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Adrenal insufficiency: Physiology, clinical presentation and diagnostic challenges

Adrenal insufficiency (AI) is a serious condition, which can arise from pathology affecting the adrenal gland itself (primary adrenal insufficiency, PAI), hypothalamic or pituitary pathology (secondary adrenal insufficiency, SAI), or as a result of suppression of the hypothalamic-pituitaryadrenal (HPA) axis by exogenous glucocorticoid therapy (tertiary adrenal insufficiency, TAI). AI is associated with an increase in morbidity and mortality and a reduction in quality of life. In addition, the most common cause of PAI, autoimmune adrenalitis, may be associated with a variety of other autoimmune disorders. Untreated AI can present with chronic fatigue, weight loss and vulnerability to infection. The inability to cope with acute illness or infection can precipitate life-threatening adrenal crisis. It is therefore a critical diagnosis to make in a timely fashion, in order to institute appropriate management, aimed at reversing chronic ill health, preventing acute crises, and restoring quality of life. In this review, we will describe the normal physiology of the HPA axis and explain how knowledge of the physiology of this axis helps us understand the clinical presentation of AI, and forms the basis for the biochemical investigations which lead to the diagnosis of AI.

An Intact ACTH LC-MS/MS Assay as an Arbiter of Clinically Discordant Immunoassay Results

BACKGROUND

Measurement of plasma adrenocorticotropic hormone (ACTH) is key in the differential diagnosis of hypothalamic-pituitary-adrenal disorders. Two-site sandwich immunoassays dominate clinical testing of ACTH in North America; however, discordant results between manufacturers have been repeatedly reported. To resolve the discrepancy, we developed a liquid chromatography–tandem mass spectrometry (LC-MS/MS) assay for the intended measurand, biologically active intact ACTH (iACTH).

METHODS

The multiple reaction monitoring LC-MS/MS assay was designed to selectively measure full-length iACTH, as well as ACTH analogs and fragments (i.e., ACTH1–24 and ACTH18–39). Epitope assignment of the Roche Elecsys antibodies was performed by MALDI-TOF mass spectrometry. A method comparison between Roche Elecsys and Siemens Immulite ACTH immunoassays was performed and clinically concordant/discordant results identified. In a subset of these samples, the iACTH concentration was determined using the LC-MS/MS method.

RESULTS

The lower limit of the measuring interval of the iACTH LC-MS/MS assay was 9 pg/mL (2 pmol/L). The assay was linear from 9 to 1938 pg/mL (2 to 427 pmol/L). Epitope mapping revealed that the Roche capture and detection antibodies bound residues 9–12 and 36–39 of ACTH, respectively. The iACTH LC-MS/MS analysis demonstrated that for discordant results between 2 immunoassays studied, only the Roche results were highly positively correlated with the iACTH concentration.

CONCLUSIONS

Immunoprecipitation of biologically active ACTH molecules followed by LC-MS/MS analysis enabled selective detection of iACTH and relevant biologically active fragments in plasma. Applied to the investigation of clinically discrepant results, this method can act as an arbiter of the concentration of iACTH present.

Falsely elevated plasma ACTH levels measured by the Elecsys assay related to heterophilic antibody in a case of secondary adrenocortical insufficiency

A 49-year-old woman with membranous nephropathy was referred to our hospital during the tapering of oral prednisolone, because of suspicion of primary adrenal insufficiency based on a plasma ACTH level of 399.1 pg/mL in the Elecsys assay and a serum cortisol level of 3.1 μg/dL. A rapid ACTH stimulation test revealed a suboptimal response, whereas a prolonged ACTH simulation test showed a sufficient increase in her urinary free cortisol. Also, big ACTH was not detected by gel exclusion chromatography. Therefore, we speculated that ACTH levels were falsely elevated due to some interference substances. Pretreatment of her plasma with either polyethylene glycol precipitation or a heterophilic blocking tube substantially reduced her ACTH values. When either the Immulite ACTH II or the TOSOH II ACTH was tried instead of the Elecsys ACTH, her plasma ACTH values turned out to be lower and appropriate for her clinical status. These results indicated that heterophilic antibodies interfered only with the Elecsys ACTH assay presumably by bridging the capture and tracer antibodies. To our knowledge, this is the first case in which the Elecsys ACTH assay yielded falsely elevated results. Regardless of the measurement system used, if there is a discordance between assay results and clinical findings, it should be considered to adopt additional procedures and/or another assay.

Heterophile Antibody to Adrenocorticotropin Hormone Interfering with the Investigation of Cushing's Syndrome

The practice of medicine depends on the accuracy of biochemical assays. The high prevalence of incidental masses on imaging necessitates a correct biochemical diagnosis before proceeding to radiological studies. Hormonal assays, tumour markers, and markers of cardiac injury are particularly susceptible to heterophile antibody interference which may lead to inaccurate and misleading results, inappropriate investigation and/or treatment, patient concern and potential harm. A case of heterophile antibody interference in the measurement of ACTH in a patient with Cushing's syndrome resulting in unnecessary invasive investigation is presented. Close collaboration and communication between laboratory and clinical staff is essential where laboratory results and the clinical picture are not congruent.

Assay-Specific Spurious ACTH Results Lead to Misdiagnosis, Unnecessary Testing, and Surgical Misadventure-A Case Series

The proper clinical evaluation of pituitary and adrenal disorders depends on the accurate measurement of plasma ACTH. The modern two-site sandwich ACTH immunoassay is a great improvement compared with older methods but still has the potential for interferences such as heterophile antibodies and pro-opiomelanocortin (POMC) and ACTH fragments. We report the cases of five patients in whom the diagnosis or differential diagnosis of Cushing syndrome was confounded by erroneously elevated results from the Siemens ACTH Immulite assay [ACTH(Immulite)] that were resolved using the Roche Cobas or Tosoh AIA [ACTH(Cobas) and ACTH(AIA), respectively]. In one case, falsely elevated ACTH(Immulite) results owing to interfering antibodies resulted in several invasive differential diagnostic procedures (including inferior petrosal sinus sampling), MRI, and unnecessary pituitary surgery. ACTH(Cobas) measurements were normal, and further studies excluded the diagnosis of Cushing syndrome. In three cases, either Cushing disease or occult ectopic ACTH were suspected owing to elevated ACTH(Immulite) results. However, adrenal (ACTH-independent) Cushing syndrome was established using ACTH(AIA) or ACTH(Cobas) and proved surgically. In one case, ectopic ACTH was suspected owing to elevated ACTH(Immulite) results; however, the ACTH(Cobas) findings led to the diagnosis of alcohol-induced hypercortisolism that resolved with abstinence. We have concluded that ACTH(Immulite) results can be falsely increased and alternate ACTH assays should be used in the diagnosis or differential diagnosis of clinical disorders of the hypothalamic-pituitary-adrenal axis.

Interference in ACTH immunoassay negatively impacts the management of subclinical hypercortisolism

PURPOSE

Low plasma corticotropin is considered a useful parameter for the diagnosis of subclinical hypercortisolism in patients with an adrenal incidentaloma. However, immunoassays are vulnerable to interference from endogenous antibodies. In this study, subjects who underwent Hypothalamus-pituitary-adrenal axis evaluation for the assessment of subclinical hypercortisolism were evaluated. The objective of the study was to ascertain whether antibody interference in corticotropin immunoassay affected the diagnostic work-up and clinical decisions.

METHODS

The 437 consecutive patients with incidentally discovered adrenal adenomas were included in this single centre study. Patients who had a combination of a nonsuppressed corticotropin concentration (>4.4 pmol/L) and a non-suppressed cortisol concentration after 1 mg overnight dexamethasone suppression test (>50 nmol/L) were selected. Eight eligible subjects without specific features of Cushing's syndrome were identified and recruited for interference studies and follow-up. Nine controls including one patient with unilateral adrenalectomy and one patient with Cushing's disease were recruited as well.

MEASUREMENTS

Eligible subjects and controls were subjected to hormonal tests and investigations for suspected interference. Interference studies included measurement of corticotropin on a different analytical platform, serial dilutions, polyethylene glycol precipitation and heterophilic antibody analysis. Patients were followed with clinical and laboratory parameters for a median duration of 30 (12-90) months.

RESULTS

Antibody interference was identified in four patients. Rheumatoid factor was responsible for the interference in one patient. Clinical management of the patients was affected by the erroneous results. Interference tests were negative in control subjects.

CONCLUSIONS

Erroneous results associated with analytical interference negatively impacted on clinical decision making in this patient group. This should be considered particularly in conditions such as subclinical hypercortisolism which decisions depend on laboratory investigations mainly. Analytical interference could explain the high variability observed both in field measurements from patients who were expected to have lower corticotropin concentrations and in subclinical hypercortisolism prevalence reported by different studies. Many problems can be resolved by ensuring good communication between clinical and laboratory staff.

Serum sample containing endogenous antibodies interfering with multiple hormone immunoassays. Laboratory strategies to detect interference

OBJECTIVES

Endogenous antibodies (EA) may interfere with immunoassays, causing erroneous results for hormone analyses. As (in most cases) this interference arises from the assay format and most immunoassays, even from different manufacturers, are constructed in a similar way, it is possible for a single type of EA to interfere with different immunoassays. Here we describe the case of a patient whose serum sample contains EA that interfere several hormones tests. We also discuss the strategies deployed to detect interference.

SUBJECTS AND METHODS

Over a period of four years, a 30-year-old man was subjected to a plethora of laboratory and imaging diagnostic procedures as a consequence of elevated hormone results, mainly of pituitary origin, which did not correlate with the overall clinical picture.

RESULTS

Once analytical interference was suspected, the best laboratory approaches to investigate it were sample reanalysis on an alternative platform and sample incubation with antibody blocking tubes. Construction of an in-house 'nonsense' sandwich assay was also a valuable strategy to confirm interference. In contrast, serial sample dilutions were of no value in our case, while polyethylene glycol (PEG) precipitation gave inconclusive results, probably due to the use of inappropriate PEG concentrations for several of the tests assayed.

CONCLUSIONS

Clinicians and laboratorians must be aware of the drawbacks of immunometric assays, and alert to the possibility of EA interference when results do not fit the clinical pattern.

Heterophilic antibody interference in immunometric assays

Immunometric assays are inherently vulnerable to interference from heterophilic antibodies, endogenous antibodies that bind assay antibodies. The consequences of such interference can be devastating. In this review, we discuss strategies that reduce the damage caused by heterophilic antibodies. Clinicians should only order blood tests that are indicated for the patient and clinical setting at hand, and have the confidence to question laboratory results discordant with the clinical picture. Laboratorians should familiarize themselves with the vulnerability of the assays they offer, and be able to perform and interpret adequate confirmatory measures correctly. When designing immunoassays, the immunoassay industry should invest the necessary resources in specific protective measures against heterophilic antibody interference. Examples include using antibody fragments and the addition of effective blockers to assay reagents. The increasing use of modified monoclonal mouse antibodies both in therapy and diagnostics could present a particular challenge in the future.