8 Special Issues: Blood Gas and Ionized Calcium Analysis
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CHAPTER 3:
P r e -A na ly t ica l V ar i ab le s
Therefore, specimens sent to the lab for ionized calcium determinations
should be handled with the same caution as other blood gas samples since
pre-analytical errors in pH will impact ionized calcium results [7].
KEY POINTS
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Errors in the clinical laboratory can occur in pre-analytical, analytical, or postanalytical steps. Most errors (almost two-thirds of all errors) occur in pre-analytical
steps.
During specimen collection, a patient must be identified by matching at least two
criteria. Blood should be collected in the correct tube following the correct order
of draw.
Correct order of drawing blood: (1) microbiological blood culture tubes (yellow
top), (2) royal blue tube (no additive) if trace metal analysis is desired, (3) citrate
tube (light blue), (4) serum tube (red top) or tube with gel separator/clot activator
(gold top or tiger top), (5) heparin tube (green top), (6) EDTA tube (purple/lavender
top), and (7) oxalate-fluoride tube (gray top).
Proper centrifugation (in the case of analyzing serum or plasma specimens) and
proper transportation of the specimen to the laboratory are required, as well as
maintaining proper storage of the specimen prior to analysis in order to avoid
artifactual changes in the analyte.
EDTA (purple top tube) is the anticoagulant of choice for the complete blood
count (CBC). The EDTA tube is also used for blood bank pre-transfusion testing,
flow cytometry, hemoglobin A1C, and most common immunosuppressive drugs
such as cyclosporine, tacrolimus, sirolimus, and everolimus; another
immunosuppressant, mycophenolic acid, is measured in serum or plasma instead
of whole blood.
Heparin (green top tube) is the only anticoagulant recommended for the
determination of pH blood gases, electrolytes, and ionized calcium. Lithium
heparin is commonly used instead of sodium heparin for general chemistry tests.
Heparin is not recommended for protein electrophoresis and cryoglobulin testing
because of the presence of fibrinogen, which co-migrates with beta-2 monoclonal
proteins.
For coagulation testing, citrate (light blue top) is the appropriate anticoagulant.
Potassium oxalate is used in combination with sodium fluoride and sodium
iodoacetate to inhibit enzymes involved in the glycolytic pathway. Therefore the
oxalate/fluoride (gray top) tube should be used for collecting specimens for
measuring glucose level.
Ideally, all blood gas specimens should be measured immediately and never
stored. A plastic syringe, transported at room temperature, is recommended if
analysis will occur within 30 minutes of collection. Otherwise, a specimen must be
stored in ice. Glass syringes are recommended for delayed analysis because glass
References
does not allow the diffusion of oxygen or carbon dioxide. Bubbles must be
completely expelled from the specimen prior to transport, as the pO2 will be
significantly increased and pCO2 decreased within 2 minutes.
REFERENCES
[1] Carraro P, Plebani M. Errors in STAT laboratory; types and frequency 10 years later. Clin
Chem 2007;53:1338À42.
[2] Murphy JE, Ward ES. Elevated phenytoin concentration caused by sampling through the
drug-administered line. Pharmacotherapy 1991;11:348À350.
[3] Dunn EJ, Morga PJ. Patient misidentification in laboratory medicine: a qualitative analysis of
227 root cause analysis reports in the Veteran Administration. Arch Pathol Lab Med
2010;134:244À55.
[4] Aleccia J. Patients still stuck with bill for medical errors. 2008 2/29/2008 8:26:51 AM ET
[cited 2012 06/28/2012]; Available from: ,http://www.msnbc.msn.com/id/23341360/ns/
health-health_care/t/patients-still-stuck-bill-medical-errors/#.T-yk5vVibJs..
[5] Lee DC, Klachko MN. Falsely elevated lithium levels in plasma samples obtained in lithium
containing tubes. J Toxicol Clin Toxicol 1996;34:467À9.
[6] Knowles TP, Mullin RA, Hunter JA, Douce FH. Effects of syringe material, sample storage
time, and temperature on blood gases and oxygen saturation in arterialized human blood
samples. Respir Care 2006;51:732À6.
[7] Toffaletti J, Blosser N, Kirvan K. Effects of storage temperature and time before centrifugation
on ionized calcium in blood collected in plain vacutainer tubes and silicone-separator (SST)
tubes. Clin Chem 1984;30(4):553À6.
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CHAPTER 4
Laboratory Statistics and Quality Control
4.1 MEAN, STANDARD DEVIATION, AND
COEFFICIENT OF VARIATION
CONTENTS
In an ideal situation, when measuring a value of the analyte in a specimen,
the same value should be produced over and over again. However, in reality,
the same value is not produced by the instrument, but a similar value is
observed. Therefore, the most basic statistical operation is to calculate the
mean and standard deviation, and then to determine the coefficient of variation (CV). Mean value is defined as Equation 4.1:
Mean ðXÞ 5
X 1 1 X 2 1 X 3 1 ?? 1 Xn
n
ð4:1Þ
Here, X1, X2, X3, etc., are individual values and “n” is the number of values.
After calculation of the mean value, standard deviation (SD) of the sample
can be easily determined using the following formula (Equation 4.2):
rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
P
SD 5
ðx1 2xÞ2
n21
ð4:2Þ
Here, X1 is the individual value from the sample and n is again the number
of observations.
Standard deviation represents the average deviation of an individual value
from the mean value. The smaller the standard deviation, the better the precision of the measurement. Standard deviation is the square root of variance.
Variance indicates deviation of a sample observation from the mean of all
values and is expressed as sigma. Therefore (Equation 4.3):
σ 5 OSD
A. Dasgupta and A. Wahed: Clinical Chemistry, Immunology and Laboratory Quality Control
DOI: http://dx.doi.org/10.1016/B978-0-12-407821-5.00004-8
© 2014 Elsevier Inc. All rights reserved.
ð4:3Þ
4.1 Mean, Standard
Deviation, and
Coefficient of
Variation................ 47
4.2 Precision and
Accuracy ............... 48
4.3 Gaussian
Distribution and
Reference Range .. 48
4.4 Sensitivity,
Specificity, and
Predictive
Value...................... 50
4.5 Random and
Systematic Errors in
Measurements...... 51
4.6 Laboratory
Quality Control:
Internal and
External ................. 52
4.7 LeveyÀJennings
Chart and Westgard
Rules ...................... 54
4.8 Delta Checks.. 56
4.9 Method
Validation/
Evaluation of a
New Method ......... 58
4.10 How to
Interpret the
Regression
Equation? .............. 59
47
48
CHAPTER 4:
Laboratory Statistics and Quality Control
4.11 BlandÀAltman
Plot ......................... 60
4.12 ReceiverÀ
Operator Curve..... 60
4.13 What is Six
Sigma? ................... 61
4.14 Errors
Associated with
Reference
Range .................... 62
4.15 Basic Statistical
Analysis: Student
t-Test and Related
Tests ...................... 63
Coefficient of variation is also a very important parameter because CV can be
easily expressed as a percent value; the lower the CV, the better the precision
for the measurement. The advantage of CV is that one number can be used
to express precision instead of stating both mean value and standard deviation. CV can be easily calculated with Equation 4.4:
CV 5 SD=Mean 3 100
ð4:4Þ
Sometimes standard error of mean is also calculated (Equation 4.5).
Standard error of mean 5 SD=On
ð4:5Þ
Here, n is the number of data points in the set.
Key Points ............. 63
References ............ 66
4.2 PRECISION AND ACCURACY
Precision is a measure of how reproducible values are in a series of measurements, while accuracy indicates how close a determined value is to
the target values. Accuracy can be determined for a particular test by analysis of an assayed control where the target value is known. This is typically provided by the manufacturer or made in-house by accurately
measuring a predetermined amount of analyte and then dissolving it in a
predetermined amount of a solvent matrix where the matrix is similar to
plasma. An ideal assay has both excellent precision and accuracy, but
good precision of an assay may not always guarantee good accuracy.
4.3 GAUSSIAN DISTRIBUTION AND
REFERENCE RANGE
Gaussian distribution (also known as normal distribution) is a bellshaped curve, and it is assumed that during any measurement values will
follow a normal distribution with an equal number of measurements
above and below the mean value. In order to understand normal distribution, it is important to know the definitions of “mean,” “median,” and
“mode.” The “mean” is the calculated average of all values, the “median”
is the value at the center point (mid-point) of the distribution, while the
“mode” is the value that was observed most frequently during the measurement. If a distribution is normal, then the values of the mean,
median, and mode are the same. However, the value of the mean,
median, and mode may be different if the distribution is skewed (not