Simple but Unrecognized: Spurious High PTH Levels from Blood Drawn from Arm with Parathyroid Autograft

Introduction: Surgical options for hyperparathyroidism uncontrolled by medical treatment include total parathyroidectomy with autotransplant of parathyroid tissue, most commonly to the forearm. Success of the procedure is demonstrated by a decrease of the systemic PTH level while successful autotransplant can be demonstrated by a higher PTH level from the ipsilateral arm. Here we noted highly variable PTH levels in an ESRD patient, which resulted from venous blood obtained from the left arm, downstream of a parathyroid autotransplant. Case Description: A 43 y/o woman with ESRD since a young age from Henoch Schoenlein Purpura and two failed diseased donor kidney transplants on chronic hemodialysis was admitted for complications of peripheral vascular disease. The patient also had a history of HTN, asthma, pulmonary hypertension, pulmonary embolism, cardiac arrest and superior vena cava syndrome. During her hospitalization the patient was dialyzed on a MWF schedule via a right upper arm arterovenous fistula. Her blood tests were notable for an initial PTH value of >2200pg/ml and cinecalcet was prescribed. The patient had parathyroidectomy with autotransplant of parathyroid tissue into her left forearm 17 years prior to presentation. Repeat PTH levels were obtained from her right arm, contralateral to the autograft, and were 110-160pg/ml, within the recommended target range. Cinacalcet was discontinued after two days. A review of laboratory data revealed at least one previous instance of a spurious high PTH. Discussion: The laterality and location of venopuncture is determined by practical concerns and is typically not documented in the test report. Highly discordant PTH levels can result if venous blood is obtained from the arm with a parathyroid autotransplant, which may lead to inappropriate treatment. There is very little literature on this phenomenon and its potential clinical implications. A case series of 4 patients inappropriately referred for parathyroidectomy to a single ENT practice suggests that it may not be infrequent. It is recommended that PTH levels are obtained from the arm contralateral to the autotransplant site to ensure that the values reflect systemic levels.