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Clinical Chemistry 46: 1662-1668, 2000;
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(Clinical Chemistry. 2000;46:1662-1668.)
© 2000 American Association for Clinical Chemistry, Inc.

Articles

Intraoperative Parathyroid Hormone Analysis: A Study of 200 Consecutive Cases

Lori J. Sokoll1,a, Helen Drew1 and Robert Udelsman2

Departments of
1 Pathology and
2 Surgery, Johns Hopkins Medical Institutions, Baltimore, MD 21287.
a Address correspondence to this author at: Department of Pathology, Johns Hopkins Medical Institutions, 600 N. Wolfe St., Meyer B-125, Baltimore, MD 21287. Fax 410-614-7609; e-mail: lsokoll{at}jhmi.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Background: Immunoassays for parathyroid hormone (PTH), with short incubation times and results available in <15 min, have allowed intraoperative monitoring of the success of parathyroid surgery. The purpose of this study was to evaluate the analytical performance of a rapid PTH assay and its clinical performance in a series of 200 patients.

Methods: PTH was measured with a modified immunochemiluminometric assay with a 7-min incubation time (QuiCk-IntraOperativeTM Intact PTH assay). The rapid assay was compared with results in a central laboratory (immunoradiometric assay) in 44 EDTA-plasma specimens. The rapid assay was used intraoperatively in 200 consecutive cases with specimens analyzed before and 5–10 min after resection of the hypersecreting parathyroid gland(s).

Results: Intraassay imprecision was 12% at 28 ng/L and 11% at 278 ng/L. Regression analysis of results of the rapid PTH assay and the IRMA PTH assay in 44 parathyroidectomy patients yielded y = 1.26x - 12 ng/L, Sy|x = 26.3 ng/L, r = 0.984, and in 40 of 44 patients with values <200 ng/L, y = 1.02x + 1.9, Sy|x = 13.9, r = 0.947. In the 195 cases using intraoperative PTH testing with complete results and defined clinical outcomes, the overall accuracy of the assay in predicting surgical success was 88% using the criterion of a 50% decrease at 5–10 min and 97% including the subset of patients with delayed decreases of PTH.

Conclusions: The rapid PTH assay had excellent analytical performance and excellent agreement with the PTH immunoradiometric assay and predicted the success of parathyroid surgery in this large series of consecutive patients.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Despite a success rate >95% for surgery to correct hyperparathyroidism, several primarily biochemical approaches have been proposed to assess surgical success to avoid reexploration, which has a higher complication rate and a lower success rate than initial surgeries (1)(2). In 1988, measurement of circulating parathyroid hormone (PTH) was proposed as an intraoperative monitor with the discovery that a highly sensitive intact PTH assay could be modified to obtain results rapidly (3). PTH is a useful measure because PTH is produced only in parathyroid glands, and the intact 84-residue molecule has a half-life of <5 min. In addition, secretion of PTH is suppressed in normal parathyroid glands after the removal of all hyperfunctioning tissue, and therefore, PTH concentrations should decline rapidly after all hypersecreting parathyroid tissue has been removed (4).

The first rapid assay developed for use intraoperatively was a modified commercial intact PTH IRMA assay (3). The incubation time of this assay was reduced to 15 min from 22 h with an increase in the limit of detection from 1 to 25 ng/L. Subsequently, other rapid assays with radioactive labels were developed using serum, plasma, or whole blood specimens with incubation times of 10–30 min and turnaround times of up to 1 h (5)(6)(7). The kinetics of these assays were altered by increasing incubation temperatures, adopting continuous shaking, and altering sample and reagent volumes. Despite the technological advances of these initial assays in decreasing incubation times, the potential of intraoperative PTH measurements was not realized until the introduction of modified immunochemiluminometric assays, with decreased signal detection times and the ability to perform testing in the operating suite (8)(9).

Endocrine surgeon George Irvin is generally credited with introducing this tool into parathyroid surgery. Since the commercialization and Food and Drug Administration approval of the assay in 1997, the assay is, as commented by Garner and Leight (10), "... now moving from the investigative arena to status as a routine clinical assay at medical centers with extensive experience with this procedure". Rapid PTH assays are now available in both manual (8)(11) and automated formats (12). Intraoperative PTH assays, in combination with other techniques, have allowed improvements to the traditional bilateral neck exploration for hyperparathyroidism, such as using a minimally invasive approach (13)(14)(15)(16)(17)(18). Minimally invasive parathyroidectomy has allowed the procedure to be performed in an outpatient setting at our institution. As we reported, in comparison to the traditional procedure, the minimally invasive approach significantly shortened length of hospital stay and lowered costs for patients undergoing surgery for primary hyperparathyroidism (13). The rapid PTH assay has been successfully used for parathyroid surgery and for venous localization before surgery (19) for >2 years at the Johns Hopkins Medical Institutions. Here we report our experience with this assay in 200 consecutive parathyroidectomy cases.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
assay
Rapid PTH was measured using the QuiCk-IntraOperativeTM Intact PTH assay (Nichols Institute Diagnostics). This immunochemiluminometric assay uses two goat polyclonal antibodies against PTH, one coated on a polystyrene bead and another labeled with an acridinium ester. In the operating room assay procedure, blood is first drawn from the patient and transferred to an EDTA-containing Vacutainer tube (3 mL) and gently mixed. Aliquots of 700 µL of blood are pipetted into two microcentrifuge tubes and centrifuged for 30 s at 3000g. Plasma (200 µL) is then pipetted into duplicate 12 x 75-mm glass tubes, and 100 µL of the acridinium-labeled signal antibody and the capture antibody-coated polystyrene bead are added. Tubes are incubated at 45 °C for 7 min while shaking at 400 rpm in a heater-shaker apparatus (QuiCk-PakTM Kinetic Enhancer). After incubation, beads are washed three times with 2 mL of saline. Beads are transferred to clean 12 x 75 tubes and counted for 2 s in a single-tube luminometer (QuiCk-Pak Quantifier). The total time to perform this assay, including specimen processing, is 12–14 min. Approximately 1 h of technician time before the surgery is required in the laboratory to perform instrument performance checks, to generate the calibration curve, and to assay quality-control material. A cart that can be transported into or adjacent to the operating suite holds the instrumentation required for the assay (microcentrifuge, heater shaker apparatus, bead washer, and luminometer) and provides a small work area.

interference study
The effect of the sedative-hypnotic agent propofol (DIPRIVAN®) on the rapid PTH assay was assessed by comparing a patient specimen diluted with propofol (10 g/L) to dilutions made with saline and calculating percent recovery. Dilutions of propofol ranged from 0.01% to 40%. Propofol is formulated in a white, oil-in-water emulsion and is administered intravenously as a bolus dose or a continuous infusion.

assay comparison
We analyzed 44 EDTA-plasma specimens by the rapid PTH assay in the operating room and by the Nichols Intact PTH IRMA assay in the central laboratory. The rapid assay was also compared with the central laboratory assay using a subset (n = 37) of paired plasma (rapid assay-preferred specimen) and serum (central laboratory assay-preferred specimen) specimens.

parathyroidectomy patients
Two hundred patients underwent parathyroid exploration with intraoperative PTH measurements by one endocrine surgeon between October 1997 and December 1999. Serum intact PTH and calcium concentrations were used to confirm the diagnosis of hyperparathyroidism preoperatively. The mean age of the patients was 58 ± 14 years (range, 13–88 years), and 143 (72%) were female. Intact PTH was measured in the operating suite using the rapid assay at baseline before exploration and at 5–10 min after gland resection. A 50% decline in PTH concentration from baseline indicated that the affected gland was removed successfully (8). In this study, failure to cure hyperparathyroid disease was defined as failure to achieve normal serum calcium concentrations within 1 week of the operation. Since inception of the rapid assay, 69% (125 of 180) of patients have undergone outpatient minimally invasive parathyroidectomy (13), whereas the remainder have undergone traditional bilateral exploration with general anesthesia.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
analytical performance
The analytical performance of the QuiCk-IntraOperative Intact PTH assay was evaluated with respect to intraassay precision, assay detection limit, propofol interference, and agreement with the in-house PTH IRMA assay. Intraassay imprecision (CV) was 12% at 28 ng/L (2.97 pmol/L) and 11% at 278 ng/L. The detection limit, defined as the 95% confidence limit of 20 replicates of the zero standard, was 11 ng/L, higher than that for the central laboratory assay (1 ng/L).

Interference of the intravenous anesthetic propofol used in outpatient parathyroid surgery on the rapid PTH assay was also evaluated. A negative interference was noted when propofol (10 g/L) was present at dilutions of 5% to 40% with measured PTH from 90% to 53% of expected.

Deming regression analysis of results of the rapid PTH assay and the IRMA in the central laboratory in 44 plasma specimens from 8 parathyroidectomy patients yielded y = 1.26x - 12 ng/L, Sy|x = 26.3 ng/L, r = 0.984, and for 40 of 44 patients with values <200 ng/L, y = 1.02x + 1.9, Sy|x = 13.9, r = 0.947. There was also good agreement in a subset (n = 37) of paired specimens analyzed using plasma specimens in the rapid assay and serum specimens in the IRMA assay: y = 1.18x - 17, Sy|x = 24.8, r = 0.988.

clinical results
Fig. 1 shows the distribution of the first 200 parathyroidectomies in which the rapid PTH assay was used. Patients were divided into those patients with primary hyperparathyroidism (n = 189; 94%) and those with secondary-tertiary hyperparathyroidism (n = 11; 6%) and subdivided into whether the patients had undergone one or more previous cervical explorations. Non-reoperative primary hyperparathyroid patients were further subdivided into patients with uniglandular or multiglandular disease. Of the 200 patients (Table 1 ), 196 had complete results, whereas in 4 cases, sampling was discontinued at the discretion of the surgeon: 2 cases before resection because of very low baseline concentrations of 35 and <11 ng/L attributed to specimen dilution with saline; and 2 cases where obtaining additional specimens was determined to be uninformative.



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  		Figure 1. Distribution of the first 200 parathyroidectomies in which the rapid PTH assay was used.


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  		Table 1. Rapid PTH results and patient outcomes in 200 parathyroid surgeries.

In 169 of 196 cases, there was a 50% decrease in intact PTH concentrations at 5–10 min or at the first postresection time point. In 26 of the total cases, the first specimen postresection was drawn at >10 min (73% by 15 min) because of difficulties with intravenous access. PTH concentrations and the percentage change in PTH from baseline in the 149 primary hyperparathyroid patients with single adenomas with expected 50% declines after gland excision are presented in Figs. 2 and 3. Fig. 4 illustrates similar utility of the intraoperative assay in three patients with multiglandular disease. Fig. 4A shows a patient with a double adenoma. Lack of a decline after the first resection suggested that another hyperfunctioning gland was present, thus requiring further exploration. After resection of the second abnormal gland, the PTH concentration decreased significantly. Fig. 4 , B and C, demonstrate patterns observed in two patients with secondary-tertiary disease undergoing subtotal or total parathyroidectomy. The majority of cases are similar to the case presented in Fig. 4B with PTH measurement made after 3.5 or 4 glands have been removed. Fig. 4C illustrates a case in which PTH measurements were made after each resection showing a stepwise decrease in plasma PTH concentrations.



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  		Figure 2. Intact PTH concentrations in 149 primary hyperparathyroid patients with single adenomas.

PTH concentrations below the assay limit of detection (11 ng/L) were assigned a value of 10 ng/L.



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  		Figure 4. Use of the intraoperative assay in three patients with multiglandular disease.

(A), patient with a double adenoma. (B) and (C), two patients with secondary-tertiary hyperparathyroidism who underwent subtotal or total parathyroidectomy with PTH measurements made after 3.5 or 4 glands had been removed (B) or PTH measurements made after each resection (C).

In 17 patients, PTH concentrations declined 50%, but only after a delay of >10 min. This is illustrated in Fig. 5 in 14 single adenoma patients. The majority of patients (13 of 14) showed a 50% decline by 30 min. There was no clear evidence for delayed declines in PTH concentrations after adequate tumor resection in the majority of cases; however, in a subset of cases theoretical explanations include tumor manipulation during mobilization of the gland causing artificial increases (n = 3), baseline concentrations below the upper limit (65 ng/L) of the intact PTH reference range (n = 1), sampling from neck veins downstream from normal or abnormal parathyroid glands, and specimen integrity problems, such as the presence of hemolysis (n = 1).



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  		Figure 5. Percentage change in rapid intact PTH concentrations from baseline in patients with single adenomas who did not demonstrate a 50% decrease at the initial 5–10 min sample.

Fourteen patients (dashed lines) had delays in the drop in PTH concentrations to 50% of baseline, whereas four patients (solid lines) were normocalcemic after surgery, but a 50% decline in PTH concentrations was not achieved.

Fig. 5Up displays results from four patients with uniglandular disease who were considered cured based on postsurgical calcium concentrations but in whom a 50% decline in PTH concentrations was not achieved. In three of the four patients, all with low baseline concentrations and two with suspected tumor manipulation, declines to 54%, 56%, and 60% were observed. In the fourth patient, in whom follow-up confirmed adequate resection, PTH concentrations remained constant at 66% of baseline. In patients with reoperative primary hyperparathyroidism, two patients also failed to demonstrate a 50% PTH decline. These were associated with extensive tumor manipulation in one case and a low baseline in the other.

In three cases, the PTH assay did not decline, and postoperative follow-up suggested persistent hyperparathyroidism. In one of the three cases, mild persistent disease was present despite the removal of an abnormal parathyroid gland, whereas in another case, coexisting sarcoidosis provided for a complicated clinical picture. In one final case, PTH concentrations did not decline after removal of an enlarged parathyroid gland contained in a goiter; despite minimal increases in calcium concentrations after surgery and a normal PTH concentration, it was unclear if the patient was cured of his disease.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The clinical utility of rapid PTH measurements in parathyroid surgery was first reported in 1988 using a modified intact PTH IRMA assay (3). PTH concentrations declined to a mean of 22% of baseline concentrations in measurements taken at 15 and 30 min after excision of a single adenoma in 12 patients. The authors concluded that the judgment and skill of the surgeon to determine surgical cure could be complemented with the intraoperative PTH assay. Subsequently, rapid assays were developed using radioactive (5)(6)(7) as well as nonradioactive (8)(9) formats. The use of nonradioactive formats with equipment transportable on a cart has allowed the rapid assay to be performed outside the central laboratory setting either adjacent or within the operating suite. This eliminates delays in reporting of results related to specimen transport time, and having a technologist on-site allows for interaction with the surgeon and operating team, particularly in relation to specimen collection and integrity. However, providing a dedicated technologist on-site in the operating room can place a strain on laboratories already impacted with staffing difficulties.

The QuiCk-IntraOperative Intact PTH assay used in this study has a chemiluminescent label and performs well analytically. In patient specimens, the rapid assay had a good correlation with the PTH IRMA assay used in the central laboratory. In addition, subsequent to the initial validation studies, intraoperative PTH specimens retested in the main laboratory on the limited occasions when the result did not appear to fit the clinical situation also showed good agreement with the central laboratory assay (not shown). The instrumentation has been reliable; in only one instance in a >2-year period has testing been discontinued in the operating room because of a technical problem.

As a result of the extensive on-going experience with the assay both by the technical staff and clinicians, there has been an evolution in operating room specimen protocol, adapted from those developed by Irvin and co-workers (7)(8), which can be ascertained in part from the data in Figs. 2Up and 3 . Initially, two baseline specimens were drawn with postexcision specimens drawn at 10, 20, and often 30 min. With the introduction of the minimally invasive approach to the surgery, which produced shorter operative times, the protocol was altered to include one baseline specimen and typically one specimen at 5 min after gland excision. Procedures related to the surgery have also evolved with a recent change in location from the general operating room suite to a same-day-surgery location in our outpatient facility. In this setting, the anesthetic propofol is often administered intravenously. This was discovered when several white, lipemic-looking specimens were obtained. A small study in the laboratory confirmed not only a dilutional effect of the drug but also a negative interference. Drug infusion is now stopped 5 min before sampling to reduce assay interference. The presence of technologists in the operating room allows for interaction with the anesthesiologists drawing the specimens to ensure appropriate specimen discards and timing of specimens and to request redraws when specimens are unfit for analysis.



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  		Figure 3. Percentage change in intact PTH from baseline in 149 primary hyperparathyroid patients with single adenomas.

In this study of consecutive cases using intraoperative PTH testing, the overall accuracy of the assay in predicting surgical success was 88%, using the guideline of a 50% decrease at 5–10 min, and 97% including the subset of patients with delayed PTH decreases in the 195 cases with complete results and defined clinical outcomes. The assay was found to have utility in cases of both primary and secondary-tertiary hyperparathyroidism as well as in reoperative cases. Others have also found PTH assays to be useful intraoperatively (8)(10)(20)(21)(22)(23), although there is not complete agreement (5)(24)(25). Garner and Leight (10) found that the intraoperative PTH assay predicted postoperative outcome in 96% of 130 consecutive cases of primary hyperparathyroidism, whereas Gordon et al. (22) validated assay accuracy in 72 patients undergoing bilateral exploration for primary hyperparathyroidism, using gross morphologic criteria to identify uniglandular or multiglandular disease. Clary et al. (20) studied patients with secondary hyperparathyroidism and observed a mean 85% decrease in PTH concentrations after total or subtotal parathyroidectomy in 13 patients, whereas Irvin et al. (26) compared patients who underwent reoperative parathyroidectomy for failed surgery or recurrent disease and found the success rate of the surgery increased from 76% to 94% when intraoperative PTH assays were used.

Use of intraoperative PTH assays has also played a role in the recent efforts to improve on the already highly successful bilateral neck exploration approach to parathyroidectomy to reduce incision size and extent of neck exploration as well as minimize costs such as operating room times and hospital lengths of stay. Intraoperative PTH assays combined with preoperative imaging and other techniques have allowed for minimally invasive approaches with uniformly favorable outcomes (13)(14)(15)(16)(17)(18). Miccoli et al. (17) compared conventional cervicotomy with bilateral exploration and frozen section tissue analysis in 18 patients with primary hyperparathyroidism to video-assisted surgery with intraoperative PTH measurements performed in 20 patients. Operative time was significantly shorter in the second group (57 vs 70 min), and those patients had better cosmetic results and a less painful course. In another study (14), the use of concise parathyroidectomy with added sestamibi single photon emission computed tomography and intraoperative PTH assays in primary hyperparathyroid patients decreased the length of stay (1.90 vs 1.07 days), but mean operating time and perioperative costs were not affected. At our institution, an outpatient minimally invasive technique has been adopted that consists of preoperative sestamibi single photon emission computed tomography imaging, surgeon-administered local or regional anesthesia, exploration through small incisions of 1–4 cm, and intraoperative PTH measurements (13). This technique was used in 33 consecutive patients with primary hyperparathyroidism and compared with 184 consecutive patients who underwent bilateral exploration with general anesthesia. Patient outcomes were similar with respect to cure rate and morbidity, but significant reductions were observed (13) with the new procedure in both hospital lengths of stay (0.3 vs 1.8 days) and total hospital charges ($3174 vs $6328).

Despite the success achieved using rapid PTH measurements as a monitor of parathyroid surgery, there are several limitations to the use and implementation of the assay. The first is that it is not intended to replace the experience of the surgeon performing the procedure. Clinical judgment is required in interpreting assay values that may not be entirely consistent with the patient’s clinical situation. For example, this study found several patients with postexcision values that fell close to the 50% guideline, but it was the judgment of the surgeon that these patients had been cured and the operations were suspended. Some considerations include the initial baseline PTH concentration, potential manipulation of the tumor during mobilization, and the specimen sampling location, such as possible use of neck veins. In addition, the use of a 50% decline at 5–10 min does not take into account interindividual variability in PTH half-life (27)(28) or the possibility of altered renal or hepatic status in the patient affecting PTH clearance. Like any other assay, specimen integrity is critical, and the technologist needs to work with the operating room staff to ensure that specimens are free from interferences and are not diluted with intravenous fluids.

In summary, although use of the rapid PTH assay can be an expensive proposition for the laboratory, its use can substantially alter operative management (29). The assay allows the surgeon to confirm that a functional decrement has occurred after excision of an enlarged parathyroid gland, and the assay can prevent operative failure and suggest that additional dissection is indicated to locate additional hypersecreting glands. The assay also obviates frozen sections in the vast majority of patients because the surgeon is far more interested in the functional result of parathyroid resection, rather than histological confirmation of parathyroid excision. The assay also assists in allowing the use of a minimally invasive surgical approach in an outpatient setting, which can reduce costs.


   Acknowledgments
 
We thank Willard Dunn, Phaedre Mohr, Carla Terry, Lilah Evans, and Patricia Donovan for valuable contributions.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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L. R. Johnson, G. Doherty, T. Lairmore, J. F. Moley, L. M. Brunt, J. Koenig, and M. G. Scott
Evaluation of the Performance and Clinical Impact of a Rapid Intraoperative Parathyroid Hormone Assay in Conjunction with Preoperative Imaging and Concise Parathyroidectomy
Clin. Chem., May 1, 2001; 47(5): 919 - 925.
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