Measurement of ionized calcium in canine blood samples collected in prefilled and self-filled heparinized syringes using the i-STAT point-of-care analyzer

Sample stability and heparin interference in ionized calcium and ionized magnesium measurements in horses using the Stat Profile Prime Plus co-oximetry electrolyte analyzer

BACKGROUND

The determination of iCa and iMg is important in veterinary medicine, but their immediate determination in whole blood is not always possible. Their stability in other sample types and the existence of interferences must be evaluated before its use.

OBJECTIVES

We aimed to analyze the effects of storage time on the stability of iCa, iMg, and other analytes in whole blood, plasma, and serum samples in horses and assess the interference of heparin in these measurements.

METHODS

Whole blood, heparin-plasma, and serum samples from 10 horses were stored at 4°C and analyzed 1, 2, 3, 4, 5, 6, 7, 8, 24, 48, and 168 hours after sample collection using the Stat Profile Prime Plus Vet equipment (Nova Biomedical, Waltham, MA, USA). Results were analyzed by ANOVA or mixed-effect models.

RESULTS

The concentration of iCa, iMg, total calcium (tCa), total magnesium (tMg), and the ratios iCa/tCa and iMg/tMg did not differ up to 168 hours when compared to the initial time. Total Ca, iMg, and tMg were not significantly different among sample types, but iCa concentrations were slightly but significantly lower in plasma. Freezing at -20°C did not affect iCa, iMg, tCa, and tMg. The pH increased in serum and plasma after 8 hours, and a mild negative correlation existed between plasma iCa concentration and pH. A negative correlation was observed also between the ratios iCa/tCa or iMg/tMg and pH in plasma and serum. A significant decrease in iCa and iMg was detected when comparing homemade syringes at high heparin concentration (~200-300 U heparin/mL) and commercial lithium-heparin tubes (20-30 U/mL).

CONCLUSIONS

Samples stored at 4°C can be used to determine iCa and iMg concentrations up to 7 days after collection. Other metabolites are stable for up to 8 hours; heparin interference should be taken into account if using homemade heparin syringes.

Comparison of ionised calcium measured using a portable analyser to a reference method in healthy dogs

OBJECTIVES

To compare the ionised calcium measured on a portable analyser (iSTAT, Abbott) to a reference method.

MATERIALS AND METHODS

Blood samples from 39 apparently healthy dogs were analysed in duplicate using a portable analyser and a reference method (Radiometer ABL800 FLEX). Bland-Altman plots and Passing-Bablok regression were used to assess constant and proportional bias between the two instruments. A within-assay percentage coefficient of variation and total error (TE) was calculated for both analysers. The reference interval was calculated for the portable analyser using the robust method with confidence interval bootstrapping.

RESULTS

The Bland-Altman plot showed a -0.036 mmol/L difference between the two instruments (95% confidence limit -0.08 to 0.01 mmol/L; limits of agreement -0.07 to 0.006 mmol/L). Neither the Bland-Altman plot nor the Passing-Bablock regression (slope -0.03; 95% confidence interval -0.08 to 0.19 and intercept 1; 95% confidence interval 0.83 to 1.2) showed significant proportional bias. The coefficient of variation for the portable analyser was 1.08%, compared to 0.78% for the reference method with a total error of 3.5% for the portable analyser. The estimated population-based reference interval for ionised calcium using the portable analyser is 1.23 to 1.42 mmol/L.

CLINICAL SIGNIFICANCE

For the healthy dogs in this study, compared to the reference method, the portable analyser showed no significant bias for measurement of ionised calcium. Further studies including hyper and hypocalcaemic dogs are required to determine clinical impact of the use of this analyser.

The multivariate predictive model to estimate ionized calcium concentration from serum biochemical results in dogs: External validation

BACKGROUND

Predicted ionized calcium (piCa) can be calculated from routine biochemistry variables using a recently developed predictive model in dogs. However, it has not been evaluated with variables measured from multiple laboratories.

OBJECTIVES

We aimed to (a) externally validate piCa in dogs where biochemistry results were obtained from different analyzers, and (b) compare the diagnostic performances of piCa and total calcium (tCa).

METHODS

A cross-sectional multicentric study on 138 dogs from three different hospitals was performed. The sensitivity (Sen), specificity (Spe), positive (PPV) and negative predictive values (NPV), and diagnostic discordance of piCa and tCa were calculated using logistic regression for ionized hypercalcemia and hypocalcemia. Diagnostic performance fluctuations across hospitals were also assessed.

RESULTS

For ionized hypercalcemia, the Sen (81.8%), Spe (96.1%), PPV (69.2%), NPV (97.7%), and diagnostic discordance (5.1%) of piCa were not significantly different among hospitals or from those of tCa. For ionized hypocalcemia, the Sen (range: 9.7%-53.8%) and Spe (range: 95.6%-99.6%) of piCa and tCa (Sen range: 16.2%-87.8%; Spe range: 58.3%-98.1%) varied across hospitals, although to a lesser extent for piCa. The diagnostic discordances of piCa (20.3%) and tCa (25.4%) were close. The prediction interval (PI) of piCa demonstrated high Sen to screen for ionized hypercalcemia (100%) and hypocalcemia (range: 75%-93.3%), and high Spe to diagnose ionized hypercalcemia and hypocalcemia (100% for both).

CONCLUSIONS

These results support the external validation of piCa in dogs. Its PI represents a notable advantage over tCa to help clinicians explore calcium-related disorders when ionized calcium cannot be readily measured.

Spurious Results in Mineral and Electrolyte Analysis

Assessment of health status and the course of treatment of patients are often determined by results obtained from analysis of blood parameters. Errors in results can occur and cause inappropriate interpretation of laboratory data and, therefore, disease states. Electrolytes and minerals are particularly prone to spurious results; therefore, it is critical that factors influencing inappropriate resulting be recognized, and steps taken to minimize them.

Serum Ionized Calcium Quantification for Staging Canine Periodontal Disease: A Preliminary Study

Periodontal diseases (PD) are infectious, inflammatory, progressive diseases of the oral cavity affecting people and dogs. PD takes 2 forms: gingivitis and periodontitis. Diagnosing or staging PD can be achieved only with dental x-rays and periodontal probing, both of which require the use of general anesthesia in dogs. This study aimed to determine whether serum ionized calcium ([iCa(2+)]) levels can be useful in preliminary PD staging in dogs. A sample of 40 dogs (n = 40) was divided into 4 groups (n = 10 each) based on the following PD stages: G1 (gingivitis), G2 (initial periodontitis), G3 (moderate periodontitis), and G4 (severe periodontitis). The groups were then subjected to [iCa(2+)] quantification. Statistically significant differences were observed between PD stages and [iCa(2+)] for all stages except G3 and G4. Therefore, this parameter can be used as an additional tool to establish and monitor preliminary PD status.

Use of bisphosphonates to treat severe idiopathic hypercalcaemia in a young Ragdoll cat

A 3-year-old Ragdoll cat was referred for investigation of polyuria, polydipsia, vomiting, weight loss and hypercalcaemia. Serum biochemical abnormalities included total and ionised hypercalcaemia and hypophosphataemia. Following clinical investigations a diagnosis of idiopathic hypercalcaemia was made. Because of the severity of the hypercalcaemia and the associated clinical signs, treatment for hypercalcaemia was commenced with pamidronate. Major electrolyte abnormalities were detected but, remarkably, were accompanied by minimal clinical signs. The cat was subsequently treated with oral alendronate and is clinically normal 15 months later. Reports of the use of bisphosphonates in cats are limited and close monitoring of patients is recommended.

A comparison of total calcium, corrected calcium, and ionized calcium concentrations as indicators of calcium homeostasis among hypoalbuminemic dogs requiring intensive care

OBJECTIVE

(1) To evaluate whether total calcium (tCa) correlates with ionized calcium (iCa) in hypoalbuminemic dogs; (2) to evaluate whether calcium adjusted for albumin (Alb), or total protein (TP), or both accurately predict iCa concentrations and hence can be used to monitor calcium homeostasis in critically ill hypoalbuminemic dogs; and (3) to evaluate factors associated with any potential discrepancy in calcium classification between corrected total and ionized values.

DESIGN

Prospective observational clinical study.

SETTING

Small animal intensive care unit in a veterinary medical teaching hospital.

ANIMALS

Twenty-eight client-owned dogs with hypoalbuminemia.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

iCa was determined using ion-specific electrode methodology, on heparinized plasma. The tCa concentration was adjusted for Alb and TP using published equations. In total 29% (8/28) of the hypoalbuminemic, critically ill dogs in this study were hypocalcemic at intensive care unit admission, as determined by iCa measurement. Corrected calcium values failed to accurately classify calcium status in 67.9% and 64.3% of cases, according to whether the Alb-adjusted or TP-adjusted values, respectively, were used. The sensitivity and specificity of the tCa to evaluate hypocalcemia was 100% and 47%, respectively. The sensitivity and specificity of the correction formulae were 37.5% and 79% for the Alb-adjusted values and 37.5% and 74% for TP-adjusted values. tCa overestimated the presence of hypocalcemia and underestimated the presence of normocalcemia, while corrected calcium values overestimated the presence of normocalcemia and underestimated the presence of hypocalcemia.

CONCLUSIONS

Calcium homeostasis in hypoalbuminemic critically ill dogs should be evaluated by iCa concentrations rather than tCa or calcium adjusted for Alb or TP. Given that tCa has 100% sensitivity for detecting hypocalcemia in this population it is recommended that all hypoalbuminemic and critically ill patients with low tCa should be evaluated with an iCa measurement.