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Letters to the Editor |
1 Dipartimento di Medicina, Chirurgia e Odontoiatria, Università di Milano, Milano, Italy
2 Laboratorio di Analisi, Ospedale San Paolo Milano, Italy
3 Unità di Chirurgia Endocrina, Ospedale San Paolo Milano, Italy
aAddress correspondence to this author at: Dipartimento di Medicina, Chirurgia e Odontoiatria, Università di Milano, 20142 Milano, Italy. Fax 39-0289128221; e-mail alessandra.barassi{at}unimi.it.
To the Editor:
Intact (1-84) parathyroid hormone (PTH) has a short half-life and can be monitored during surgery to confirm the removal of all hyperfunctioning parathyroid tissue, but exact criteria for timing of sample collection and number of samples that best indicate a surgical cure are still to be perfected (1). We report the degree of accuracy of our protocol of performing only 2 intraoperative PTH tests in patients undergoing parathyroidectomy.
Sixty patients (43 women, 17 men; mean age 59.8 years, range 22–79 years), without renal failure, who underwent a minimally invasive surgical operation for sporadic primary hyperparathyroidism, participated in the study. Blood was collected from a peripheral vein before skin incision (basal or t-0) and 10 min after parathyroidectomy (t-10). The removal of the hyperfunctioning gland(s) was considered effective when the PTH decrease was 
50% from the t-0 value (2). We measured the t-0 concentration before skin incision because physical manipulation of the parathyroid glands during surgery often leads to false increases in PTH concentrations. Thus baseline PTH obtained after incision may be falsely increased, and PTH may show a misleading decrease and appear to meet cutoff criteria compared with the t-10 concentration, which might actually represent the true baseline PTH (3). When t-10 PTH did not decrease, we collected another blood sample, generally 35 min after parathyroidectomy (t-35). While awaiting the result of the last sample, we performed bilateral neck exploration and removed any enlarged glands. A subtotal parathyroidectomy was performed if the last sample did not show a PTH decrease or no other adenomas were found.
We measured PTH with an electrochemiluminescence immunoassay (Roche Diagnostics) performed on an Elecsys-2010 (Roche Diagnostics). The PTH assay detection limit was 1.2 ng/L, the assay analytical range was 1.2–5000 ng/L, and the reference interval was 15–65 ng/L. The total imprecision values (n = 12) were 5.9% and 4.3% at PTH concentrations of 1.8 ng/L and 11.7 ng/L, respectively. The definition of operative success was eucalcemia for 6 months or longer after parathyroidectomy (4).
With a considerable decrease of PTH concentration of 50% at t-10, the procedure had 1 (1.5%) false positive, 5 (8%) false negatives, 8 (13%) true negatives, and 46 (77%) true positives with 90% sensitivity, 89% specificity, 98% positive predictive value, 62% negative predictive value, and 90% overall accuracy. For the false positive result, the PTH decrease was 72%, and for the false negatives, true negatives, and true positives, respectively, the median (range) PTH decreases were 27% (5%–42%), 16% (5%–33%), and 82% (53%–92%). Considering the decrease at t-35, for the patients without a PTH decrease of 50%, the procedure had 1 (1.5%) false positive, 0 false negative, 0 true negative, and 59 (98.5%) true positives. The median (range) PTH decrease was 81% (53%–94%) for the true positives. In 59 of 60 patients the presented protocol led to correct prediction of postoperative eucalcemia with 100% sensitivity, 98.5% positive predictive value, and 98.5% overall accuracy. Thus the rapid PTH assay correctly identified all but 1 case of solitary adenoma and of multigland disease, including 3 cases of hyperplasia.
Because the last sample must be collected 10 min after excision and transport of samples to the laboratory took 5 min, the average total waiting time after parathyroidectomy was 
35 min.
The cost of 2 intraoperative PTH determinations was $13.28 on the Elecsys-2010 already in use in the laboratory. The cost with a STAT-IntraOperative-System (Future Diagnostics), was $1035.51 (2 samples and 6 calibrators plus 2 controls, all performed in duplicate) with 15 min of processing time making a total of 25 min after the parathyroidectomy. Thus, the overall costs for a paired intraoperative PTH assay were $89.86 ($13.28 for the reagents and $76.58 for the operating room, allowing for an additional 10 min awaiting laboratory results) and $1059.94 ($1035.51 for the reagents and $24.43 for 2 h of technician time) when performed in the laboratory and in the operating theater, respectively. If the intervention lasted until t-35, the overall costs for 3 intraoperative PTH assays was $96.49 ($19.91 for the reagents and $76.58 for 10 min use of the operating room) and $1194.63 ($1164.10 for the reagents and $30.53 for 2.5 h of technician time) when performed in the laboratory and in the operating theater, respectively.
The suggested procedure with only 2 intraoperative PTH tests performed in the laboratory seems the most appropriate to obtain the highest quality results at the lowest cost if the laboratory is in close proximity to the surgical site (transport time of 5 min).
References
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