Assays / Hormones / Adrenocorticotrophin (plasma)

Clinical Use

1 Establishment of the aetiology of Cushing's syndrome.

2 Assessment of treatment of patients with Cushing's syndrome.

3 Differentiation of primary from secondary causes of adrenal insufficiency.

4 Localisation of excess ACTH production.

Applications
1 Aetiology of Cushing's syndrome.
Adrenocorticotrophin (ACTH) estimations are used to establish aetiology in patients with proven Cushing's syndrome. For this purpose, ACTH is usually measured on a single sample or on several samples during a dynamic test such as the CRH test.

2 Treatment of patients with Cushing's syndrome.

ACTH estimations help to:

• predict the likelihood of development of Nelson's syndrome after bilateral adrenalectomy;
• assess the effectiveness of pituitary irradiation, chemotherapy or surgery.
  

3 Differentiation between primary and secondary adrenal insufficiency.

In patients with proven adrenal insufficiency, measurement of ACTH in an 09.00h plasma sample permits distinction between pituitary and adrenal causes.

4 Localisation of excess ACTH production.

Selective venous sampling is helpful in locating the source of ACTH in patients where imaging is equivocal.

Estimation of ACTH is rarely indicated in the diagnosis of Cushing's syndrome, congenital adrenal hyperplasia, hirsutes, delayed puberty, hypopituitarism or during an insulin-induced hypoglycaemia test.

Patient Preparation
1,2,3 None. Between 09.00h and 10.00h, take blood (20 mL, St. Bart's; 7-10 mL, St.Thomas' and Newcastle) from a forearm vein using a plastic syringe. Failure to obtain blood at the first attempt may invalidate the results of subsequent successful venepuncture. In particularly anxious patients the ACTH concentration may rise to twice the upper limit of normal. In such patients sampling 30 min after the insertion of a butterfly is advised. Where patients are receiving replacement therapy with corticosteroids, the sample should be taken immediately before the morning dose of steroid. The time of the previous dose must be recorded.

4 This procedure should be carried out by a radiologist and endocrinologist experienced in the technique. Catheters are inserted into both inferior petrosal veins, via the femoral vein. A canula is also inserted into a peripheral vein. Protocols vary, but generally blood is taken simultaneously from the three sites before and 2, 5, 10 and 15 min after an intravenous injection of 100 mg of corticotrophin releasing hormone (CRH). Blood samples must be placed immediately in ice and, at the end of the procedure, taken to the laboratory for immediate separation. The plasma should be divided into two portions and stored at -200C.

Sample Preparation
Samples to be sent to London (St. Bart's):

Transfer 20 mL blood to two plastic tubes containing heparin. Within 10 min of venepuncture, separate the plasma using a refrigerated centrifuge. Transfer the plasma to two plastic tubes and immediately freeze both portions at -20C. Visible haemolysis invalidates the assay.

Samples to be sent to Newcastle and London (St. Thomas'):

Transfer the blood to two cooled plastic tubes or glass vacutainers containing EDTA. Within 10 min of venepuncture, separate the plasma using a refrigerated centrifuge. Transfer the plasma to two plastic tubes and immediately freeze at -20C. Falsely low results are caused by the presence of heparin. Samples in which this anti-coagulant has been used will be discarded. The minimum sample volume is 1.5 mL.

Send one sample from the pair to the SAS laboratory. Ensure the sample remains frozen during transport. Hold the remaining portion of the plasma frozen in reserve. Record on the SAS request form the time of day at which the sample was taken and the time and dose of any previous corticosteroid treatment.

Notes: Work at St. Thomas' showed that plastic and glass (siliconised and non-siliconised) vacutainers may be used for the collection of blood for their assay. The blood specimen should be separated as described above.

Reference Ranges:
For interpretation of ACTH results during dynamic tests, consult the appropriate Centre.

Inferior petrosal sinus sampling after CRH stimulation.
A gradient of the ACTH concentration of more than two-fold between one petrosal sinus and peripheral samples indicates a pituitary adenoma secreting excess ACTH.

Centres offering this assay
London (St. Bart's), London (St. Thomas'), Newcastle.

References
Landolt AM, Valavanis A, Girard J, Eberle AN. Corticotrophin-releasing factor-test used with bilateral, simultaneous inferior petrosal sinus blood-sampling for the diagnosis of pituitary-dependent Cushing's disease. Clin Endocrinol 1986; 25: 687-696

Oldfield EH, Doppman JL, Nieman LK, et al. Petrosal sinus sampling with and without corticotropin-releasing hormone for the differential diagnosis of Cushing's syndrome. N Eng J Med 1991; 325: 897 - 905.

Rees LH, Cook DM, Kendal JW, et al.. A radioimmunoassay for rat plasma ACTH. Endocrinology 1971; 89: 254-261.

Trainer PJ and Grossman A. The diagnosis and differential diagnosis of Cushing's syndrome. Clin Endocrinol 1991; 34: 317-330.

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