Pheochromocytomas are a rare type of secretory tumor that arise from chromaffin cells within the adrenal glands. In the early stages of disease, patients are often asymptomatic, but as the tumor grows, individuals may experience symptoms such as hypertension, headaches, increased sweating, and episodes of unexplained generalized anxiety or dread. Pheochromocytomas secrete the hormone epinephrine, also known as adrenaline, and norepinephrine — collectively referred to as catecholamines. Patients with pheochromocytomas are at increased risk for experiencing a “catecholamine storm” that presents itself as a severe hypertensive crisis requiring emergency intervention.
Early identification is essential for patient care, but diagnosing a pheochromocytoma is challenging given the diffuse and sporadic nature of symptoms, a high proportion (50–60%) of asymptomatic individuals, and low population prevalence. Pheochromocytomas are diagnosed from the biochemical measurement of metanephrines, the metabolites of catecholamines, followed by imaging studies. As such, it is critically important that pheochromocytoma testing is accurate, sensitive, and specific.
To ensure high-fidelity results are released, clinical laboratorians must understand the biological role and properties of catecholamines and metanephrines and apply this knowledge to build processes that protect result integrity.
PREANALYTICAL SPECIMEN HANDLING: THE FOUNDATION FOR ACCURATE RESULTS
Preanalytical factors such as diet, medication use, stress, collection practice, and posture all affect catecholamine secretion. Exogenous factors that increase catecholamine secretion include caffeine, nicotine, strenuous exercise, acetaminophen, decongestants, and tricyclic antidepressants.
When placing an order, providers should discuss how these factors may alter test results and advise patients to avoid ingestion of products that stimulate catecholamine release. Catecholamines are produced in response to stress, which may even be noticeable from difficult or traumatic venipuncture. Phlebotomists should note difficult collections so that they can be viewed along with the result and clinical context. Venipuncture should be performed in the supine position, as standing and upright postures increase catecholamine secretion. Best practices state there should be a pause between needle insertion and blood collection to allow the patient time to recover from the stress of the initial puncture.
For metanephrines testing, the same collection practices should be observed. However, metanephrines are significantly less responsive to preanalytical variability compared to catecholamines. The timing of patient presentation also affects the measurement of catecholamines and metanephrines. In the setting of a secretory tumor, catecholamines are produced in a pulsatile fashion that correlates with symptomatic onset. Individuals who present in the absence of acute symptoms often have normal concentrations of catecholamines, which limits their diagnostic utility. Metanephrines, however, are produced at a relatively constant rate that is independent of symptomatic presentation. Due to decreased variability in response to preanalytical variables and constant rate of production, measurement of metanephrines is the preferred first-line test for diagnosis of pheochromocytoma.
Following specimen collection, blood samples should be placed on ice and spun down within the hour to prevent analyte degradation. Room temperature transport or delayed processing will falsely lower catecholamine concentrations, which may result in a missed or delayed diagnosis of a new or recurring tumor. By using the laboratory information system (LIS), laboratories can layer process improvement strategies throughout the sample collection protocol.
For example, the laboratory can program popup windows to alert the phlebotomist of special collection and handling instructions. Labels also may be programmed to signal to laboratory staff that samples should arrive on ice and be processed immediately upon arrival. Building a system with multiple checkpoints educates and empowers staff across the hospital to follow best practices and rapidly identify problems that affect specimen integrity.
ANALYTICAL TECHNIQUES INFORM CLINICAL INTERPRETATION
The analytical methodology is also an important consideration when following a patient with a newly diagnosed or established pheochromocytoma. The Endocrine Society provides diagnostic guidelines for pheochromocytoma, which endorse the measurement of metanephrines and catecholamines by either high-performance liquid chromatography (HPLC) or liquid chromatography-tandem mass spectrometry (LC-MS/MS).
Compared to HPLC and LC-MS/MS, immunoassays for catecholamines suffer from reduced analytical sensitivity, higher limits of detection, and an overall negative bias. The diagnosis of pheochromocytoma requires accurate testing with a wide dynamic measuring range, because patients with pheochromocytomas can have metanephrine or catecholamine levels 1,000 times higher than the reference range. On the other hand, monitoring recurrence requires a low limit of quantitation, as even small changes in metanephrine and/or catecholamine content may suggest early recurrence.
To optimize clinical sensitivity, pheochromocytomas are diagnosed by performing plasma metanephrine testing followed by imaging. Routine monitoring of diagnosed adrenal tumors can be performed by collection of catecholamines with or without metanephrines, a decision that should be made on a case-by-case basis and dictated by clinical history.
Laboratories can measure catecholamines and metanephrines in a single blood sample or a 24-hour urine collection. Due to the temporal variability in catecholamine secretion, spot urine tests are of little clinical value. From a practical standpoint, 24-hour urine collections may be more time intensive than plasma measurements; however, urine collections offer an alternative for patients with difficult vascular access or anxiety about venipuncture. Avoiding a difficult draw also preserves specimen integrity by eliminating stress as a source of preanalytical variability.
As demonstrated in the case of catecholamine testing, clinical sensitivity of endocrinology testing is influenced by preanalytical variability and the analytical approach. Designing robust systems that mitigate sources of variability and encourage interdepartmental collaboration serves to further improve laboratory testing and support evidence-based medicine.
Ashley Rackow, PhD, is a clinical chemistry fellow at Johns Hopkins University. Her clinical and research interests include main chemistry, endocrinology, toxicology, and therapeutic drug monitoring. +Email: [email protected]