Measurement of Intraoperative Parathyroid Hormone Predicts Long-term Operative Success FREE

METHODS FOR the intraoperative measurement of intact parathyroid hormone (PTH) were developed during the early 1990s as valuable adjuncts to surgical skill in parathyroid surgery.^1- 5 In recent years, quick intraoperative PTH monitoring has become a popular method to confirm the complete removal of hyperfunctioning parathyroid tissue. With this technique, minimally invasive and/or focused parathyroid surgery^6- 12 has challenged the traditional bilateral approach.¹³ However, whereas a sufficient decline in the intraoperative PTH level has been shown to predict early operative success, defined as postoperative normocalcemia,^{6- 7,14- 15} the long-term results have been questioned. The purpose of this study was to determine the accuracy of the intraoperative measurement of PTH in predicting long-term success after surgery for primary hyperparathyroidism (pHPT).

Consecutive patients who underwent an operation for pHPT with intraoperative PTH measurement were followed up yearly for serum levels of total calcium, intact PTH, and creatinine.

The study included 102 consecutive patients, 25 men and 77 women, with a clinical and biochemical diagnosis of sporadic pHPT who underwent neck surgery with intraoperative PTH measurement at the Department of Surgery, Lund University Hospital, Lund, Sweden. Eighteen patients who underwent parathyroidectomy without intraoperative PTH measurement during the same period were excluded. The mean ± SD patient age was 63 ± 13 years. The mean ± SD serum calcium level was 11.16 ± 0.84 mg/dL (2.79 ± 0.21 mmol/L) with a mean intact serum PTH level of 120.6 ± 178.5 pg/mL (12.7 ± 18.8 pmol/L). Seven patients had slightly elevated levels of serum creatinine before surgery. The median weight of the excised parathyroid adenomas was 0.66 g (range, 0.11-10.10 g).

Except for 8 asymptomatic patients with total serum calcium levels of more than 10.4 mg/dL (2.60 mmol/L), all patients had symptomatic pHPT. There were no differences in preoperative clinical and biochemical variables between the 102 patients with intraoperative PTH measurement who were included in the study and the 18 excluded patients without a measurement of intraoperative PTH.

The criterion for parathyroidectomy was symptomatic pHPT or significantly increased calcium levels in asymptomatic cases. All patients underwent neck exploration according to the principles of unilateral approach.¹⁶ Briefly, if an adenoma is found on the first side explored, it is excised. If the adenoma is not found on the first side or if the results of intraoperative PTH measurement are inconclusive, comprehensive bilateral exploration is performed. In our cohort, 58 patients underwent unilateral exploration, and 44 underwent bilateral exploration. The skewed distribution of numbers between the 2 groups is due to a variety of localization procedures that were investigated during the study period.

The decline in the level of intact serum PTH was determined during surgery. The criteria used to decide when to terminate exploration have been described previously.⁵ A decline 15 minutes after parathyroid gland excision of 60% or more below baseline (obtained when the first enlarged gland is visualized) highly predicts parathyroid adenoma and a return to postoperative normal or low levels of serum calcium. All blood samples were obtained from peripheral veins.

A diagnosis of pHPT was established in each patient using conventional histopathologic criteria¹⁷: adenoma in 98 cases and multiglandular disease in 4 cases. All 18 patients who were excluded had a diagnosis of solitary adenoma.

All blood samples were collected after an overnight fast. Preoperative data were obtained from blood samples collected the day before surgery. Levels of intact serum PTH were measured using an assay for intact PTH (Incstar, Stillwater, Minn) (reference range, 9.5-47.5 pg/mL [1.0-5.0 pmol/L]). The sensitivity of this assay is 1.23 pg/mL (0.13 pmol/L). The interassay coefficient of variation is less than 11%, and the intra-assay variation is less than 6%. The method has a coefficient of variation of 2.2% at a value of 47.5 pg/mL (5 pmol/L).

To shorten the time for intraoperative analysis, the assay was modified as previously described.⁵ The intra-assay variation between 7.6 and 95.0 pg/mL (0.8 and 10 pmol/L) is less than 8% for the rapid method. The correlation between the methods is 0.99.

Levels of total serum calcium (reference range, 8.8-10.4 mg/dL [2.20-2.60 mmol/L]), creatinine (reference range in men, 0.62-1.31 mg/dL [55-116 µmol/L], and in women, 0.51-1.13 mg/dL [45-100 µmol/L]), and phosphate (reference range, 2.17-5.0 mg/dL [0.70-1.60 mmol/L]) were all analyzed using routine tests (Ektachem 700XR-C; Eastman Kodak Co, Rochester, NY).

Results are expressed as mean ± SD unless stated otherwise. For assessments repeated across time, repeated-measures analysis of variance was used. Efficacy of measurement of intraoperative PTH for long-term operative success was calculated and expressed as sensitivity, specificity, positive predictive value, and overall accuracy. P<.05 was considered significant.

Eight weeks after the operation, all patients had normal or low calcium levels (mean ± SD, 9.2 ± 0.6 mg/dL [2.30 ± 0.15 mmol/L]) with a mean ± SD level of intact serum PTH of 38.9 ± 22.7 pg/mL (4.1 ± 2.4 pmol/L).

The mean ± SD decline in intraoperative PTH level in 98 patients with primary exploration as a result of a solitary adenoma was 83% ± 10% after 15 minutes. In all but 4 patients, the decline 15 minutes after parathyroid gland excision was 60% or more below baseline. In these 4 patients the PTH concentration decreased by 40%, 52%, 54%, and 55%, suggesting that they did not have a solitary adenoma. However, these patients were bilaterally examined with no evidence of more than 1 enlarged parathyroid gland. Furthermore, they had a histopathological diagnosis of parathyroid adenoma, and their levels of serum calcium (8.96, 9.88, 9.48, and 9.16 mg/dL [2.24, 2.47, 2.37, and 2.29 mmol/L], respectively) and PTH (44.6, 58.9, 35.1, and 25.6 pg/mL [4.7, 6.2, 3.7, and 2.7 pmol/L], respectively) were normalized 5 years after surgery. Thus, the false-negative rate was 4%.

Patients examined because of multiglandular disease (n = 4) were correctly predicted not to have an adenoma using the intraoperative PTH measurement. In these patients the PTH level declined by 24%, 30%, 35%, and 50%, respectively, 15 minutes after the first parathyroid gland excision. After further exploration, in all cases a second enlarged parathyroid gland was found on the contralateral side and removed. The decreased level of serum PTH at 15 minutes after the removal of the second gland predicted that no hyperfunctioning tissue was left. All 4 cases were classified as hyperplasia in the histopathologic reports.

The annual mean ± SD total serum calcium level and mean ± SD intact serum PTH level up to 5 years after surgery are shown in Figure 1. The demonstrated increase across time in total serum calcium was statistically significant (P = .003). Further analyses showed that the difference was between mean serum calcium levels at 1 year and 5 years after surgery (P = .001). In contrast, levels of intact serum PTH did not change with time (P = .35).

Twenty-two patients (22%) were unavailable for 5-year follow-up: 16 had died and 6 refused. However, these patients were followed up for 2 to 48 months (median, 24 months; mean ± SD, 27 ± 18 months), and none developed recurrent pHPT.

Of the remaining 80 patients (78%), all but 1 had normal (n = 71) or slightly decreased (n = 8) serum calcium levels (mean ± SD, 9.24 ± 0.4 mg/dL [2.31 ± 0.10 mmol/L]), with a mean ± SD serum PTH level of 35.1 ± 14.2 pg/mL (3.7 ± 1.5 pmol/L) at the 5-year follow-up. No patients received treatment with vitamin D analogues.

One patient with hypercalcemia (10.6 mg/dL [2.65 mmol/L]) had an elevated serum PTH level (104.4 pg/mL [11.0 pmol/L]), suggesting recurrent disease. However, the serum creatinine had increased from a preoperative level of 1.62 mg/dL to 7.75 mg/dL (143 µmol/L to 685 µmol/L) at the 5-year follow-up. During this period, serum phosphate levels also increased, from 3.0 mg/dL to 5.85 mg/dL (0.97 mmol/L to 1.89 mmol/L). This patient was interpreted to have developed renal failure with secondary hyperparathyroidism.

Thirty-four patients had elevated serum PTH levels at least once during the postoperative study period, with normal or slightly decreased calcium concentrations (range, 8.04-10.16 mg/dL [2.01-2.54 mmol/L]) (Table 1) and normal serum creatinine levels. All 34 patients had undergone surgery for sporadic adenoma. For this subgroup of patients, serum PTH levels did not significantly change across time (P = .24), whereas serum calcium concentrations increased (P = .01). Further analyses demonstrated that the difference was between the mean serum calcium concentrations at 1 year and 5 years postoperatively (9.08 ± 0.4 mg/dL vs 9.36 ± 0.32 mg/dL [2.27 ± 0.10 mmol/L vs 2.34 ± 0.08 mmol/L]; P = .001).

Sixteen percent (16 of 97 patients) had elevated PTH levels 1 year after surgery, whereas 11% (9 of 80 patients) had elevated levels at 5 years. However, most of the 34 patients showed fluctuations in their PTH levels throughout the study period. Twelve of the 16 patients with elevated serum PTH levels at 1 year were available for 5-year follow-up. In 9 of these patients, the increased PTH level resolved with time. Thus, only 3 of 12 patients with elevated PTH concentrations 1 year after surgery still had elevated PTH levels at the 5-year follow-up.

A decline of intraoperative PTH of 60% or more 15 minutes after gland excision had a 100% positive predictive value for cured pHPT 5 years after pHPT surgery. Furthermore, using our biochemical definition of parathyroid adenoma, the method had an overall accuracy of 95%, a sensitivity of 95%, and a specificity of 100%.

The aim of pHPT surgery is a return to normocalcemia after the operation. Although a sufficient decline in intraoperative PTH level has been associated with excellent early (at least 6 months after surgery) results,^{6- 7,14- 15} long-term results have been questioned. A major concern is that the reported proportion of patients with multiglandular disease is lower in series based on intraoperative PTH level (3%-5%)^7,18- 19 than in studies in which the extent of parathyroid tissue resection is based on bilateral neck exploration, gross morphologic characteristics, and frozen section (9%-24%).^7,20- 21 Therefore, some investigators have suggested that with time there will be many recurrences because of nonvisualized enlarged parathyroid glands. However, it is well known in other fields of medicine that size is not always correlated with function or hyperfunction. It seems reasonable that not all enlarged parathyroid glands will result in pHPT. In our study, multiglandular disease was present in 4% of the patients. No late recurrence of pHPT was found, although 1 patient developed renal insufficiency with suspected secondary hyperparathyroidism. Thus, our study was able to show that the efficacy of measurement of intraoperative PTH is as good for long-term results as it is for early results. This is in agreement with Carneiro and Irvin,⁷ who found a recurrent rate of 1% in a similar study in which patients were followed up for an average time of 2.3 years.

Several studies have reported that intraoperative PTH measurement has a varying proportion of false-positives results^19,21- 22; that is, patients with a sufficient decline in intraoperative PTH level after the removal of 1 enlarged gland despite multiglandular disease. In contrast, in our study, patients with multiglandular disease were correctly predicted not to have an adenoma using intraoperative PTH measurement. This means that with a sufficient decline in intraoperative PTH, one does not even have to visualize a normal ipsilateral gland.

In this context, it may be appropriate to discuss some technical aspects of intraoperative PTH measurement. In the literature, blood samples are reportedly obtained from different sites (central and/or peripheral veins or even arteries) and at different specified intervals.^4,19,21,23 We always obtain our samples in a peripheral vein according to a strict protocol. A major concern with samples obtained from the jugular vein is that a parathyroid tumor upstream or downstream from the sample site will influence the PTH level differently. This may be helpful in localizing a hyperfunctioning gland, but that is another issue. During dissection, manipulation of the parathyroid tumor may affect the PTH obtained from a central vein. However, parathyroid gland manipulation may also affect the peripheral PTH value. Therefore, it is of utmost importance to perform a meticulous dissection of the enlarged gland without pressure before its vessels are ligated or clipped. Furthermore, the baseline sample should be obtained when the enlarged gland is first visualized and not before the induction of anesthesia; we have previously shown that the PTH level increases after general anesthesia.²⁴ A second and third sample are collected 5 minutes and 15 minutes after excision. Finally, in our hands the efficacy of the method relies primarily on a decline of PTH at 15 minutes of 60% or more below the baseline value.⁵ Most other series use a PTH decrease of 50% or more after 10 minutes as a criterion to predict postoperative normocalcemia.^7,19,21- 22

A false-negative rate of 4% was noted in our study (ie, 4 of 98 patients underwent unnecessary extended exploration). Others have reported similar early results.^21- 22 This rate, which ideally should be as low as possible, is of special interest when the intraoperative measurement of PTH is used to guide minimally invasive or focused pHPT surgery because it leads to additional (unnecessary) exploration and/or conversion to an open procedure.

There was an increase in total serum calcium across time, whereas PTH and renal function, estimated as serum creatinine, did not change. The reason for this finding is unknown. Altered vitamin D and/or bone metabolism after pHPT surgery could be a factor. To the best of our knowledge, the postoperative trend of rising serum calcium (albeit within the normal range) has not been described previously and therefore warrants further investigation.

A postoperative elevated intact PTH level after successful parathyroid surgery has been documented before, irrespective of surgical strategy.^23,25- 28 This phenomenon is associated with older age and advanced pHPT.²⁶ Furthermore, these patients have normal parathyroid and renal function but exhibit remineralization of cortical bone, decreased calcium absorption, and low levels of vitamin D.²⁸ Interestingly, in our study, most of the 34 patients who showed increased postoperative levels of serum PTH demonstrated fluctuations in their PTH level throughout the study period. Only 3 patients had elevated PTH levels at both the 1-year and 5-year follow-up. In addition, although the proportion of patients with elevated PTH concentrations decreased between the 1-year and 5-year follow-up, we found no evidence of a significant change in PTH levels across time.

Our study focused specifically on the accuracy of the intraoperative measurement of PTH in predicting long-term success after pHPT surgery. It does not address whether it is justifiable to use this method in routine pHPT surgery. This question can be properly answered only by a prospective randomized trial.

In conclusion, the measurement of intraoperative PTH during surgery for pHPT accurately differentiates between single and multiple gland disease and ensures good 5-year results. In terms of predicting 5-year operative success, the overall accuracy of this method is 95%.

Corresponding author and reprints: Johan Westerdahl, MD, PhD, Department of Surgery, Lund University Hospital, S-221 85 Lund, Sweden (e-mail: johan.westerdahl@kir.lu.se).