Recognizing discrepancies between blood tests of adenocorticotropic hormone (ACTH) levels and patients’ clinical data is important for clinicians to avoid unnecessary testing or misdiagnosis, a case series highlights.
The study, “Corticotropin hormone assay interference: A case series,” was published in the journal Clinical Biochemistry.
Cushing’s syndrome comprises a group of disorders characterized by abnormally high levels of cortisol in the body. Its main cause is the long-term exposure to cortisol-like medications, called exogenous Cushing’s syndrome, but it can also be caused by the abnormal production of cortisol by the body, called endogenous Cushing’s syndrome.
Endogenous Cushing’s syndrome may be caused by a tumor in the pituitary gland that secretes large amounts of ACTH, which, in turn, overstimulates the adrenal glands to produce cortisol — called Cushing’s disease. It can also be caused by a cortisol-producing tumor in the adrenal gland or by ACTH-producing tumors outside the pituitary gland.
The blood levels of ACTH are different between Cushing’s syndrome patients, depending on the cause of the disease. Patients with Cushing’s syndrome due to adrenal tumors have low blood ACTH levels — as their healthy pituitary gland detects cortisol levels and decreases ACTH production when cortisol is high — while patients with pituitary tumors or ACTH-producing tumors have high levels of ACTH.
Thus, precise and accurate measurements of blood ACTH levels are essential to help distinguish between adrenal and pituitary problems and clarify the type of Cushing’s syndrome in a patient.
ACTH is measured by a “sandwich immunoassay,” where two different molecules, or antibodies, are used against ACTH to calculate its levels in a blood sample. However, patients’ own antibodies and medications can interfere with the results of such assays — known as assay interference — and lead to misdiagnosis and unnecessary or incorrect testing or procedures.
While the frequency of potential interfering agents is reported to be high, clinically relevant interference has been reported to occur in only 0.5-3% of samples. Nevertheless, the recognition of potentially inaccurate results is crucial to providing the best clinical practice.
Researchers have now reported the cases of 12 patients (eight women and three men at a mean age of 43) in whom discrepancies between the clinical data and ACTH results raised concerns of assay interference.
Of these patients, nine were being investigated for possible Cushing’s syndrome and the remaining three for adrenal insufficiency. However, all patients showed high ACTH blood levels according to results of an ACTH assay.
To clarify whether assay interference was affecting these results, the team conducted several tests and measured ACTH blood levels using a different platform that uses different antibodies to detect ACTH.
For all patients, repeating the same assay using serial dilutions of the blood sample showed a nonlinear dose response, and measuring ACTH levels with a different platform resulted in low blood ACTH levels.
These results strongly indicated that assay interference led to falsely elevated blood ACTH levels in all patients. And as a consequence, several patients underwent unnecessary laboratory testing or radiological imaging.
The team noted that among the several tests conducted, repeating the measurement of ACTH levels using serial dilutions and using another platform most reliably confirmed the presence of interfering agents, and should be used when discrepancies between the clinical data and ACTH results are noted.
“These cases illustrate the importance of clinical vigilance and laboratory communication when results are incompatible with the clinical picture — a step than can avoid unnecessary costly investigations,” the researchers wrote.
While the tests could not clarify the nature of the interfering agent, the researchers noted that the data pointed to the presence of interfering antibodies, which may be neutralized with a specific compound called polyethylene glycol.