A complete blood count, also known as a full blood count, is a set of medical laboratory tests that provide information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cells and platelets, the concentration of hemoglobin, and the hematocrit. The red blood cell indices, which indicate the average size and hemoglobin content of red blood cells, are also reported, and a white blood cell differential, which counts the different types of white blood cells, may be included.
The CBC is
often carried out as part of a medical assessment and can be used to monitor
health or diagnose diseases. The results are interpreted by comparing them to
reference ranges, which vary with sex and age. Conditions like anemia and
thrombocytopenia are defined by abnormal complete blood count results. The red
blood cell indices can provide information about the cause of a person's anemia
such as iron deficiency and vitamin B12 deficiency, and the results of the
white blood cell differential can help to diagnose viral, bacterial and
parasitic infections and blood disorders like leukemia. Not all results falling
outside of the reference range require medical intervention.
The CBC is
usually performed by an automated hematology analyzer, which counts cells and
collects information on their size and structure. The concentration of
hemoglobin is measured, and the red blood cell indices are calculated from
measurements of red blood cells and hemoglobin. Manual tests can be used to
independently confirm abnormal results. Approximately 10–25% of samples require
a manual blood smear review, The complete blood count evaluates the three
cellular components of blood. Some medical conditions, such as anemia or
thrombocytopenia, are defined by marked increases or decreases in blood cell
counts.
The CBC is
often used to screen for diseases as part of a medical assessment. It is also
called for when a healthcare provider suspects a person has a disease that
affects blood cells, such as an infection, a bleeding disorder, or some
cancers. People who have been diagnosed with disorders that may cause abnormal
CBC results or who are receiving treatments that can affect blood cell counts
may have a regular CBC performed to monitor their health, The results may
indicate a need for a blood or platelet transfusion.
The complete
blood count has specific applications in many medical specialties. It is often
performed before a person undergoes surgery to detect anemia, ensure that
platelet levels are sufficient, and screen for infection, as well as after
surgery, so that blood loss can be monitored. In emergency medicine, the CBC is
used to investigate numerous symptoms, such as fever, abdominal pain, and
shortness of breath, and to assess bleeding and trauma. Blood counts are
closely monitored in people undergoing chemotherapy or radiation therapy for
cancer, because these treatments suppress the production of blood cells in the
bone marrow and can produce severely low levels of white blood cells, platelets
and hemoglobin. Regular CBCs are necessary for people taking some psychiatric
drugs, such as clozapine and carbamazepine, which in rare cases can cause a
life-threatening drop in the number of white blood cells. Because anemia during
pregnancy can result in poorer outcomes for the mother and her baby, the
complete blood count is a routine part of prenatal care; and in newborn babies,
a CBC may be needed to investigate jaundice or to count the number of immature
cells in the white blood cell differential, which can be an indicator of
sepsis.
The complete
blood count is an essential tool of hematology, which is the study of the
cause, prognosis, treatment, and prevention of diseases related to blood. The
results of the CBC and smear examination reflect the functioning of the
hematopoietic system—the organs and tissues involved in the production and
development of blood cells, particularly the bone marrow. For example, a low
count of all three cell types can indicate that blood cell production is being
affected by a marrow disorder, and a bone marrow examination can further
investigate the cause. Abnormal cells on the blood smear might indicate acute
leukemia or lymphoma,
The
reference ranges for the complete blood count represent the range of results
found in 95% of apparently healthy people. This is particularly likely if such
results are only slightly outside the reference range, if they are consistent
with previous results, or if there are no other related abnormalities shown by
the CBC. When the test is performed on a relatively healthy population, the
number of clinically insignificant abnormalities may exceed the number of
results that represent disease. For this reason, professional organizations in
the United States, United Kingdom and Canada recommend against pre-operative
CBC testing for low-risk surgeries in individuals without relevant medical
conditions. Repeated blood draws for hematology testing in hospitalized
patients can contribute to hospital-acquired anemia and may result in
unnecessary transfusions. The blood is usually taken from a vein, but when this
is difficult it may be collected from capillaries by a fingerstick, or by a
heelprick in babies. Testing is typically performed on an automated analyzer,
but manual techniques such as a blood smear examination or manual hematocrit
test can be used to investigate abnormal results. Cell counts and hemoglobin
measurements are performed manually in laboratories lacking access to automated
instruments.
Automated
On board the
analyzer, the sample is agitated to evenly distribute the cells, then diluted
and partitioned into at least two channels, one of which is used to count red
blood cells and platelets, the other to count white blood cells and determine
the hemoglobin concentration. Some instruments measure hemoglobin in a separate
channel, and additional channels may be used for differential white blood cell
counts, reticulocyte counts and specialized measurements of platelets. The
cells are suspended in a fluid stream and their properties are measured as they
flow past sensors in a technique known as flow cytometry. Hydrodynamic focusing
may be used to isolate individual cells so that more accurate results can be
obtained: the diluted sample is injected into a stream of low-pressure fluid,
which causes the cells in the sample to line up in single file through laminar
flow.
To measure
the hemoglobin concentration, a reagent chemical is added to the sample to
destroy the red cells in a channel separate from that used for red blood cell
counts. On analyzers that perform white blood cell counts in the same channel
as hemoglobin measurement, this permits white blood cells to be counted more
easily. Hematology analyzers measure hemoglobin using spectrophotometry and are
based on the linear relationship between the absorbance of light and the amount
of hemoglobin present. Chemicals are used to convert different forms of
hemoglobin, such as oxyhemoglobin and carboxyhemoglobin, to one stable form,
usually cyanmethemoglobin, and to create a permanent colour change. The
absorbance of the resulting colour, when measured at a specific
wavelength—usually 540 nanometres—corresponds with the concentration of
hemoglobin.
Sensors
count and identify the cells in the sample using two main principles:
electrical impedance and light scattering. Impedance-based cell counting
operates on the Coulter principle: cells are suspended in a fluid carrying an
electric current, and as they pass through a small opening, they cause
decreases in current because of their poor electrical conductivity. The
amplitude of the voltage pulse generated as a cell crosses the aperture
correlates with the amount of fluid displaced by the cell, and thus the cell's
volume, while the total number of pulses correlates with the number of cells in
the sample. The distribution of cell volumes is plotted on a histogram, and by
setting volume thresholds based on the typical sizes of each type of cell, the
different cell populations can be identified and counted.
In light
scattering techniques, light from a laser or a tungsten-halogen lamp is
directed at the stream of cells to collect information about their size and
structure. Cells scatter light at different angles as they pass through the
beam, which is detected using photometers.
Radiofrequency-based
methods can be used in combination with impedance. These techniques work on the
same principle of measuring the interruption in current as cells pass through
an aperture, but since the high-frequency RF current penetrates into the cells,
the amplitude of the resulting pulse relates to factors like the relative size
of the nucleus, the nucleus's structure, and the amount of granules in the
cytoplasm. Small red cells and cellular debris, which are similar in size to
platelets, may interfere with the platelet count, and large platelets may not
be counted accurately, so some analyzers use additional techniques to measure platelets,
such as fluorescent staining, multi-angle light scatter and monoclonal antibody
tagging. Another calculation, the red blood cell distribution width, is derived
from the standard deviation of the mean cell volume and reflects variation in
cellular size.
After being
treated with reagents, white blood cells form three distinct peaks when their
volumes are plotted on a histogram. These peaks correspond roughly to
populations of granulocytes, lymphocytes, and other mononuclear cells, allowing
a three-part differential to be performed based on cell volume alone. More
advanced analyzers use additional techniques to provide a five- to seven-part
differential, such as light scattering or radiofrequency analysis, or
myeloperoxidase, an enzyme found in cells of the myeloid lineage. Basophils may
be counted in a separate channel where a reagent destroys other white cells and
leaves basophils intact. The data collected from these measurements is analyzed
and plotted on a scattergram, where it forms clusters that correlate with each
white blood cell type. which uses artificial intelligence to classify white
blood cells from photomicrographs of the blood smear. The cell images are
displayed to a human operator, who can manually re-classify the cells if
necessary.
Most
analyzers take less than a minute to run all the tests in the complete blood
count. However, some abnormal cells may not be identified correctly, requiring
manual review of the instrument's results and identification by other means of
abnormal cells the instrument could not categorize.
Point-of-care
testing
Point-of-care
testing refers to tests conducted outside of the laboratory setting, such as at
a person's bedside or in a clinic. Hemoglobin and hematocrit can be measured on
point-of-care devices designed for blood gas testing, but these measurements
sometimes correlate poorly with those obtained through standard methods. There
are simplified versions of hematology analyzers designed for use in clinics
that can provide a complete blood count and differential.
Manual
The tests
can be performed manually when automated equipment is not available or when the
analyzer results indicate that further investigation is needed. or numerical
results that fall outside set thresholds. The appearance of the red and white
blood cells and platelets is assessed, and qualitative abnormalities are
reported if present. Changes in the appearance of red blood cells can have
considerable diagnostic significance—for example, the presence of sickle cells
is indicative of sickle cell disease, and a high number of fragmented red blood
cells requires urgent investigation as it can suggest a microangiopathic
hemolytic anemia. In some inflammatory conditions and in paraprotein disorders
like multiple myeloma, high levels of protein in the blood may cause red blood
cells to appear stacked together on the smear, which is termed rouleaux. Some
parasitic diseases, such as malaria and babesiosis, can be detected by finding
the causative organisms on the blood smear, and the platelet count can be
estimated from the blood smear, which is useful if the automated platelet count
is inaccurate. This gives the percentage of each type of white blood cell, and
by multiplying these percentages by the total number of white blood cells, the
absolute number of each type of white cell can be obtained. Manual counting is
subject to sampling error because so few cells are counted compared with
automated analysis,
The
hematocrit can performed manually by filling a capillary tube with blood,
centrifuging it, and measuring the percentage of the blood that consists of red
blood cells. or severe leukocytosis. Manual cell counts are labour-intensive
and inaccurate compared to automated methods, so they are rarely used except in
laboratories that do not have access to automated analyzers. Sometimes a stain
is added to the diluent that highlights the nuclei of white blood cells, making
them easier to identify. Manual platelet counts are performed in a similar
manner, although some methods leave the red blood cells intact. Using a
phase-contrast microscope, rather than a light microscope, can make platelets
easier to identify. The manual red blood cell count is rarely performed, as it
is inaccurate and other methods such as hemoglobinometry and the manual
hematocrit are available for assessing red blood cells; but if it is necessary
to do so, red blood cells can be counted in blood that has been diluted with
saline.
Hemoglobin
can be measured manually using a spectrophotometer or colorimeter. To measure
hemoglobin manually, the sample is diluted using reagents that destroy red
blood cells to release the hemoglobin. Other chemicals are used to convert
different types of hemoglobin to one form, allowing it to be easily measured.
The solution is then placed in a measuring cuvette and the absorbance is
measured at a specific wavelength, which depends on the type of reagent used. A
reference standard containing a known amount of hemoglobin is used to determine
the relationship between the absorbance and the hemoglobin concentration,
allowing the hemoglobin level of the sample to be measured.
In rural and
economically disadvantaged areas, available testing is limited by access to
equipment and personnel. At primary care facilities in these regions, testing
may be limited to examination of red cell morphology and manual measurement of
hemoglobin, while more complex techniques like manual cell counts and
differentials, and sometimes automated cell counts, are performed at district
laboratories. Regional and provincial hospitals and academic centres typically
have access to automated analyzers. Where laboratory facilities are not
available, an estimate of hemoglobin concentration can be obtained by placing a
drop of blood on a standardized type of absorbent paper and comparing it to a
colour scale.
Quality
control
Automated
analyzers have to be regularly calibrated. Most manufacturers provide preserved
blood with defined parameters and the analyzers are adjusted if the results are
outside defined thresholds. To ensure that results continue to be accurate,
quality control samples, which are typically provided by the instrument
manufacturer, are tested at least once per day. The samples are formulated to
provide specific results, and laboratories compare their results against the known
values to ensure the instrument is functioning properly. For laboratories
without access to commercial quality control material, an Indian regulatory
organization recommends running patient samples in duplicate and comparing the
results. A moving average measurement, in which the average results for patient
samples are measured at set intervals, can be used as an additional quality
control technique. Assuming that the characteristics of the patient population
remain roughly the same over time, the average should remain constant; large
shifts in the average value can indicate instrument problems.
In addition
to analyzing internal quality control samples with known results, laboratories
may receive external quality assessment samples from regulatory organizations.
While the purpose of internal quality control is to ensure that analyzer
results are reproducible within a given laboratory, external quality assessment
verifies that results from different laboratories are consistent with each
other and with the target values. The expected results for external quality
assessment samples are not disclosed to the laboratory. External quality
assessment programs have been widely adopted in North America and western
Europe, Logistical issues may make it difficult for laboratories in
under-resourced areas to implement external quality assessment schemes.
Included
tests
The CBC
measures the amounts of platelets and red and white blood cells, along with the
hemoglobin and hematocrit values. Red blood cell indices—MCV, MCH and
MCHC—which describe the size of red blood cells and their hemoglobin content,
are reported along with the red blood cell distribution width, which measures
the amount of variation in the sizes of red blood cells. A white blood cell
differential, which enumerates the different types of white blood cells, may be
performed, and a count of immature red blood cells is sometimes included.
Red blood
cells, hemoglobin, and hematocrit
An example
of CBC results showing a low hemoglobin, MCV, MCH and MCHC. The person was
anemic. The cause could be iron deficiency or a hemoglobinopathy.
Red blood
cells deliver oxygen from the lungs to the tissues and on their return carry
carbon dioxide back to the lungs where it is exhaled. These functions are
mediated by the cells' hemoglobin. The analyzer counts red blood cells,
reporting the result in units of 106 cells per microlitre of blood or 1012
cells per litre, and measures their average size, which is called the mean cell
volume and expressed in femtolitres or cubic micrometres. Hemoglobin, measured
after the red blood cells are lysed, is usually reported in units of grams per
litre or grams per decilitre. Assuming that the red blood cells are normal,
there is a constant relationship between hemoglobin and hematocrit: the
hematocrit percentage is approximately three times greater than the hemoglobin
value in g/dL, plus or minus three. This relationship, called the rule of
three, can be used to confirm that CBC results are correct.
Two other
measurements are calculated from the red blood cell count, the hemoglobin
concentration, and the hematocrit: the mean corpuscular hemoglobin and the mean
corpuscular hemoglobin concentration. These parameters describe the hemoglobin
content of each red blood cell. The MCH and MCHC can be confusing; in essence
the MCH is a measure of the average amount of hemoglobin per red blood cell.
The MCHC gives the average proportion of the cell that is hemoglobin. The MCH
does not take into account the size of the red blood cells whereas the MCHC does.
Collectively, the MCV, MCH, and MCHC are referred to as the red blood cell
indices. Another parameter is calculated from the initial measurements of red
blood cells: the red blood cell distribution width or RDW, which reflects the
degree of variation in the cells' size.
An
abnormally low hemoglobin, hematocrit, or red blood cell count indicates
anemia. Anemia is not a diagnosis on its own, but it points to an underlying
condition affecting the person's red blood cells. Anemia reduces the blood's
ability to carry oxygen, causing symptoms like tiredness and shortness of
breath. If the hemoglobin level falls below thresholds based on the person's
clinical condition, a blood transfusion may be necessary.
An increased
number of red blood cells, leading to an increase in the hemoglobin and
hematocrit, is called polycythemia. Dehydration or use of diuretics can cause a
"relative" polycythemia by decreasing the amount of plasma compared
to red cells. A true increase in the number of red blood cells, called absolute
polycythemia, can occur when the body produces more red blood cells to
compensate for chronically low oxygen levels in conditions like lung or heart
disease, or when a person has abnormally high levels of erythropoietin, a
hormone that stimulates production of red blood cells. In polycythemia vera,
the bone marrow produces red cells and other blood cells at an excessively high
rate.
Evaluation
of red blood cell indices is helpful in determining the cause of anemia. If the
MCV is low, the anemia is termed microcytic, while anemia with a high MCV is
called macrocytic anemia. Anemia with a low MCHC is called hypochromic anemia.
If anemia is present but the red blood cell indices are normal, the anemia is
considered normochromic and normocytic. An elevated MCHC can also be a false
result from conditions like red blood cell agglutination or highly elevated
amounts of lipids in the blood.
Microcytic
anemia is typically associated with iron deficiency, thalassemia, and anemia of
chronic disease, while macrocytic anemia is associated with alcoholism, folate
and B12 deficiency, use of some drugs, and some bone marrow diseases. Acute
blood loss, hemolytic anemia, bone marrow disorders, and various chronic
diseases can result in anemia with a normocytic blood picture. The MCV serves
an additional purpose in laboratory quality control. It is relatively stable
over time compared to other CBC parameters, so a large change in MCV may
indicate that the sample was drawn from the wrong patient.
A low RDW
has no clinical significance, but an elevated RDW represents increased
variation in red blood cell size, a condition known as anisocytosis.
White blood
cells
The complete
blood count is interpreted by comparing the output to reference ranges, which
represent the results found in 95% of apparently healthy people. Based on a
statistical normal distribution, the tested samples' ranges vary with sex and
age.
On average,
adult females have lower hemoglobin, hematocrit, and red blood cell count
values than males; the difference lessens, but is still present, after
menopause. CBC results for children and newborn babies differ from those of
adults. Newborns' hemoglobin, hematocrit, and red blood cell count are
extremely high to compensate for low oxygen levels in the womb and the high
proportion of fetal hemoglobin, which is less effective at delivering oxygen to
tissues than mature forms of hemoglobin, inside their red blood cells. The MCV
is also increased, and the white blood cell count is elevated with a
preponderance of neutrophils. The red blood cell count and related values begin
to decline shortly after birth, reaching their lowest point at about two months
of age and increasing thereafter. The red blood cells of older infants and
children are smaller, with a lower MCH, than those of adults. In the pediatric
white blood cell differential, lymphocytes often outnumber neutrophils, while
in adults neutrophils predominate. The type of analyzer used to run the CBC
affects the reference ranges as well. Reference ranges are therefore established
by individual laboratories based on their own patient populations and
equipment.
Limitations
Some medical
conditions or problems with the blood sample may produce inaccurate results. If
the sample is visibly clotted, which can be caused by poor phlebotomy
technique, it is unsuitable for testing, because the platelet count will be
falsely decreased and other results may be abnormal. Samples stored at room
temperature for several hours may give falsely high readings for MCV, because
red blood cells swell as they absorb water from the plasma; and platelet and
white blood cell differential results may be inaccurate in aged specimens, as
the cells degrade over time.
Samples
drawn from individuals with very high levels of bilirubin or lipids in their plasma
may show falsely high readings for hemoglobin, because these substances change
the colour and opacity of the sample, which interferes with hemoglobin
measurement. This effect can be mitigated by replacing the plasma with saline.
Another
antibody-mediated condition that can affect complete blood count results is red
blood cell agglutination. This phenomenon causes red blood cells to clump
together because of antibodies bound to the cell surface. Red blood cell
aggregates are counted as single cells by the analyzer, leading to a markedly
decreased red blood cell count and hematocrit, and markedly elevated MCV and
MCHC.
History
Before
automated cell counters were introduced, complete blood count tests were
performed manually: white and red blood cells and platelets were counted using
microscopes. The first person to publish microscopic observations of blood
cells was Antonie van Leeuwenhoek, who reported on the appearance of red cells
in a 1674 letter to the Proceedings of the Royal Society of London. Jan
Swammerdam had described red blood cells some years earlier, but did not
publish his findings at the time. Throughout the 18th and 19th centuries,
improvements in microscope technology such as achromatic lenses allowed white
blood cells and platelets to be counted in unstained samples.
The
physiologist Karl Vierordt is credited with performing the first blood count.
The hemocytometer, introduced in 1874 by Louis-Charles Malassez, simplified the
microscopic counting of blood cells. Malassez's hemocytometer consisted of a
microscope slide containing a flattened capillary tube. Diluted blood was
introduced to the capillary chamber by means of a rubber tube attached to one
end, and an eyepiece with a scaled grid was attached to the microscope,
permitting the microscopist to count the number of cells per volume of blood.
In 1877, William Gowers invented a hemocytometer with a built-in counting grid,
eliminating the need to produce specially calibrated eyepieces for each
microscope.
In the
1870s, Paul Ehrlich developed a staining technique using a combination of an
acidic and basic dye that could distinguish different types of white blood
cells and allow red blood cell morphology to be examined.
The first
techniques for measuring hemoglobin were devised in the late 19th century, and
involved visual comparisons of the colour of diluted blood against a known
standard. Attempts to automate this process using spectrophotometry and
colorimetry were limited by the fact that hemoglobin is present in the blood in
many different forms, meaning that it could not be measured at a single
wavelength. In 1920, a method to convert the different forms of hemoglobin to
one stable form was introduced, allowing hemoglobin levels to be measured
automatically. The cyanmethemoglobin method remains the reference method for
hemoglobin measurement and is still used in many automated hematology
analyzers.
Maxwell
Wintrobe is credited with the invention of the hematocrit test. In 1929, he
undertook a PhD project at the University of Tulane to determine normal ranges
for red blood cell parameters, and invented a method known as the Wintrobe
hematocrit. Hematocrit measurements had previously been described in the
literature, but Wintrobe's method differed in that it used a large tube that
could be mass-produced to precise specifications, with a built-in scale. The
fraction of red blood cells in the tube was measured after centrifugation to
determine the hematocrit. The invention of a reproducible method for
determining hematocrit values allowed Wintrobe to define the red blood cell
indices. Wallace's patent application was granted in 1953, and after
improvements to the aperture and the introduction of a mercury manometer to
provide precise control over sample size, the brothers founded Coulter Electronics
Inc. in 1958 to market their instruments. The Coulter counter was initially
designed for counting red blood cells, but with later modifications it proved
effective for counting white blood cells.
After basic
cell counting had been automated, the white blood cell differential remained a
challenge. Throughout the 1970s, researchers explored two methods for
automating the differential count: digital image processing and flow cytometry.
Using technology developed in the 1950s and 60s to automate the reading of Pap
smears, several models of image processing analyzers were produced. These
instruments would scan a stained blood smear to find cell nuclei, then take a
higher resolution snapshot of the cell to analyze it through densitometry. They
were expensive, slow, and did little to reduce workload in the laboratory
because they still required blood smears to be prepared and stained, so flow
cytometry-based systems became more popular, and by 1990, no digital image
analyzers were commercially available in the United States or western Europe.
These techniques enjoyed a resurgence in the 2000s with the introduction of
more advanced image analysis platforms using artificial neural networks.
Early flow
cytometry devices shot beams of light at cells in specific wavelengths and
measured the resulting absorbance, fluorescence or light scatter, collecting
information about the cells' features and allowing cellular contents such as
DNA to be quantified. One such instrument—the Rapid Cell Spectrophotometer,
developed by Louis Kamentsky in 1965 to automate cervical cytology—could
generate blood cell scattergrams using cytochemical staining techniques.
Leonard Ornstein, who had helped to develop the staining system on the Rapid
Cell Spectrophotometer, and his colleagues later created the first commercial
flow cytometric white blood cell differential analyzer, the Hemalog D.
Introduced in 1974, this analyzer used light scattering, absorbance and cell
staining to identify the five normal white blood cell types in addition to
"large unidentified cells", a classification that usually consisted
of atypical lymphocytes or blast cells. The Hemalog D could count 10,000 cells
in one run, a marked improvement over the manual differential. In 1981,
Technicon combined the Hemalog D with the Hemalog-8 analyzer to produce the
Technicon H6000, the first combined complete blood count and differential
analyzer. This analyzer was unpopular with hematology laboratories because it
was labour-intensive to operate, but in the late 1980s to early 1990s similar
systems were widely produced by other manufacturers such as Sysmex, Abbott,
Roche and Beckman Coulter.
Explanatory
notes
References
Citations
General
bibliography
Bibliography:
Wikipedia
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