Measurement of ionised calcium in body fluids-a review

All-Solid-State Calcium Sensors Modified with Polypyrrol (PPY) and Graphene Oxide (GO) as Solid-Contact Ion-to-Electron Transducers

Reliable, cost-effective, and robust screen-printed sensors were constructed and presented for Ca2+ ions determination. The sensors were based on the use of bilirubin (1,3,6,7-tetramethyl-4,5- dicarboxyethy-2,8-divinyl-(b-13)-dihydrobilenone) as a recognition sensory material in plasticized poly (vinyl chloride) (PVC) membranes. Polypyrrol (PPY) and graphene oxide (GO) were used as ion-to-electron transducers, where the effects of anionic excluder, pH, and selectivity were investigated. In a 50 mM tris buffer solution of pH 5, the electrodes offered a potential response for Ca2+ ions with a near-Nernstian slopes of 38.1 ± 0.4 (r2 = 0.996) and 31.1 ± 0.6 (r2 = 0.999), detection limits 3.8 × 10−6 (0.152 μg/mL) and 2.3 × 10−7 M (8.0 ng/mL), and linear concentration ranges of 7.0 × 10−6–1.0 × 10−2 (400–0.28 μg/mL) and 7.0 × 10−7–1.0 × 10−2 M (400–0.028 μg/mL) for sensors based on PPY and GO, respectively. Both sensors revealed stable potentiometric responses with excellent reproducibility and enhanced selectivity over a number of most common metal ions, such as Na+, K+, Li+, NH4+, Fe2+, Mg2+, and Ba2+. Impedance spectroscopy and chronopotentiometric techniques were used for evaluating the potential drift and the interfacial sensor capacitance. The proposed sensors offered the advantages of simple design, ability of miniaturization, good potential stability, and cost-effectiveness. The developed electrodes were applied successfully to Ca2+ ion assessment in different pharmaceutical products, baby-food formulations, and human blood samples. The results obtained were compared with data obtained by atomic absorption spectrometry (AAS).

Conformational comparability of factor IX-Fc fusion protein, factor IX, and purified Fc fragment in the absence and presence of calcium

A long lasting recombinant factor IX -Fc fusion protein (rFIX-Fc) is being developed for the treatment of hemophilia B and is currently in late stage clinical investigation. By limiting injection frequency and maintaining efficacy, rFIX-Fc shows promise as a new therapeutic option for hemophilia B patients. However, before gaining regulatory approval, rFIX-Fc must undergo rigorous analytical and biological testing, in addition to clinical trials. Included in this testing is the need to understand this protein's higher-order structure and dynamics. In this study, we investigated and compared the biophysical properties of rFIX-Fc, rFIX, and Fc using hydrogen/deuterium exchange mass spectrometry and differential scanning calorimetry. Within the limits of these techniques, our results show that structural comparability exists between rFIX and the FIX region of rFIX-Fc. In addition, changes in the structure and dynamics of both proteins, in response to calcium binding, a requirement for FIX function, are also highly comparable. In the case of Fc and Fc region of rFIX-Fc, conformational comparability is also established. These biophysical results further support the conclusion that fusing an immunoglobulin gamma 1 Fc to rFIX does not significantly alter the higher-order structure of FIX or Fc, Ca binding to FIX, or Fc functionality.

Calcium influxes into brain and cerebrospinal fluid are linearly related to plasma ionized calcium concentration

Unidirectional Ca influxes into brain and cerebrospinal fluid (CSF) were measured at different plasma concentrations of ionized Ca ([Ca]i) in pentobarbital-anesthetized rats. Plasma [Ca]i was varied acutely from 0.6 to 3.0 mumol/ml by intravenous infusion of EGTA, NaCl or CaCl2 or by thyroparathyroidectomy. Ca influx was determined from the 15-min uptake of 45Ca after intravenous injection. There were significant regional differences in 45Ca uptake into the CNS, with a approximately 20-fold greater rate into ventricular CSF than into frontal cortex. Autoradiographs of 45Ca uptake demonstrated that uptake into frontal cortex reflects primarily transport across the cerebral capillaries, whereas uptake into ventricular CSF reflects transport across the choroid plexuses. At both sites, Ca influx was a linear function of plasma [Ca]i and extrapolated to zero at [Ca]i = 0. Infusion of EGTA or CaCl2 did not alter the integrity of the blood-brain barrier, as determined by the permeability to [14C]sucrose. These results indicate that Ca influx into the CNS is not regulated by a saturable mechanism that is sensitive to acute changes in plasma [Ca]i. The proportionality between influx and concentration is suggestive of passive diffusional transport. The brain is protected from acute changes in plasma [Ca]i by the low cerebrovascular permeability to Ca, approximately 5 X 10(-8) cm/s.

Assessment of calcium homeostasis in the critically ill surgical patient. The diagnostic pitfalls of the McLean-Hastings nomogram

Hypocalcemia is a common problem in critically ill surgical patients. We prospectively evaluated whether measurement of the total serum calcium (Ca) concentration or calculation of the serum ionized Ca level (by the McLean-Hastings nomogram) accurately reflects the measured serum ionized Ca level. Although 71% and 58% of 156 predominantly surgical intensive care unit (ICU) patients were hypocalcemic by the total serum Ca or calculated ionized Ca level, respectively, only 12% were hypocalcemic by directly measured serum ionized Ca measurement. The total serum Ca and calculated ionized Ca concentrations were sensitive (95% and 89%, respectively) but lacked specificity (32% and 46%, respectively) in predicting ionized hypocalcemia. Analyses of Ca binding to albumin in the serum of surgical ICU patients and normal subjects suggested that there is a circulating factor in critically ill patients that increases the binding of Ca to albumin. These observations may explain why the McLean-Hastings nomogram underestimates the protein-induced changes in serum Ca in critically ill surgical subjects. We conclude that: total serum Ca and calculated ionized Ca concentrations are poor indicators of the true serum ionized Ca status in critically ill surgical patients, and we recommend direct measurement of serum ionized Ca levels in these patients; and variability in the affinity of Ca for binding proteins in critical illness may explain the poor correlation between serum total and ionized Ca measurements.

Electrodes in clinical chemistry

On theoretical grounds it would appear preferable to use calibrants which are not adjusted to a constant ionic strength, whether concentration or activity is being estimated directly. For indirect methods, when sample and calibrant are diluted in an ionic strength adjusting buffer, there is probably little difference between adjusted and unadjusted calibrants. In this case a simple calculation of concentration is possible. A simple algorithm may be used for activity estimation. The algorithm for direct concentration estimation is more complex. If concentration, rather than activity, is required an indirect method must be used. If activity is required a direct method must be used. The quantitative effects of variations in calibration procedure may be smaller than variations in junction potential.

Free ionised calcium--a critical survey

It is our aim to summarise and discuss procedures for the evaluation of the concentration of free ionised calcium in serum or plasma. Stress is laid upon the interrelations and relative validity of the most common algebraic expressions to appraise the calcium status. The multitude of formulae proposed in the literature are, by mathematical discussion, reduced to variations on a single theme. A second topic is the direct potentiometric measurement of free ionised calcium concentration. Finally we review the literature on the clinical utility of measuring or calculating the free ionised calcium concentration.

The effect of ionized calcium, pH, and temperature on bioactive parathyroid hormone during and after open-heart operations

Normal myocardial function is dependent on the metabolic balance of a number of electrolytes and hormones. The calcium ion plays a major role in muscle contraction and is rigorously controlled within narrow limits. Open-heart surgery imposes metabolic disturbances on both electrolytes and hormones, especially ionized calcium. Normally, ionized calcium levels are controlled by parathyroid hormone with a negative feedback from the ionized calcium controlling the system, but the results from this study suggest that during open-heart procedures, ionized calcium does not impose its normal negative feedback on bioactive parathyroid hormone secretion. The low blood pH levels that occurred during the operative conditions of the patients studied and the level of hypothermia imposed on the circulating blood during cardiopulmonary bypass appeared to influence the control of parathyroid hormone secretion, causing high levels of hormone to be secreted during this period.

A calcium hypothesis of antidepressant action

The biogenic amine hypothesis of affective disorders has served as a valuable model for investigations concerning the mechanism of action of antidepressant drugs. Despite numerous publications on the subject, the manner in which tricyclic antidepressants elicit their therapeutic effect remains unknown. Recent findings which involve tricyclic antidepressant interactions with thyroid hormone, along with an appraisal of the older literature concerned with dyscalcemias and affective disorders, provides the basis for an hypothesis featuring a role for calcium in mediating antidepressant responses.

Preparation of material to control precision of calcium selective electrodes

A simple procedure is described for the preparation of a stable precision quality control material for use in the measurement of level of ionised calcium in serum at or near the reference range. Repeat analyses on a Nova 2 ionised calcium analyser of serum pools stored at different temperatures over a period of three months showed coefficients of variation less of less than 4%.

Ionized calcium in body fluids

This article will review the methods currently available to the clinician and research worker for measuring the concentration of ionized calcium in various body fluids including whole blood, serum, plasma, urine, cerebrospinal fluid, milk, and synovial fluid. The methods to be reviewed are based on procedures involving bioluminescence, colorimetry and ion-selective electrodes. Emphasis will be given to the precision and, wherever possible, accuracy of each technique. Possible sources of error and interfering agents will be identified. Attention will be given to the recommended conditions for measuring ionized calcium in each body fluid. An assessment will be made of the theoretical and practical importance of measuring ionized calcium rather than total calcium and of its value in clinical medicine.

Calcium measurements in serum and plasma--total and ionized

This article will review the methods currently employed for measuring the concentrations of total and ionized calcium in serum or plasma. As far as total calcium is concerned, various techniques such as atomic absorption spectrometry, spectrophotometry, fluorometry, complexometric titration, and flame photometry will be described and compared. Particular emphasis will be given to the accuracy and precision of each technique. Possible sources of error and interfering agents will be identified and the various procedures for the taking and handling of blood samples evaluated. Inter-laboratory variation in the measurement of calcium will be studied. An assessment will be made of a new reference method for measuring total calcium in serum using isotope-dilution mass spectrometry. The usefulness of the total calcium measurement in clinical medicine will be briefly discussed. Within the last decade the refinement of spectrophotometric techniques and the improvements in ion-selective electrode technology have revolutionized the measurement of ionized calcium in serum, such that it may now be possible to replace total calcium measurements with ionized calcium measurements on a routine basis. The various techniques currently in use for measuring ionized calcium will be described and evaluated. Particular attention will be paid to the preparation of standards, the procedures for taking blood samples, and the handling of the samples prior to and during measurement. An assessment of the relative value of measuring total and ionized calcium will be presented.