I. Subject: Ionized Calcium (Ca++) Analysis Whole Blood

Transcription

1 I. Subject: Ionized Calcium (Ca++) Analysis Whole Blood II. Method: i-stat III. Principle: A. Ca++: is measured by ion-selective electrode potentiometry. Concentrations are calculated from the measured potential through the Nernst equation. Ionized calcium (Ca++) is the physiologically active form of calcium, which comprises approximately 45% of the total calcium in plasma. It is essential for the contractility of smooth vascular muscle, and, it plays a vital part in cardiovascular function. It is also important in muscle function, nerve function, and bone formation, and it is a cofactor in many cellular hormone and enzyme reactions. The action of the parathyroid hormone (PTH)- 1,25 dihydroxyvitamin D (1,25D)- and calcitonin closely controls the concentration of calcium in extracellular fluid, and regulates the transport of calcium across the gastrointestinal tract, kidney, and bone. Calcium is one of the most tightly controlled analytes in the body with fluctuations of less than 5% occurring about the mean during a 24-hour period. Clinically, hypocalcemia can result from a deficiency of PTH or 1,25 D, which can be causedby malabsorption of vitamin D, hypoparathyroidism, or chronic renal failure. Hypercalcemia, which occurs more frequently than hypocalcemia, is commonly caused by primary hyperparathyroidism and malignant disease. The elevated calcium resulting from both of these conditions can produce abnormal cardiovascular rhythms. In critical care situations, especially where large amounts of blood are being transferred, ionized calcium levels should be monitored closely. Transfused blood typically contains citrate as an anticoagulant that can bind ionized calcium and affect its level in the blood. Although total calcium levels may increase, ionized calcium may decrease and lead to cardiac and neuromuscular malfunction. When measuring ionized calcium, ph should be measured. Because hydrogen ions compete with calcium for calcium binding sites, a change in sample ph can have a direct effect on calcium levels. For example, a change in ph of 0.1 can cause a change in calcium of 0.2 mg/dl, which exceeds the span of the normal range. Its 1

2 effects, if not taken into account, are clearly significant. Clinical differences between arterial capillary, and venous specimens are small and insignificant. IV. Specimen: A. Patient preparation: It is essential that the patient sample is drawn when the patient is at steady state. 1) Have the patient maintain a relaxed state and normal breathing for at least ten (10) minutes. 2) Have the patient maintain a sitting or lying position for at least five (5) minutes before collecting blood. 3) Ideally, collection should not be made for four (4) hours after eating. B. Specimen type: Whole Blood 1) Specimen is commonly venous, but may be arterial if drawn with blood gases. 2) Specimen must be anti-coagulated with dry lithium heparin. The following choices in heparin preparations may be used: C. Sample size: a) dry lithium heparin of concentration < 10 IU/mL. b) calcium-titrated heparin preparations < 70 IU/mL. c) heparin balanced with appropriate amounts of Ca++, Na+, K+, and H+ < 40 IU/mL d) heparin containing zinc < 20 IU/mL. Adult/Pediatric Neonatal Optimum: ml 0.25 ml Minimum: 95 ul 90 ul 2

3 D. Collection Containers: Plastic or glass syringe containing anti-coagulate, or calcium titrated capillary tube. E. Sample Stability: Specimen will be stable for 30 minutes of collection. F. Unacceptable specimens: 1) air contaminated specimen 2) inadequate sample volume 3) coagulated specimen 4) un-iced sample (longer than ten (30) minutes), iced > 1 hour 5) evidence of stasis (tourniquet left longer than one (1) minute before venipuncture 6) evidence of extra muscle activity (fist pumping) 7) sample hemolysis 8) improperly labeled or improperly requisitioned specimen 9) any specimen posing a safety hazard to lab personnel. (Refer to department policy on "Handling of Improper Blood Gas Specimens") G. Specimen Collection: Refer to the policy and procedure on Blood Gas Sampling for information on arterial blood collection and capillary tube collection. 1) Standard Precautions must be observed at all times in collecting and handling specimens. Gloves must always be worn. Gowns and protective eye wear and fluid barrier body shields should be worn if there is the 3

4 possibility of splashing or spraying of blood. Use extreme caution to avoid needle stick injury. Use needle protective device. Never re-cap needles. Dispose of needles in Sharps container. Samples must be transported in containers labeled as Hazardous materials. 2) Determine appropriate site for peripheral venous collection. Always draw specimen below site of intravenous infusions. 3) Place tourniquet on upper arm to distend vein. DO NOT ALLOW PATIENT TO EXERCISE ARM. 4) Wipe area with alcohol wipe. Allow to dry for thirty (30) seconds. 5) Penetrate the lumen of the vein at a oblique angle with a gauge needle and withdraw specimen. DO NOT UNDO TOURNIQUET DURING BLOOD COLLECTION. 6) Remove the needle from the arm and apply firm pressure to puncture site with a clean gauze. H. Handling and Transport Conditions: 1) Using a one-handed technique, place needle tip into a needle protective device. Remove needle and discard into Sharps container. Expel any air bubbles. Cap syringe. 2) Anticoagulant must be thoroughly mixed with specimen by rolling syringe gently between both hands. 3) Samples should be run as soon as possible because of damages that may occur due to continued blood cell respiration and subsequent changes in ph. Therefore, specimens must be run within thirty (30) minute of collection. 4) The specimen itself must be labeled if not performed at point-of-care. The label must have the patient's name and unique identification number, and the collection date and time. Non-washable ink should be used. 5) A written or computer generated requisition must accompany the specimen. 4

5 (Refer to the department policy on Requisitioning of Blood Gases). V. Reagents: Refer to i-stat Analysis policy. VI. Calibration: Refer to i-stat Analysis policy. VII. Quality Control: Refer to i-stat Analysis policy VIII. Procedure: Refer to i-stat Analysis policy IX. Calculations: (Automated) X. Results: A. References Ranges: TEST UNITS NORMALS Ca++ mmol/l ph B. Critical Values: Ca++ mmol/l <0.78 or > 1.58 Refer to the department policy on Blood Gas Critical Values. C. Technical Ranges: TEST UNITS TECH LIMIT Ca++ mmol/l Technical ranges have been established by validation of instrument accuracy over the reported ranges using a commercially prepared and assayed calibration verification material, traceable to NBS standard reference materials. 5

6 D. Reporting: 1) Record patient demographics. 2) Record results with units as follows: Ca++ x.xx mmol/l ph x.xxx Specimen type 3) Record date and time of collection, receipt, and reporting. 4) Record specimen type and any pertinent clinical information. 5) Record the patient name, hospital number, test accession number, and the machine cup number on the accession log. 6) Results that fall outside established technical ranges will be reported a > or < the technical limit exceeded. XI. Procedural Limitations: A. Refer to the i-stat Operator s Manual policy and procedure for instrument performance specifications. B. Ionized calcium varies significantly with the time of day, with changes reported ranging from 4 to 10%. These changes could be due to the effects of meals, daily variation in acid-base balance, and sleep. Because a study has shown significant differences in the circadian variation of ionized calcium between the sexes, hormonal variations might also have an influence. 6

7 C. Interfering Substances: ANALYTE INTERFEREN T INTERFERENT CONCENTRATION EFFECT ON ANALYTE RESULT. Ionized Calcium Magnesium - hydroxybutyrat e Lactate Salicylate 1.0 mmol/l above normal 20 mmol/l 20 mmol/l 4.34 mmol/l Increase ( ) ica by 0.04 mmol/l Decrease ( ) ica by 0.1 mmol/l Decrease ( ) ica by 0.05 mmol/l Decrease ( ) ica by 0.1 mmol/l XII. References: NCCLS, Considerations in the Simultaneous Measurement of Blood Gases, Electrolytes, and Related Analytes in Whole Blood; Proposed guideline, November NCCLS, Ionized Calcium Determinations: Precollection Variables, Specimen Choice, Collection, and Handling; Approved Guideline, December i-stat Operator s Manual,