September 2020 — Clinical Chemistry

The correct answer is Variation in blood draw site.

The function of parathyroid hormone (PTH) is to regulate the concentration of calcium and phosphorus in the blood by acting upon several target organs. PTH increases the release of calcium and phosphorus from the bones and increases calcium reabsorption while promoting phosphorous excretion from the kidneys. PTH also stimulates the production of the active form of vitamin D (1,25-dihydroxyvitamin D) in the kidneys, which increases absorption of calcium and phosphorus from the intestine (1).

Pathophysiological increases in parathyroid hormone are associated with hyperparathyroidism, which can be classified as primary, secondary, or tertiary. Secondary hyperparathyroidism most commonly occurs as a result of renal failure or vitamin D deficiency but can also occur in malabsorption syndromes (i.e. Crohn’s disease, celiac disease), pseudohyperparathyroidism, or with the use of certain medications (i.e. bisphosphonates, anticonvulsants) (2). Secondary hyperparathyroidism of renal origin proceeds by two main mechanisms. First, impaired glomerular filtration leads to decreased renal excretion of phosphate and subsequent hypocalcemia. Second, individuals with renal disease produce less 1,25-vitamin D, causing a decrease in calcium absorption from the intestine. Hypocalcemia stimulates the parathyroid gland to release more PTH, manifesting as hyperparathyroidism (3).

Physiological explanations for the spuriously elevated PTH result for this patient are unlikely. This patient has a longstanding history of secondary hyperparathyroidism of renal origin. Although his high serum creatinine concentrations do show a gradual decline in kidney function, his serum creatinine was higher a month prior than at the time of the questionable PTH result. Moreover, aside from PTH, all other laboratory results between the last two draws are consistent. If the PTH concentration had truly increased by more than four times the previous result, one would expect to see marked aberrations in other analytes as well. The second most common cause of secondary hyperparathyroidism, vitamin D deficiency, can also be ruled out. Vitamin D deficiency presents with high PTH, low serum calcium, and low or normal levels of phosphorus. At the time of the questionable PTH result, the patient had hyperphosphatemia.

After eliminating physiological explanations, pre-analytical or analytical reasons need to be considered. Analytical interference due to prescription and over-the-counter medications or supplements such as biotin can cause erroneous results in immunoassays that use biotinylated antibodies to capture analytes of interest, including assays for PTH (4,5). The direction of interference depends on the assay format. Competitive immunoassays may have falsely elevated results while non-competitive sandwich immunoassays will have falsely decreased results (6). There are several strategies to investigate biotin interference, including dilution studies or rerunning the sample on an alternate platform. To prevent biotin interference, patients should avoid taking supplements with biotin before sample collection. In this case, the questionable PTH result was measured using a sandwich immunoassay. The result in question was markedly increased compared to previous results, so biotin interference can be ruled out.

There are a number of pre-analytical factors that can influence a laboratory result. Fasting state, diet, patient posture, prolonged tourniquet use, tube additives, order of draw, and collection technique including draw site are just a few such variables. In this case, none of these variables are expected to cause the magnitude of change observed for this patient. The key to this case is that the patient had undergone a parathyroidectomy 16 years prior to manage his secondary hyperparathyroidism. It is common during total parathyroidectomies for the surgeon to implant the parathyroid gland into the patient’s forearm, known as auto-transplantation (7). The sample in question was drawn from the patient’s right forearm where his parathyroid tissue was implanted, resulting in a significantly elevated PTH concentration of >4100 pg/mL.

References
1. Tietz Textbook of Clinical Chemistry
2. Yang, Lang & Arnold, A & Brandi, M & Brown, E & D'Amour, P & Hanley, David & Rao, Sudhaker & Rubin, M & Goltzman, David & Silverberg, Shonni & Marx, S & Peacock, Munro & Mosekilde, L & Bouillon, Roger & Lewiecki, E.. (2009). Diagnosis of Asymptomatic Primary Hyperparathyroidism: Proceedings of the Third International Workshop. The Journal of clinical endocrinology and metabolism. 94. 340-50. 10.1210/jc.2008-1758.
3. McPherson, Richard and Pincus, Matthew R., "Henry's Clinical Diagnosis and Management by Laboratory Methods" (2017). Faculty Bookshelf. 81. https://hsrc.himmelfarb.gwu.edu/books/81
4. Waghray A, Milas M, Nyalakonda K, Siperstein AE. Falsely low parathyroid hormone secondary to biotin interference: a case series. Endocr Pract. 2013;19(3):451-455. doi:10.4158/EP12158.OR
5. Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150-1160.
6. Colon P, Greene D. Biotin Interference in Clinical Immunoassays. JALM. 2018;2(6):941-951.
7. Conzo G, Della Pietra C, Tartaglia E, et al. Long-term function of parathyroid subcutaneous autoimplantation after presumed total parathyroidectomy in the treatment of secondary hyperparathyroidism. A clinical retrospective study. Int J Surg. 2014;12 Suppl 1:S165-S169. doi:10.1016/j.ijsu.2014.05.019