Ionized Calcium Test Volume at Duke Medical Center

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Data from Dr. John Toffaletti, Duke Univ. Blood Gas and Clinical Pediatric Lab

Major Clinical Uses of Ionized Calcium Measurements

To maintain hemodynamic function: cardiac contractility, vasoactive tone, etc.

For monitoring patients in critical care: In sepsis, mortality is higher among hypocalcemic patients

(hypoparathyroid condition) Neonates Patients with pancreatitis, renal disease, etc

To diagnose / treat hypercalcemic and hypocalcemic conditions:

Hyper- or hypoparathyroidism, malignancy, etc.

Free Calcium Ions Appear to be the Physiologically-Active Form of Calcium

Ca ion entry into myocardial cells initiates contraction

Ca ions act as intracellular “2nd messengers” which initiate cellular production of many enzymes and hormones

Ca ion entry into smooth muscle cells controls blood pressure

The parathyroid gland responds to the ionized Ca concentration

What Are Ionized, Complex-Bound, Protein-Bound and Ultrafiltrable Calcium?

Protein-Bound Ca

80% Ca-Albumin

20% Ca-Globulin

UnassociatedCalcium Ions

Complex-Bound Ca

35-45 % of Total Ca

10-15% of Total Ca

45-50% of Total Ca

Ult

rafi

ltra

ble

Ca

To

tal

Ca

lciu

m

Ion

ized

Ca

Calcium Equilibria in Blood

higher pH

lower pH

HCO3

Citrate

HPO4

Lactate

Complex-Bound

Calcium

Ca ++

Ca ++

Ca ++

Ca ++

Ca ++

Ionized Calcium

Ca ++

Ca ++

Ca ++

Ca ++

Ca ++

Protein-Bound Calcium

Ca ++ Ca ++

Ca ++ Ca ++

Albumin

How Does pH Affect Ionized Calcium?

Ionized calcium changes about 0.06 mmol/L for each 0.1 change in pH:

Note the concentration of albumin is constant in these examples. pH affects the amount of Ca ions that bind to albumin.

pH ion Ca (mmol/L) Alb (g/dL)7.20 1.35 4.07.30 1.30 4.07.40 1.25 4.07.50 1.20 4.07.60 1.15 4.0

Arch Path Lab Med 2002; 126: 947-950.

Normal

The Albumin Concentration Also Affects the ion Ca/ pH Relationship

Since pH affects Ca binding by albumin: Less albumin (protein) means ion Ca

changes less for each 0.1 change in pH:

Albumin (g/dL) ion Ca / 0.1 pH

2.5 0.040

3.0 0.048

3.5 0.055

4.0 0.062

4.5 0.069

Arch Path Lab Med 2002; 126: 947-950.

Normal

Why Was the pH-Corrected Ionized Calcium Developed (circa 1981)?

Original purpose was to allow samples that had been exposed to air (i.e., typical general chemistry samples) to be analyzed for ionized calcium instead of total calcium.

Clin Chem 1981; 27: 1264-1267.

Is Correcting Ionized Calcium to pH 7.40 Clinically Useful?

Certainly useful if ionized Ca is ordered on a specimen that has been exposed to air or otherwise had a pH change.

Usually happens to samples collected at outside clinics. Also, patients typically have normal pH.

What about other situations: In acidotic patients, the pH-corrected ionized Ca might

tell a physician the ionized Ca if pH is normalized. pH 7.20, ionized Ca 1.20 mmol/L (normal)

Ionized Ca could decrease to 1.10 mmol/L if pH increased to 7.40

Is Correcting Ionized Calcium to pH 7.40 Clinically Useful?

What about pH-corrected ionized Ca used by itself?

Generally not useful and can be confusing when presented along with the actual ionized Ca result!

Actual ionized Ca preferable in hemodialysis patients Clin Chem 1992; 38: 1384.

Protein concentrations affects the pH-correction factor

Is Correcting Ionized Calcium to pH 7.40 Clinically Useful?

In 1990, the same group that developed pH-corrected ionized Ca determined that pH-corrected ionized Ca agreed with actual ionized Ca in ~97% of non-critical care patients:

Hyperparathyroidism, malignancy, Paget’s disease, osteoporosis, sarcoidosis, etc.

However, these patients usually have fairly normal pH.

What About Correcting Total Calcium Results to Give an Equivalent Ionized

Calcium?

Why Not Always Measure Ionized Calcium Instead of Total Calcium?

The ionized Ca concentration is much more “changeable” than total Ca:

Normal serum processing changes pH: Cell metabolism generally decreases pH Exposure of blood to air increases pH Clotting appears to change pH unpredictably Ionized Ca changes inversely about 0.05-0.06 mmol/L

for each 0.1 unit change in pH. Anticoagulants typically bind Ca ions:

Modern “balanced” heparins minimize this effect

History of Methods for Measuring Ionized Calcium

1935: McLean and Hastings perfused isolated frog hearts with Ca-containing fluids and plasma. The amplitude of contraction was proportional to ionized Ca activity.

Developed nomogram for calculating ionized Ca from total Ca and total protein.

1960s: Various colorimetric methods for ultrafiltrable/ionized Ca were developed.

History of Methods for Measuring Ionized Calcium

1970: Moore used an ion-selective electrode from Orion to measure ionized Ca.

1975: Orion SS-20 analyzer commercially available.

1980: Radiometer ICA-1 becomes available.

1990: Ionized Ca available from many companies as part of blood gas analyzer: Nova, Radiometer, AVL, IL, Corning, et al.

Over the Years, Physicians Have Tried to “Correct” Total Calcium to Make It

Equivalent to Ionized Calcium

In the past, total Ca test much more available than ionized Ca test.

Not so true today Still, total Ca is part of routine test panels while

ionized Ca requires specific order and sample collection.

Algorithms to “correct” total Ca are still in use.

Significant Events in the History of Correcting Total Calcium and Calculating

Ionized Calcium

McLean and Hastings (1935-37), conclude that frog heart contractions are sensitive to ionized Ca but not to protein-bound Ca.

Developed nomogram that predicted ionized Ca from total Ca and total protein levels.

Significant Events in the History of Correcting Total Calcium and Calculating

Ionized Calcium

Many other equations developed through the years.

Ladenson, et al (1978) report that corrected total Ca could not reliably predict Ca status as measured by ionized Ca.

Evaluated 13 correction equations: none appreciably better than uncorrected tot Ca.

Correlations of Total Ca or Corrected Ca vs Measured Ionized Ca (Ladenson, et al 1978)

Algorithm r Avg Deviation(mg/dL)

% of Discrepancies with ion Ca

None (measured tot Ca)

0.87 0.24 24.0

Moore (1970) 0.87 0.27 27.0

Kelly (1976) 0.85 0.24 22.8

McL & Hast (1935)

0.82 0.27 28.0

Orell (1971) 0.83 0.28 26.5

Examples of Equations Used to Calculate Either Ionized Ca or “Corrected” Total Ca

Caion = [0.9 + (0.55 x CaT) - (0.3 x Alb)]

4(Forster, 1985)

Examples:

CaT = 10.0 mg/dL and Alb = 4.5: Caion = 1.26 mmol/L

CaT = 10.0 mg/dL and Alb = 3.0: Caion = 1.38 mmol/L

CaT = 9.0 mg/dL and Alb = 3.0: Caion = 1.24 mmol/L

Examples of Equations Used to Calculate Either Ionized Ca or “Corrected” Total Ca

Cacorr = CaT – 0.71 (Alb – 3.4) (Orell, 1971)

Examples:

CaT = 10.0 mg/dL and Alb = 4.5: Cacorr = 9.2 mg/dL (2.3 mmol/L)

CaT = 10.0 mg/dL and Alb = 3.0: Cacorr =10.3 mg/dL(2.6 mmol/L)

CaT = 9.0 mg/dL and Alb = 3.0: Cacorr = 9.3 mg/dL (2.3 mmol/L)

Ionized Ca is Clearly Superior to Total Calcium for Monitoring Ca Status in Critical

Care and Surgical Patients

Albumin and total protein levels are often abnormal, usually low:

Fluids, nutrition, etc. Patients may have received citrated blood

products Citrate binds Ca ions tightly.

Ionized Ca is Clearly Superior to Total Calcium for Monitoring Ca Status in Critical

Care and Surgical Patients

Normal ionized Ca levels especially critical for the heart.

Parathyroid function may be compromised: Sepsis and pancreatitis promote hypoparathyroid

conditions.

Many Recent Reports Have Re-Confirmed that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

Among 50 stable patients on hemodialysis, none of 4 published equations for correcting total Ca greatly improved agreement with ionized Ca beyond simply using total Ca.

Clase CM, et al. Nephrol Dial Transplant 2000; 15; 1841-46.

Many Recent Reports Have Re-Confirmed that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

Albumin-adjusted Ca cannot be used in an intensive care setting to reliably monitor the Ca levels in critically-ill patients. Ionized Ca should be measured.

Slomp J, et al. Crit Care Med 2003; 31: 1389-93.

Many Recent Reports Have Re-Confirmed that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

Direct measure of ion Ca is indicated for assessing Ca status in trauma patients.

Equations underestimate hypocalcemia Hypoalbuminemia associated with hypocalcemia Dickerson RN, et al. J Parenteral Nutr 2004; 28: 133-141.

More Recent Reports Confirming that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

Among 34 patients on chronic hemodialysis, albumin-corrected Ca had 4 false negatives for hypocalcemia and 7 false positives for hypercalcemia.

Goransson LG. Nephrol Dial Transplantation 2005; 20: 2126-29.

More Recent Reports Confirming that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

Among 97 patients with malignant disorders, ionized Ca indicated hypercalcemia in 38% of patients, versus only 11% for corrected Ca and 8.4% for total Ca.

Ijaz A, et al. J Coll Physicians Surg Pak 2006; 16: 49-52.

More Recent Reports Confirming that Corrected Total Ca Does Not Reliably

Predict Ionized Ca

In a study of 237 critically-ill surgical patients, calcium homeostasis should be evaluated by ionized Ca measurements rather than by corrected Ca.

Corrected Ca missed 62 of 158 patients classified as hypocalcemic by ion Ca.

Byrnes MC, et al. Am J Surgery 2005; 189: 310-14.

Why Don’t Correction Algorithms Work?

Do not consider binding by small anions: Citrate, bicarbonate, phosphate, lactate

Most do not consider effect of pH: Loss of CO2, cell metabolism, clotting (during

processing) affect pH and Ca binding by proteins. ion Ca/0.1 pH affected by albumin concentration.

Why Don’t Correction Algorithms Work?

Albumin binding of Ca also affected by drugs, lipids, etc.

Analytical variability affects the corrected Ca result:

CaT, albumin, TP, etc measurements have variability

Study Showing the Effect of Albumin Variability on Corrected Calcium

Result

Labriola L, et al.

Nephrol Dial Transplant 2009; 24: 1834-38

Remember:

Total calcium (uncorrected) is still useful

and will continue to peacefully coexist

with ionized calcium.

Some examples:

1.50 1.75 2.00 2.25 2.50 2.75 3.00 Total Ca

0.90 1.00 1.10 1.20 1.30 1.40 1.50 Ionized Ca

Calcium (mmol/L)

Inta

ct P

TH

(n

g/L

)

Interpretation of PTH and Calcium: Either Ionized or Total Ca Measurements are Useful

PRIMARY HYPERPARATHYROIDISM

RENAL FAILURE

HYPOPARATHYROIDISM NORMAL RANGE MALIGNANCY

0

100

200

300

400

Total Calcium as a Predictor of Survival in Trauma ICU Patients

6

6.5

7

7.5

8

8.5

9

9.5

10

0 2 4 6 8 10

Survivors

Non-survivors

To

tal

Cal

ciu

m (

mg

/dL

)

Day of ICU Stay

Initial day Final Day

J Crit Care 2004; 19: 54-64

Appropriate Reference Ranges Are Important For Correct Interpretation of Total and Ionized

Calcium Results! Total Calcium

Child 2.20 - 2.68 mmol/L (8.8 - 10.7 mg/dL) Adult 2.10 - 2.55 mmol/L (8.4 - 10.2 mg/dL)

Ionized Calcium * Neonatal 1.20 - 1.48 mmol/L (4.8 - 5.9 mg/dL) Child 1.20 - 1.38 mmol/L (4.8 - 5.5 mg/dL) Adult 1.16 - 1.32 mmol/L (4.6 - 5.3 mg/dL)

* Reference ranges are for either serum or for whole blood collected with neutralized heparin (such as electrolyte-balanced) that has minimal effect on ionized calcium.

Conclusions

pH-corrected ionized Ca should (almost) always be reported with the actual ionized Ca and the pH.

pH-corrected ionized Ca especially useful when sample has been exposed to air and patient likely has normal acid-base status.

Ionized Ca should be measured in critical-care patients.

Conclusions

Total Ca and ionized Ca measurements will continue to peacefully coexist.

Correction algorithms tend to lose effectiveness when used at other institutions.

Appropriate reference ranges for actual or corrected Ca results are important for correct interpretation.

Corrected Ca results tend to make physician feel better. They may or may not make the patient feel better.