a 250 µg vial. In conclusion, the use of the low-dose SST to diagnose HPA insufficiency not justified; the standard-dose SST should continue to be used, with assay-specific cut-off levels.

References1 Stewart PM et al. (1998) A rational approach for

assessing the hypothalamo–pituitary–adrenal axis. Lancet 1: 1208–1210

2 Dickstein G et al. (1991) Adrenocorticotropin stimulation test: effects of basal cortisol level, time of day, and suggested new sensitive low-dose test. J Clin Endocrinol Metab 72: 773–789

3 Mayenknecht J et al. (1998) Comparison of low- and high-dose corticotropin stimulation tests in patients with pituitary disease. J Clin Endocrinol Metab 83: 1558–1562

4 Dorin RI et al. (2003) Diagnosis of adrenal insufficiency. Ann Intern Med 139: 194–204

5 Kazlauskaite R et al. (2008) Corticotropin tests for hypothalamic-pituitary-adrenal axis insufficiency: a meta-analysis. J Clin Endocrinol Metab [doi: 10.1210/jc.2008-0710]

6 Abdu TA et al. (1999) Comparison of the low-dose short synacthen test (1 µg), the conventional-dose short synacthen test (250 µg), and the insulin-tolerance test for assessment of the hypothalamo-pituitary-adrenal axis in patients with pituitary disease. J Clin Endocrinol Metab 84: 838–843

7 Agha A et al. (2006) The long-term predictive accuracy of the short synacthen (corticotropin) stimulation test for assessment of the hypothalamic-pituitary-adrenal axis. J Clin Endocrinol Metab 91: 43–47

Practice PointOn the basis of current evidence the 1 µg low-dose corticotropin-stimulation test should not be used to diagnose hypothalamo–pituitary–adrenal insufficiency. The 250 µg standard-dose corticotropin-stimulation test, used with assay-specific cut-off levels, should continue to be advocated.

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