Intra-operative parathyroid hormone monitoring in patients with parathyroid cancer.

BACKGROUND: Intra-operative parathyroid hormone (PTH) monitoring (IPM) is 97% accurate in predicting postoperative eucalcemia in sporadic primary hyperparathyroidism (SPHPT). However, its usefulness in parathyroid cancer has not been demonstrated. This study reports IPM accuracy during surgical resections for parathyroid cancer. METHODS: Eight of 556 consecutive patients with SPHPT underwent parathyroidectomy using IPM and had parathyroid cancer. Operative success was defined as eucalcemia > six months and operative failure/persistent cancer as hypercalcemia within six months of parathyroidectomy. The IPM criterion for operative success was defined as a >50% decrease of peripheral PTH levels from the highest either pre-incision or pre-excision values, 10 minutes after resection. RESULTS: In eight patients, 11 operations were performed. Ten operations (91%) resulted in >50% intra-operative PTH decrease. However, in only seven (70%) of these resections, eucalcemia was achieved for >6 months with five of these seven (71%) procedures being initial en bloc resections. The remaining 3/10 (30%) operations with >50% intra-operative PTH decrease resulted in operative failures. In the last operation, intraoperative parathormone monitoring (IPM) correctly predicted operative failure. IPM sensitivity, specificity, positive predictive value, negative predictive value, and overall accuracy in predicting outcome were 100, 40, 70, 100, and 75%, respectively. CONCLUSIONS: IPM with the criterion of >50% PTH drop from the highest level is less accurate in predicting operative success in parathyroid cancer when compared to SPHPT. A >50% intra-operative PTH level decrease in patients with parathyroid cancer, particularly in reoperative cases, is less predictive of complete resection. The initial recognition of this disease followed by proper resection remains essential in the treatment of parathyroid cancer.

"Limited" parathyroidectomy in geriatric patients.

OBJECTIVE: Elderly patients with primary hyperparathyroidism accompanied by other diseases are often denied referral for parathyroidectomy because of the associated risks of general anesthesia and bilateral neck exploration. However, marked symptomatic improvement is recognized after successful parathyroidectomy. The purpose of this report is to examine the postoperative outcome of geriatric patients undergoing "limited" parathyroidectomy. METHODS: Since 1993, 291 consecutive patients with primary hyperparathyroidism were treated with "limited" parathyroidectomy guided by preoperative localization and intraoperative parathyroid hormone assay. In 34 of the procedures (29 initial, 5 reoperations), the patient was 75 years or older; these patients are the subject of this report. Patients were followed up for serum calcium, parathyroid hormone levels, and symptomatology. RESULTS: Twenty-seven patients were followed up for 31 (range 6-84) months: all remained normocalcemic after single gland excision guided by intraoperative parathyroid hormone assay. Another six patients in the immediate postoperative period had normocalcemia. One patient had persistent hypercalcemia. Unilateral neck exploration was possible in 29 patients. The average operating time for initial parathyroidectomy was 50 (range 20-130) minutes. Nineteen patients were eligible for ambulatory surgery. Seven were discharged without an overnight stay, 11 had a 23-hour "social" admission, and one was kept overnight after a prolonged surgical procedure. Permanent hypoparathyroidism and laryngeal nerve injury were not observed. The mortality rate related to the procedure was 0%; there was one postoperative (do not resuscitate) death caused by colonic hemorrhage. With an average follow-up of 2 years, 64% of the patients had marked improvement of symptoms. CONCLUSION: Adjunctive use of preoperative localization and intraoperative parathyroid hormone assay has made "limited" parathyroidectomy a safe, effective treatment option in geriatric patients with primary hyperparathyroidism.

Late parathyroid function after successful parathyroidectomy guided by intraoperative hormone assay (QPTH) compared with the standard bilateral neck exploration.

BACKGROUND: Controversy continues between bilateral neck exploration and limited parathyroidectomy. One approach depends on gland size and histopathologic factors; the other approach limits excision to only hypersecreting glands. Both have excellent early operative success, but late recurrence rates with limited exploration are unknown. METHODS: Three hundred twenty consecutive patients with primary hyperparathyroidism were followed 6 to 313 months after successful parathyroidectomy. One hundred seventy-six patients had bilateral neck exploration with excision of enlarged glands (group I); 144 patients had glands excised based on hyper-secretion (group II). Calcium and intact parathyroid hormone (iPTH) levels were measured yearly. Parathyroid gland hypersecretion was determined by elevated iPTH levels. RESULTS: In group I, 1 gland was excised in 160 patients (91%); 19 of 176 patients (11%) had elevated iPTH levels. In group II, 139 patients (97%) had 1 gland excised; 19 of 144 patients (13%) had high iPTH levels. The number of patients with more than 1 gland excised in group I (9%) is 3 times higher than in group II (3%) (P <.05). There was no significant difference in the incidence of recurrent hyperfunctioning glands between the 2 different operative approaches (chi-squared test). CONCLUSIONS: Late parathyroid gland function was comparable with both approaches. Multiple gland excision based on size alone may lead to excision of normal functioning glands.

The evolution of parathyroidectomy failures.

BACKGROUND: Reported operative failure rates for primary hyperparathyroidism range from 5% to 10%. Failure has been due to multiglandular disease, ectopic parathyroid glands, errors in frozen section, and missed diagnoses. Recently, our operative approach has changed from bilateral cervical exploration to direction by preoperative localization and intraoperative quick parathyroid hormone assay. The purpose of this study is to examine the causes and rates of failure in this evolving approach to parathyroidectomy. METHODS: Among 447 consecutive cases of primary hyperparathyroidectomy, 20 operative failures were examined. Three different operative approaches were compared with respect to causes and rates of failure. RESULTS: From 1969 to 1989, with bilateral neck exploration, failure was due to missed diagnoses, ectopic glands, multiglandular disease, and unknown causes, with a failure rate of 5%. From 1990 to 1993, with bilateral neck exploration and quick parathyroid hormone assay, failure was due to ectopic mediastinal glands, misinterpretation of frozen section, and operative judgment, with a failure rate of 10%. From 1993 to 1998, with preoperative localization and quick parathyroid hormone assay, the two operative failures (1.5%) were due to operative judgment and misinterpretation of the quick parathyroid hormone assay. CONCLUSIONS: The new surgical approach combining preoperative localization studies and intraoperative parathyroid hormone monitoring has eliminated the most common causes of parathyroidectomy failure and has significantly decreased the operative failure rate.

Parathyroidectomy: new criteria for evaluating outcome.

Following successful parathyroidectomy, subjective improvement in recognized symptoms and in the overall "well being" of asymptomatic primary hyperparathyroid patients has been well documented. Because quantitative methods for measuring parathyroid hormone (PTH) and normal reference ranges of serum calcium have changed in recent years, a revised biochemical criteria for evaluating postoperative outcome has become necessary. Two hundred seventy-one selected patients were followed for an average of 6.3 years after parathyroidectomy. Although 257 patients had serum calcium levels <10.6 mg/dL during the entire follow-up period, 15 per cent of them had elevated intact PTH (iPTH) levels. Fourteen patients had calcium levels > or =10.6 mg/dL at some point during follow-up, with nine patients (64%) showing high iPTH levels and eight (57%) of them developing recurrent hyperparathyroidism (calcium > or =11 mg/dL and iPTH > or =68 pg/mL). Of the 14 remaining patients, 5 had hypercalcemia with normal iPTH levels. In patients with successfully treated primary hyperparathyroidism, the recommended annual follow-up is: 1) monitor total serum calcium only if serum calcium level is <10.6 mg/dL, or if serum calcium level is > or =10.6 mg/dL; and 2) monitor serum calcium and PTH levels, because these patients have an increased incidence of hyperfunctioning parathyroid glands, which may point to late recurrence.

Improved success rate in reoperative parathyroidectomy with intraoperative PTH assay.

OBJECTIVE: The clinical usefulness of preoperative localization and intraoperative PTH assay (QPTH) in primary hyperparathyroidism have been established. However, without the use of QPTH, the parathyroidectomy failure rate remains 5% to 10% in large reported series and is probably much higher in the hands of less experienced parathyroid surgeons. Persistent hypercalcemia requires another surgical procedure. The authors compared the outcomes in 50 consecutive patients undergoing more difficult secondary parathyroidectomy with and without the adjunctive support of QPTH. METHODS: Two groups of similar patients underwent reoperative parathyroidectomy for failed surgery or recurrent disease. The successful return to normocalcemia in group I, with QPTH used to localize and confirm complete excision of all hyperfunctioning glands, was compared with group II, who did not have this intraoperative adjunct. RESULTS: In 31/33 patients in group I, calcium levels returned to normal. With good preoperative localization studies, 17 patients underwent successful straightforward parathyroidectomies as predicted by QPTH. In the other 14 patients, QPTH assay proved extremely beneficial by facilitating localization with differential venous sampling; measuring the increase in hormone secretion after massage of specific areas; recognizing suspicious nonparathyroid tissue excised without a decrease in hormone levels, avoiding frozen-section delay; and correctly identifying the excision of abnormal tissue despite false-positive/false-negative sestamibi scans. In group II, who underwent surgery before QPTH was available, 4 of 17 patients (24%) remained hypercalcemic after extensive reexploration. CONCLUSION: With the intraoperative hormone assay used to facilitate localization and confirm excision of all hyperfunctioning tissue, the success rate of reoperative parathyroidectomy has improved from 76% to 94%.

Incidence of multiglandular disease in primary hyperparathyroidism determined by parathyroid hormone secretion.

BACKGROUND: Successful parathyroidectomy depends on recognition and excision of all hyperfunctioning parathyroid glands. Because histologic definition is limited, multiglandular disease (MGD) is usually determined grossly by means of estimation of gland size and the experience of the surgeon, resulting in frequency varying from 8% to 33%. Normalization of elevated intraoperative intact parathyroid hormone (iPTH) levels after excision of all hyperfunctioning glands is necessary for postoperative normocalcemia and indicates normal secretion of remaining parathyroids. Abnormal hormone secretion measured during operation has been used to define the extent of excision and the incidence of MGD. METHODS: One hundred ten consecutive parathyroidectomy patients with no previous neck surgery or history of multiple endocrine neoplasia had intraoperative iPTH assays performed before and after excision of any suspected abnormal parathyroid gland(s). A drop in iPTH level after gland excision predicted postoperative normal calcium levels. RESULTS: All patients except one had normalization of serum calcium levels (average follow-up, 15 months). One hundred five patients had only one hyperfunctioning gland removed, and all have remained normocalcemic. Five (5%) patients had more than one gland involved: four had two or more hyperfunctioning parathyroids and one patient, who had a large parathyroid cyst removed, remained hypercalcemic. CONCLUSIONS: By using a biochemical assay, instead of estimated size, to predict which parathyroid glands are hypersecreting, the incidence of MGD in primary hyperparathyroidism was found to be 5%.

Intraoperative parathyroid hormone monitoring as an adjunct to parathyroidectomy.

BACKGROUND: Parathyroidectomy has a success rate of greater than 95% in the hands of experienced surgeons. To maintain this result in a more cost-effective way, intraoperative monitoring of intact parathyroid hormone (iPTH) has been used to decrease operative times. This technique signals when all hyperfunctioning tissue has been excised or when further dissection is necessary. METHODS: Eighty-nine consecutive patients with hyperparathyroidism had plasma samples measured for iPTH levels during parathyroidectomy. Nine patients had previous neck explorations. Perioperative iPTH measurements using immunochemiluminescent assays with a turnaround time of 10 minutes were done after excision of each suspected abnormal parathyroid gland. RESULTS: All patients except one returned to and maintained normal calcium levels during the follow-up period of 8 months (range, 1 to 25 months). Prediction of postoperative calcium levels by means of quick immunochemiluminescent assay has a sensitivity of 97%, specificity of 100%, and an overall accuracy of 97%. Specific influence on surgical judgment was noted in four patients with multiglandular disease, in seven with difficult localization problems, and in one patient in whom the hyperfunctioning parathyroid tissue was not recognized. Monitoring the plasma iPTH levels during parathyroidectomy directly aided the surgeon's operative approach in these 12 patients. CONCLUSIONS: Intraoperative iPTH assay is useful with predictive accuracy of 97%. It influenced or changed the operative approach in 13% of patients.

Ambulatory parathyroidectomy for primary hyperparathyroidism.

OBJECTIVE: To evaluate whether the combined application of preoperative localization and intraoperative monitoring of intact parathyroid hormone (iPTH) levels could facilitate safe outpatient parathyroidectomy. DESIGN: Consecutive patients, who had no antecedent social or medical conditions mandating hospitalization, were prospectively offered ambulatory parathyroidectomy with a mean follow-up of 7 months (range, 1-25 months). SETTING: Tertiary care referral center PATIENTS: From 85 patients who had primary hyperparathyroidism with hypercalcemia and elevated iPTH levels, 57 were offered outpatient parathyroidectomy. Nineteen patients were asymptomatic, 3 had hypercalcemic crisis, and the others gave a history of renal stones or had complaints consistent with bone disease. INTERVENTIONS: Technetium Tc 99m sestamibi scintiscans were used for preoperative localization. Monitoring iPTH levels during parathyroidectomy quantitatively assured the surgeon (G.L.I. only) when all hyperfunctioning glands were excised. MAIN OUTCOME MEASURE: The number of patients without complications and with short operative times who were discharged without hospital admission or overnight stay. RESULTS: The combination of preoperative localization of abnormal parathyroid glands and a decline in circulating iPTH levels predicting postoperative normocalcemia after excision of all hyperfunctioning glands resulted in successful parathyroidectomy in 84 of 85 patients. A decreased operative time (average, 52 minutes) with minimal neck dissection permitted outpatient parathyroidectomy in 42 of 57 eligible patients. CONCLUSIONS: The combination of preoperative parathyroid scintiscan localization and iPTH level monitoring during surgery permitted successful parathyroidectomy in an ambulatory setting in half of a consecutive series of patients with primary hyperparathyroidism. The safety, success, and likely cost savings of this approach suggest wider application.

Efficient parathyroidectomy guided by SPECT-MIBI and hormonal measurements.

UNLABELLED: Parathyroidectomy is a difficult and lengthy operation which is noncurative in 6% to 10% of cases. To improve the efficiency of this operation, a new dual diagnostic approach was prospectively applied. METHODS: Preoperative tomographic 99mTc-sestamibi (MIBI) scintography and intraoperative measurements of circulating parathyroid hormone (PTH) levels by a quick assay (QPTH) were used. Scintigraphy comprised immediate and delayed planar and SPECT of the neck and chest, following 20 mCi MIBI. The presence and location of persistent foci of abnormal activity found within the neck mediastinum on volume-rendered reprojection (RPJ) of the SPECT data were reported. The surgion, guided by the three-dimensional MIBI-SPECT/RPJ images, identified and excised the single or most prominent scintigraphic focus and applied the QPTH. If PTH levels fell from baseline by at least 50%, the operation was concluded. RESULTS: The operative time of primary parathyroidectomy was reduced from an average of 90 min (before the introduction of scintigraphy and intraoperative PTH measurements) to 57 min. All but two patients became normocalcemic. In 58 consecutive patients with hyperparathyroidism, MIBI-SPECT/RPJ correctly and precisely identified 51 of 53 (96%) primary parathyroid adenomas, 14 to 15 secondary hyperplasias and 2 of 3 hyperplastic glands in MEN (sensitivity 94%, specificity 92%). QPTH verified the excision of the primary parathyroid adenomas and predicted normocalcemia in 50 of 52 patients. In 6 patients with misleading scintigraphy, QPTH was especially useful and guided the surgeon to continue the operation until the abnormal parathyroid tissue was found and excised. CONCLUSION: MIBI-SPEC/RPJ and QPTH sequentially applied improved the efficiency of parathyroidectomy.

A new, practical intraoperative parathyroid hormone assay.

BACKGROUND: The clinical usefulness of intraoperative parathyroid hormone (PTH) monitoring has been shown using an immunoradiometric assay (IRMA) with several significant limitations. PTH measurement by immunochemiluminometric assay (ICMA) is a nonradioisotopic technique that is more practical for use during parathyroidectomy. METHODS: Plasma from a 15-second microcentrifugation was mixed with 2 antibodies, incubated at 45 degrees C, shaken at 400 rpm for 7 minutes, washed and counted for 2 seconds on a portable luminometer; PTH level was reported in 10 minutes. RESULTS: Sixteen patients had multiple samples taken during parathyroidectomy. PTH levels measured 5 minutes after excision of a suspected abnormal gland were compared with preoperative or preexicision samples and either confirmed complete excision or indicated the need for more exploration in each patient. Correlation of 88 ICMA samples with standard 24-hour IRMA controls was excellent (r = 0.9218, P < 0.0001). The sensitivity of the test in predicting postoperative calcium levels was 94%. CONCLUSION: This new assay can serve as a very practical adjunct for the parathyroid surgeon.

A new approach to parathyroidectomy.

OBJECTIVE: To decrease the operative time for parathyroidectomy in patients with hypercalcemic (primary) hyperparathyroid disease, a combination of preoperative localization of a parathyroid tumor with an effective nuclear scan (scintigram) and intraoperative monitoring of parathyroid hormone (quick parathyroid hormone measurement) to ensure excision of all hyperfunctioning tissue was studied. SUMMARY BACKGROUND DATA: For many years, persistent hypercalcemia after parathyroidectomy (3% to 10%) has been constant and is usually due to the surgeon's failure to remove all hyperfunctioning glands. A marked decrease in parathormone level after excision of a single large gland predicts operative success and a return to normal calcium levels. Conversely, persistent high levels of parathyroid hormone indicate excess secretion by another gland(s) and the need for further exploration. Recently Tc-99m-sestamibi (MIBI) scintigraphy was shown to be more effective in localizing parathyroid tumors than previous methods. A combination of both techniques could be useful to the surgeon if they improve the operative success rate and are cost-effective. METHODS: Parathyroidectomy was performed on 18 patients with primary hyperparathyroid disease, with tumors localized by MIBI scintigrams. When excision of the identified parathyroid gland was accomplished, the operation was terminated and quick parathyroid hormone was measured to confirm that all hyperfunctioning tissue was removed. RESULTS: Sixteen patients with positive results of scintigram had successful parathyroidectomies confirmed by quick parathyroid hormone measurement with a cervical approach. Two patients with mediastinal tumors localized by MIBI scintigraphy could not be resected using this approach. One false-positive/false-negative scintigram was obtained. Compared with patients having parathyroidectomy without localization and hormone monitoring, the average operative time was shortened from 90 to 36 minutes. CONCLUSIONS: Localization and successful excision of parathyroid tumors with confirmation that no other hyperfunctioning glands were present by quick parathyroid hormone monitoring can predict a return to normal calcium levels and a decrease in operative time in parathyroidectomy.

Clinical usefulness of an intraoperative "quick parathyroid hormone" assay.

BACKGROUND: Intraoperative assays of parathyroid hormone (PTH) in the surgical management of hyperparathyroidism have been limited by an extended "turnaround" time, making it impractical for the operating surgeon. With our modification of a standard immunoradiometric assay for intact PTH, results are reported in 12 minutes. The operative usefulness and the ability of this "quick PTH" assay to predict postoperative serum calcium levels are reported here. METHODS: Quick PTH levels from whole blood samples taken 10 minutes after excision of hyperfunctioning parathyroid glands were compared with preoperative and preexcision samples in patients undergoing 63 parathyroidectomies. Patients were divided into two groups with assay incubation times of 10 and 6 minutes. The latter was clearly not sensitive enough and resulted in a 20% false-negative rate. However, with a 10-minute incubation time, a decrease of 54% or more in quick PTH levels resulted in postoperative normocalcemia in patients with primary hyperparathyroidism. RESULTS: With these criteria used to predict the postoperative return to normocalcemia in 29 patients with primary hyperparathyroidism, the quick PTH assay had a sensitivity of 96%, specificity of 100%, and overall accuracy of 97%. CONCLUSIONS: The quick PTH assay was especially helpful in predicting postoperative calcium levels when multiple excisions were necessary to remove all hyperfunctioning tissue or some normal parathyroid glands were not visualized.

Operative monitoring of parathyroid gland hyperfunction.

With a 20-year experience of more than 700 parathyroidectomies, our persistent hypercalcemic postoperative failure rate of 7% has remained constant. Reasons for failure have been misdiagnosis or inability of the surgeon to detect and excise all hypersecreting glands. We have modified a commercially available immunoradiometric assay for intact parathyroid hormone (PTH) resulting in a 15-minute turnaround time. Since intact PTH has a half-life measured in minutes, whole blood samples taken 10 minutes after gland excisions were monitored intraoperatively to confirm significant changes in circulating hormone. Quantitative evidence that all hyperfunctioning parathyroid tissue had been ablated during operation was obtained in 19 of 21 patients. Less than four glands each were identified in 53% of these patients. The PTH "quick" test correctly pointed to an inadequate excision requiring further parathyroid ablation in two patients, made bilateral neck exploration unnecessary in two patients who had previously undergone parathyroidectomy, and predicted persistent hypercalcemia in two patients with complications.

Pheochromocytoma. Lateral versus anterior operative approach.

The possibility of bilateral, extra-adrenal, and malignant tumors has dictated a thorough abdominal exploration through an anterior incision in the management of patients with pheochromocytomas. Careful visualization or palpation of the sites known to harbor secondary tumors is still recommended by many surgeons. The present study contrasts the results and morbidity of the retroperitoneal approach with that of the intraperitoneal operative approach for resection of pheochromocytoma. In the last 14 years, 37 patients had successful total resections of their pheochromocytomas, excluding one patient with metastasis to the liver at the time of surgery who died 10 years after operation. After preoperative localization of their tumors, 17 patients were explored anteriorly and 20 underwent resection using a lateral approach. Thirty-one patients have been followed from 2 to 141 (average 56) months. All patients have either returned to a normotensive state on no medication (27 patients) or, while requiring medication (9 patients), have had normal urinary metanephrine/catecholamine levels, except for the one patient with metatastic disease. There were substantial differences in morbidity rates between the two groups, however. Four patients (20%) had minor postoperative complications, following retroperitoneal resection that included pleural effusion, urinary retention, pulmonary congestion, and fever. Nine patients (53%) had complications when the anterior approach was used, including splenectomy in two, pneumonia, and postoperative fever. Postoperative hospital stay averaged 9.8 days (range, 4 to 21 days) for the anterior group and 6.1 days (range, 4 to 12 days) when a lateral approach was used (p = 0.002). Our data suggest that, with accurate unilateral localization, the flank, retroperitoneal approach for resection of pheochromocytoma can be used successfully with less morbidity.

DNA patterns in parathyroid disease predict postoperative parathyroid hormone secretion.

Flow cytometric analysis of the nuclear deoxyribonucleic acid (DNA) content of parathyroid glands excised from patients with hypercalcemic hyperparathyroidism has identified three distinct DNA patterns. The most frequent pattern showed a high percentage of cells with tetraploid DNA, which indicated an increase in the G2 and M phase of the cell cycle. Thirty-four patients were found to have abnormal tetraploid DNA content. One patient had a normal diploid pattern, and seven were found to have an aneuploid DNA population in their excised parathyroid glands. This unexpected finding of aneuploid DNA appears to be an unique feature of these endocrine glands because they have no histologic or clinical characteristics of malignant change. All patients have remained normocalcemic and clinically well after excision of only grossly enlarged glands. Postoperative parathyroid hormone (PTH) levels were correlated in 17 patients with DNA analyses of biopsy specimens from 30 normal-sized glands which were left in situ. Seven patients with elevated PTH postoperatively had high tetraploid or aneuploid DNA in all 13 glands from which biopsy specimens had high tetraploid or aneuploid DNA in all 13 glands from which biopsy specimens had been taken. In 10 patients with normal PTH levels, six had normal diploid patterns, whereas four had high tetraploid DNA in their gland biopsy specimens. DNA content present in biopsy specimens of normal-sized, in situ glands was predictive (p less than 0.042) of parathyroid gland secretory activity. These findings suggest that the stimulus for parathyroid gland hyperfunction often affects more than a single enlarged gland and persists after clinical cure, as shown by a more rapid cell turnover in some remaining glands and continued hypersecretion of hormone.

Measurement of cell-cycle phase-specific cell death using Hoechst 33342 and propidium iodide: preservation by ethanol fixation.

We developed a rapid technique for preservation of Hoechst 33342/propidium iodide-stained cells, using ethanol as a fixative. Combined staining with these dyes makes possible analysis of cell-cycle phase-specific cell death. The technique relies on exclusion of propidium iodide from the viable cells, whereas Hoechst stains all of the cells. The bivariate histograms resulting from the flow cytometric analysis contain the equivalent of two single-parameter DNA histograms, one of the living and the other of the dead cell population. Preservation of staining involved addition of 25% ethanol in PBS after propidium iodide staining and before Hoechst staining. The separation between the living and the dead cell populations was maintained for over 3 days at 4 degrees C. This technique will be valuable for quantitative evaluation of the cell-cycle phase-specific effects of cytostatic or cytotoxic agents, particularly in situations where a lag period between staining and analysis is unavoidable.