Transcript Calcium Equilibria in Blood

Ionized Calcium Test Volume at
Duke Medical Center
100000
90000
80000
Workload
70000
60000
CPED Lab
BG Lab
50000
40000
30000
20000
10000
93
-4
94
-5
95
-6
96
-7
97
-8
98
-9
99
-0
20
00
20 1
01
-2
20
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-3
20
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-4
20
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-5
20
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-6
20
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-7
20
07
-8
20
08
-9
0
Fiscal Year
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 ions
The parathyroid gland
responds to the ionized
Ca concentration
Ca ion entry into smooth muscle cells
controls blood pressure
What Are Ionized, Complex-Bound,
Protein-Bound and Ultrafiltrable Calcium?
Protein-Bound Ca
80% Ca-Albumin
35-45 % of Total Ca
Ionized Ca
Ultrafiltrable Ca
Total Calcium
20% Ca-Globulin
Complex-Bound Ca
10-15% of Total Ca
Unassociated
Calcium Ions
45-50% of Total Ca
Calcium Equilibria in Blood
Complex-Bound
Calcium
Protein-Bound Calcium
Ionized Calcium
Ca ++
Ca ++
Ca ++
Ca ++
Ca ++
lower pH
Albumin
Ca ++
HCO3
Ca ++
Ca ++
Citrate
higher pH
Ca ++
Ca ++
Ca ++
Ca ++
Ca ++
Ca ++
HPO4
Lactate
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
7.20
7.30
7.40
7.50
7.60
ion Ca (mmol/L)
1.35
1.30
1.25
1.20
1.15
Alb (g/dL)
4.0
4.0
4.0
4.0
4.0
Normal
Arch Path Lab Med 2002; 126: 947-950.
The Albumin Concentration Also Affects
the D 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)
D 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
Normal
Arch Path Lab Med 2002; 126: 947-950.
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
 Protein
Chem 1992; 38: 1384.
concentrations affects the pH-correction factor
Is Correcting Ionized Calcium to pH 7.40
Clinically Useful?

In 1990, the same group that developed pHcorrected 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
(mg/dL)
% of Discrepancies
with ion 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
None
r
Avg Deviation
(measured tot Ca)
Examples of Equations Used to Calculate
Either Ionized Ca or “Corrected” Total Ca
Caion = [0.9 + (0.55 x CaT) - (0.3 x Alb)]
(Forster, 1985)
4
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
46.
CM, et al. Nephrol Dial Transplant 2000; 15; 1841-
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
 Dickerson
associated with hypocalcemia
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
2126-29.
LG. Nephrol Dial Transplantation 2005; 20:
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.

D 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:
Interpretation of PTH and Calcium: Either Ionized
or Total Ca Measurements are Useful
400
Intact PTH (ng/L)
300
RENAL
FAILURE
PRIMARY
HYPERPARATHYROIDISM
200
100
HYPOPARATHYROIDISM
NORMAL
RANGE
MALIGNANCY
0
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)
Total Calcium as a Predictor of
Survival in Trauma ICU Patients
Total Calcium (mg/dL)
10
9.5
9
8.5
Survivors
Non-survivors
8
7.5
7
6.5
6
Initial
0 day
2
4
6
8
Final
10Day
Day of ICU Stay
J Crit Care 2004; 19: 54-64
Appropriate Reference Ranges Are Important For
Correct Interpretation of Total and Ionized
Calcium Results!

Total Calcium
 Child
 Adult

2.20 - 2.68 mmol/L (8.8 - 10.7 mg/dL)
2.10 - 2.55 mmol/L (8.4 - 10.2 mg/dL)
Ionized Calcium *
 Neonatal
 Child
 Adult
1.20 - 1.48 mmol/L (4.8 - 5.9 mg/dL)
1.20 - 1.38 mmol/L (4.8 - 5.5 mg/dL)
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.
