Preanalytical influences on DPC IMMULITE 2000 intact PTH assays of plasma and serum from dialysis patients

Case Report of a Satin Guinea Pig with Fibrous Osteodystrophy That Resembles Human Pseudohypoparathyroidism

A case report of a 2-year-old female satin guinea pig with a history of dental overgrowth and lameness and radiological lesions of fibrous osteodystrophy is presented. The most relevant clinical findings were bone demineralization, high level of parathyroid hormone (PTH), normophosphatemia, normal ionized calcium, and low total thyroxine (tT4) with a normal renal function. Long-term treatment was based on teeth coronal reduction and maintaining a balanced diet. PTH measurement was performed with a kit suitable for rats to test 4 different paired samples of guinea pigs and resulted in similar results for each pair of measurements. Two kits routinely employed in dogs and cats failed in measuring PTH in guinea pig serum samples. The ionized calcium, PTH, and tT4 values, not previously reported in similar cases, were obtained. The determination of tT4 could be useful in the diagnosis of fibrous osteodystrophy in guinea pigs. The observed findings show similarity with human pseudohypoparathyroidism type Ia, a disease caused by an inactivating heterozygous mutation of the stimulatory G protein α subunit from the maternal genome that induces multiple hormone resistance and that courses with a syndrome called Albright hereditary osteodystrophy. Naturally occurring pseudohypoparathyroidism in animals has been reported previously only in a ferret.

Analytical verification and method comparison of the ADVIA Centaur® Intact Parathyroid Hormone assay

OBJECTIVES

This study is the first verification of the novel iPTH Siemens ADVIA Centaur® Intact Parathyroid Hormone (iPTHm) chemiluminescence immunoassay based on monoclonal antibodies. We also compared the iPTH results obtained using this assay with the previous ADVIA Centaur® Parathyroid Hormone assay (iPTHp) based on polyclonal antibodies.

DESIGN AND METHODS

The analytical performance study of iPTHm assay included LoD, LoQ, intra- and inter-assay reproducibility, and linearity. A comparison study was performed on 369 routine plasma samples. The results were analyzed independently for patients with normal and abnormal GFR, as well as patients on hemodialysis. In addition, clinical concordance between assays was assessed. Finally, we studied PTH stability of plasma samples at 4°C.

RESULTS

For the iPTHm assay LoD and LoQ were 0.03pmol/L and 0.10pmol/L, respectively. Intra- and inter-assay CV were between 2.3% and 6.2%. Linearity was correct in the range from 3.82 to 203.08pmol/L. Correlation studies showed a good correlation (r=0.99) between iPTHm and iPTHp, with bias of -2.55% (IC -3.48% to -1.62%) in the range from 0.32 to 117.07pmol/L. Clinical concordance, assessed by Kappa Index, was 0.874. The stability study showed that differences compared to basal iPTH concentration did not exceed 20% in any of the samples analyzed.

CONCLUSIONS

The iPTHm assay demonstrated acceptable performance and a very good clinical concordance with iPTHp assay, currently used in our laboratory. Thus, the novel iPTHm assay can replace the previous iPTHp assay, since results provided by both assays are very similar. In our study, the stability of iPTH is not affected by storage up to 14days.

Interpretation of plasma PTH concentrations according to 25OHD status, gender, age, weight status, and calcium intake: importance of the reference values

CONTEXT

Reference values for plasma PTH assessment were generally established on small samples of apparently healthy subjects, without considering their 25-hydroxyvitamin D (25OHD) status or other potential modifiers of PTH concentration.

OBJECTIVE

Our objective was to assess ranges of plasma PTH concentration in a large sample of adults, stratifying by 25OHD status, age, gender, weight status, and calcium intake.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional survey is based on 1824 middle-aged Caucasian adults from the Supplémentation en Vitamines et Minéraux Antioxydants study (1994).

MAIN OUTCOME MEASURES

Plasma PTH and 25OHD concentrations were measured by an electrochemoluminescent immunoassay. Extreme percentiles of plasma PTH concentrations were assessed specifically in subjects who had plasmatic values of 25OHD of 20 ng/mL or greater and 30 ng/mL or greater.

RESULTS

Among subjects with 25OHD status of 20 ng/mL or greater, the 97.5th percentile of plasma PTH concentration was 45.5 ng/L. By using this value as a reference, 5% of the subjects with plasma 25OHD less than 20 nmol/L had a high plasma PTH level, reflecting secondary hyperparathyroidism. Among vitamin D-replete subjects (25OHD status of 20 ng/mL or greater), the 97.5th percentile of plasma PTH was higher in overweight/obese subjects (51.9 vs 43.5 ng/L among normal weight subjects).

CONCLUSIONS

The reference value for plasma PTH defined in this vitamin D-replete population was far below the value currently provided by the manufacturer (65 ng/L) and varied according to overweight status. These results may contribute to improve the diagnosis of primary and secondary hyperparathyroidism and subsequent therapeutic indication.

Variability in parathyroid hormone assays confounds clinical practice in chronic kidney disease patients

BACKGROUND

Intact parathyroid hormone (iPTH) measurements are used to guide therapy in renal patients, but variability in results can occur depending on the assay used. This study has investigated iPTH assay variation in North West England and paired data with regional audit data to determine clinical relevance of assay variability.

METHODS

Thirty-seven haemodialysis patients had blood taken (EDTA plasma, and serum), and samples were processed at 17 laboratories that analyse iPTH for North West dialysis patients. Correction factors were calculated and applied to the iPTH assay results to enable direct comparisons. These correction factors were also applied to Regional Audit data to determine if iPTH assay variability explains the variation in unit performance in achieving PTH targets.

RESULTS

The iPTH results from the 37 patients were significantly different when either analysed by different assays and/or different laboratories (P < 0.001). The Abbott Architect method consistently produced the highest iPTH results. Of the 37 patients, between 49% and 65% would achieve the Kidney Disease: Improving Global Outcomes (KDIGO) iPTH target depending on the assay used. When results were adjusted using correction factors, 21% of the patients would require a change of management according to guidelines. Data from all haemodialysis units submitted for the regional audit were adjusted to the Roche assay and this led to a small change in achievement of KDIGO iPTH targets in individual units when compared to each other.

CONCLUSIONS

A combination of iPTH assay variability and diversity in clinical management leads to variation in achieving iPTH targets. Both need to be improved and/or standardized to improve patient care.

Primary hyperparathyroidism with low intact PTH levels in a 14-year-old girl

CONTEXT

Primary hyperparathyroidism (PHPT) is usually associated with hypercalcemia and inappropriately elevated serum PTH.

OBJECTIVE

Our objective was to identify the reason(s) for a low serum intact PTH in a child with PHPT.

SUBJECT AND METHODS

A 14-yr-old Caucasian girl presented with pancreatitis, nephrolithiasis, hypercalcemia ranging from 13.2 to 17.5 mg/dl, and a low serum intact PTH. She had an ultrasound and technetium-99m-sestamibi scintigraphy confirming the presence of a parathyroid adenoma.

RESULTS

The preoperative serum intact PTH assays performed at Quest Diagnostics, Nichols Institute, were low even after serial dilutions, whereas the intraoperative turbo PTH assay was elevated at 3618 pg/ml. C-terminal and cyclase-activating PTH assays for PTH were also elevated. PTH gene sequence analysis performed from DNA extracted both from the parathyroid adenoma and the patient's peripheral blood leukocytes was negative for a mutation in the PTH gene sequence.

CONCLUSIONS

The contrasting values on the intact PTH assay and the turbo PTH assay suggest that the adenoma was producing an aberrant PTH molecule that was not detected by the routine intact PTH assay. Because there was no change in PTH gene sequence, this could be indicative of a posttranslational change in the PTH molecule that would not be recognized solely by DNA sequencing. Therefore, a low or normal PTH measurement against the backdrop of clinical and biochemical hyperparathyroidism needs measurement with a variety of assays.

Performance characteristics of six intact parathyroid hormone assays

The aim of this study was to evaluate the performance characteristics of 6 intact parathyroid hormone assays: Access 2 (Beckman Coulter, Fullerton, CA), ARCHITECT i2000(SR) (Abbott Diagnostics, Abbott Park, IL), ADVIA Centaur (Siemens Healthcare Diagnostics, Deerfield, IL), Modular E170 (Roche Diagnostics, Indianapolis, IN), IMMULITE 2000 (Siemens Healthcare Diagnostics), and LIAISON (DiaSorin, Stillwater, MN). Sample collection tubes and storage conditions were compared. Imprecision studies were performed using commercial quality control materials. Linearity was assessed using pools prepared from samples. For method comparison, serum and EDTA plasma samples were tested by all methods, and the ARCHITECT was used as the comparison method. Reference intervals were determined using various vitamin D cutoffs. The types of collection tubes and storage conditions are more important for some methods than others. Total coefficients of variation were 10.9% or less. The maximum deviation from the target recovery for linearity ranged from 5.0% to 82.2%. Bland-Altman plots demonstrated percentage biases ranging from -36.3% to 24.4%. The lower limit of the reference interval was not influenced by vitamin D status, whereas the upper reference limit was affected.

Estimation of the stability of parathyroid hormone when stored at -80 degrees C for a long period

BACKGROUND AND OBJECTIVES

Stability of parathyroid hormone (PTH) at -80 degrees C for long storage periods has never been studied. This can be of importance for the conclusions of studies where blood banks have been constituted. The study's aim was to evaluate stability of PTH when stored as serum or plasma EDTA samples at -80 degrees C.

DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS

Samples were collected from 16 chronic hemodialysis patients using EDTA and gel-separator tubes. Plasma and serum were aliquoted; one aliquot was assayed with Elecsys and Liaison methods to determine the "baseline" values and another aliquot after 1, 3, 6, and 12 mo. The factors "method," "tubes," "subjects," and "time" were included in a mixed linear model to evaluate their effects on measured PTH values. The prediction interval methodology was used to assess where a future result could be obtained with a defined probability.

RESULTS

With the Liaison method, the maximum storage times with either dry or EDTA tubes were estimated to be 9 and 2 mo, respectively. With the Elecsys method, samples could be stored at least 2 yr with acceptable level of degradation.

CONCLUSION

PTH stability at -80 degrees C is not infinite. Maximum storage time and acceptance limits (30%) were defined, showing that with one method, samples should be stored for not more than 2 mo, whereas the other could be stored for up to 2 yr. With any PTH assay, the maximum storage time should be evaluated to ascertain that samples will keep their initial reactive profile after prolonged storage periods.

Vitamin D and parathyroid hormone in general populations: understandings in 2009 and applications to chronic kidney disease

Vitamin D is now recognized as an important prohormone in health and disease. Its role in immunoregulation and cardiovascular and bone health has become topical in the lay press and the medical press in the past 5 yr. The target audience for this review includes the interested clinician and researchers. The prevalence of chronic kidney disease in the general population has further increased the interest and perhaps the applicability of findings of population studies. The basic physiology of vitamin D and receptor activation and biologic importance is reviewed, as well as various vitamin D analogues and nomenclature. Issues related to measurement of vitamin D and parathyroid hormone have the potential to complicate the clinical use of these tests and should be understood by all clinicians so as to ensure informed decision making and stimulate interest in participation in clinical trials. The epidemiology of vitamin D deficiency and supplementation in association with health status and disease status is reviewed, and issues related to association versus causation are highlighted. Some recommendations for pragmatic approaches and study design are suggested.

Normal parathyroid hormone levels in a diabetic patient with parathyroid adenoma

OBJECTIVE

The incidence of diabetes mellitus in patients with primary hyperparathyroidism and, conversely, primary hyperparathyroidism in diabetic patients are approximately threefold higher than the respective expected prevalence in the general populace. The diagnosis is straightforward when the patient presents hypercalcemia and inappropriately elevated serum parathyroid hormone (PTH) levels. We report a case of parathyroid adenoma in a diabetic patient with persistent hypercalcemia and normal PTH levels.

PATIENT

A 50-year-old female patient who was referred to our outpatient clinic presented with persistent hypercalcemia (serum Ca levels between 10.5 and 11 mg/dl) with a normal serum intact PTH level of 46.1 pg/ml. Her blood pressure was 120/80 mmHg, and she was being treated with antihypertensive therapy. Her HbA1c was 7.2%, and her triglycerides were in the normal range. A bone densitometry exam revealed osteopenia of radius -1.39, femoral neck -1.39, and the total hip -1.04. A neck ultrasound revealed a mass of 13 mm next to the inferior and posterior of the right thyroid lobe. A dual phase Tc-99m-sestamibi scan revealed an area of increased uptake in the same region, which is indicative of a parathyroid adenoma. The parathyroid adenoma was removed, which resulted in the achievement of normocalcemia.

CONCLUSION

Diabetic patients should be evaluated for hyperparathyroidism as associated hypertension can complicate the course of the disease. These patients should be evaluated for primary hyperparathyroidism when they exhibit persistent hypercalcemia and when clinical suspicion is aroused even if the serum PTH levels are within the normal range.

Variation in serum and plasma PTH levels in second-generation assays in hemodialysis patients: a cross-sectional study

BACKGROUND

Previous reports show that parathyroid hormone (PTH) concentrations may vary widely depending on the assay used to assess PTH. In this cross-sectional study, we aim to determine the usefulness of standardizing blood handling for optimal interpretation of PTH in patients with chronic kidney disease.

STUDY DESIGN

Diagnostic test study.

SETTING & PARTICIPANTS

Predialysis blood was sampled in 34 long-term hemodialysis patients at a single academic medical center.

INDEX TEST

PTH was measured by using 6 different automated second-generation assays (Elecsys, Advia Centaur, LIAISON, Immulite, Architect, and Access assays), 3 blood specimen types (serum, EDTA plasma, and citrate plasma), and 2 consecutive days of measurement (after thawing and 18 hours later with samples having been let at room temperature).

REFERENCE TEST

None.

RESULTS

A mixed statistical analysis model showed that the nature of the assay (P < 0.001) and nature of the blood sample (P < 0.001) significantly influenced variability in PTH concentrations, whereas day of measurement (day 1 or 2) did not (P = 0.5). Most PTH variability was caused by observations (96.8%), then manufacturer's kit (2.5%), and last, specimen type (0.7%). PTH concentrations measured in citrate plasma were lower with every assay method used than those observed in serum or EDTA plasma. The interaction between manufacturer and specimen type was of moderate statistical significance (P = 0.04). To evaluate the potential clinical consequence of PTH measure variability, we classified patients according to Kidney Disease Outcomes Quality Initiative cutoff values (PTH < 150 pg/mL; PTH, 150 to 300 pg/mL; and PTH > 300 pg/mL). Overall, statistical classification agreement was moderate to high for comparison between assays and high to very high between different blood samples and between days of measurement. However, we found that up to 11 of 34 patients were classified in different categories with some assays (LIAISON versus Architect) and up to 7 of 34 in different categories with different blood specimen type (citrate plasma versus serum [corrected] in LIAISON assay).

LIMITATIONS

This is a cross-sectional study that used single lots of reagents. There currently is no reference method for the measurement of PTH and no recombinant PTH standard for PTH assay.

CONCLUSION

PTH variability caused by the nature of the assay and/or blood specimen type is large enough to potentially influence clinical decision making. A specified collection method therefore should be used for PTH measurements. In routine practice, we recommend serum PTH over EDTA or citrate plasma.

False positive PTH results: An easy strategy to test and detect analytical interferences in routine practice

BACKGROUND

As other immunoassays, PTH determination is not free from interferences. Indeed, natural antibodies like heterophile antibodies (HAMA) and rheumatoid factor (RF) can induce falsely elevated results, leading to misdiagnosis and expensive unnecessary explorations. However, in routine practice, these interferences are not always obvious to detect.

METHODS

On 2084 PTH samples, we applied a validation strategy in four steps to screen for HAMA and rheumatoid factor interferences.

RESULTS

36% of our samples presented an elevated PTH. We found a clinically plausible reason for 91% of them. The remaining 63 suspicious samples were treated with HBT and 40% of them were found to be HAMA positive. RF determination was performed on the HAMA-negative samples and RF was positive in 21 of them. They were then treated with RF-Absorbent. Nine of these 21 samples presented RF interference.

CONCLUSION

Applying this strategy in our routine validation, we managed to avoid spuriously elevated PTH results, which could have caused medical errors as well as unnecessary cost-effective extra-investigations.

Stability of intact parathyroid hormone in samples from hemodialysis patients

The determination of intact parathyroid hormone levels is used for diagnosis and in the management of renal osteodystrophy. Pre-analytical and analytical conditions are important in the overall confidence of the assay. Unfortunately, there are no clear recommendations for the use of serum samples or samples anticoagulated with ethylenediaminotetraacetic acid (EDTA) for the best preservation of intact parathyroid hormone. In our study, the Roche Elecsys assay was used to measure intact hormone in both serum and EDTA plasmas from 16 hemodialysis patients over the span of a month. Parathyroid hormone stability was determined in samples kept frozen for 1-5 days or after 8-24 h at room temperature. There was no difference in hormone stability between serum and EDTA samples after 1 day in frozen storage. After 5 days frozen, hormone degradation was significantly greater after EDTA anticoagulation than in serum aliquots. When samples were stored at room temperature, intact parathyroid hormone was significantly more stable in EDTA-treated samples than in clotted serum samples, especially after 24 h. We conclude that optimum results are achieved in the measurement of intact parathyroid hormone levels depending on the workflow of the lab. If the lab works with intermittent batches of samples, frozen serum is the best. If the lab services general practitioners and/or several hospitals and has a continuous flow of samples, EDTA-treated samples stored at room temperature are the best.

Practical considerations in PTH testing

New knowledge concerning PTH biology have accumulated during the past few years. The finding that the so-called "intact" PTH assays measure a "non-1-84" PTH fragment in addition to full-length PTH has led to the development of new assays. These new assays, which were initially thought to measure 1-84 PTH only, have been shown to recognize also another PTH species called "amino-PTH". As the various names given to the different assay methods are highly confusing, there is a need for a simplified nomenclature. A simple way would be to identify the older "intact" PTH assays as second-generation assays and the new assays (Whole, CAP, BioIntact) as third-generation assays. Although of considerable potential interest for the comprehension of PTH physiology, the third-generation PTH assays have not yet proved to be superior to the second-generation assays in clinical practice. There is thus currently no recommendation to switch from the second-generation to the third-generation assays in clinical practice, or to use a ratio derived from the concommitent measurement of PTH with both assay-generation. Because second- and third-generation PTH assays are usually highly correlated, significant differences in the clinical information provided by these methods are unlikely. However, our opinion is that more definitive bone biopsy studies in dialyzed patients selected according to their bone- and calcium-related treatment are still needed to reach a consensus. Finally, we have proposed that PTH reference values should be established in healthy subjects with a normal vitamin D status. This supposes that 25OHD is measured in the reference population beforehand, and that the subjects with vitamin D insufficiency are eliminated from the reference group. Although more complicated than the usual way to establish normative data, we have shown that it decreases the upper limit of normal by 25-35%, enhancing thus the diagnostic sensitivity for hyperparathyroidism without a decrease in specificity.