Full blood count (FBC): counts the different types of cells in your blood. There are red
blood cells, which carry oxygen; white blood cells, which form part of the
immune system and defend us from infection; and platelets, which help the blood
to clot. Blood counts are used to diagnose and monitor several diseases and
conditions, including anaemia, infections, inflammatory diseases and cancers.
The red blood cell count
includes the number of red blood cells, the total amount of the oxygen-carrying
molecule haemoglobin and the amount of space red blood cells fill in blood,
called haematocrit.
Women generally have fewer red blood cells than men.
Anaemia, a deficiency of red blood cells, indicates a clinically significant
drop in the ability of blood to carry oxygen. Anaemia is not a disease, but
rather a symptom, which may indicate a condition that needs to be investigated.
The white blood cell differential
count measures five subtypes of white blood cell. These are:
- Neutrophils, which mobilise a quick response to
infection
- Lymphocytes, which co-ordinate a more
sophisticated set of responses and ‘remember’ infections
- Monocytes, which target and destroy invading
pathogens and foreign matter
- Eosinophils and basophils, which target parasites
and mount allergic reactions.
The familiar CD4
cell count measures a subset of the lymphocyte family of cells. A decrease in
white blood cell counts indicates an immune dysfunction – the kind seen in AIDS
is only one of several types – while an increase may be caused by an infection
or tumour. Increases in the different subtypes of white blood cells can be a
clue to the underlying problem.
Platelets are cell fragments
involved in blood clotting. Having too few platelets increases the risk of
uncontrolled bleeding, while having too many increases the risk of excessive
clotting, which can cause the blockages that lead to a cardiovascular crisis
such as a stroke or pulmonary embolism (a blood clot in the lung).
Blood chemistry: measures the electrolytes (salts) chloride, potassium and sodium,
bicarbonate (dissolved carbon dioxide), glucose, creatinine and urea. It often
also measures calcium and the protein albumin. These tests monitor kidney
function, blood acidity, and levels of electrolytes and sugar. Abnormal levels
are generally a sign of other underlying conditions and warrant investigation.
The protein albumin is the most abundant protein
in blood, and acts like a sponge to soak up excess fluid. Decreases in albumin
are associated with liver damage and a form of kidney disease. They also occur
when the body goes into shock or is experiencing severe inflammation. Results
from other tests distinguish between the potential causes.
Creatinine is a
waste product of muscle metabolism. Because the kidneys are responsible for
almost all removal of the compound from the body, blood creatinine levels are a
good indicator of kidney function. Creatinine levels can be used to calculate
the ‘estimated glomerular filtration rate’ (eGFR), which takes account of the age, sex and ethnic origin of
the person. A high blood creatinine level and a low eGFR indicate kidney damage
and decreased kidney function.
Levels of specific electrolytes may be abnormal in
specific disorders. Diabetes, kidney damage, some forms of heart disease and
muscle and nerve problems can all result in electrolyte imbalances.
Prescription drugs may also change levels. Other signs and further testing can
determine the condition that is causing imbalance.
Levels of blood
sugars (also called blood glucose)
are used to monitor for signs of diabetes or its precursor, insulin resistance.
Diabetes, if left untreated, can lead to serious issues such as cardiovascular
disease, kidney damage, increased risk of problems with feet and legs, and
damage to the eyes.
Blood for a glucose
level test is often drawn after you have fasted; that is, not eaten for at
least eight or nine hours. This avoids the fluctuations in glucose levels that
occur after eating and provides the most accurate measurement. If results
suggest a problem, your doctor is likely to order a more specific test, called
an oral glucose tolerance test, that evaluates the body’s ability to control blood
glucose levels.
Lipid profile: Blood tests for fats (also called lipids) measure levels of cholesterol and triglycerides. The two forms of
cholesterol of interest are low-density
lipoprotein (LDL or ‘bad’ cholesterol) and high-density lipoprotein (HDL, or ‘good’ cholesterol). Tests
will also measure total cholesterol,
as well as another type of fats called triglycerides.
Generally, levels of total cholesterol and triglycerides should be as low as
possible and so should your LDL:HDL ratio. Unhealthy levels of blood lipids are
a risk factor for cardiovascular disease.
Liver function tests: ALT (alanine aminotransferase) and AST (aspartate aminotransferase)
are two enzymes found within cells of the liver. Their presence in the blood
indicates damage to the liver cells. Results are likely to vary from test to
test and so dramatic changes, often presented as multiples of your ‘normal’
reading, are a more reliable indicator of ongoing damage. A change that is up
to five times a normal reading is considered mild impairment.
If levels of either
enzyme, or their ratio (ALT:AST), change, your doctor is likely to look for
indications of potential causes. These could include viral hepatitis
infections, starting to take certain antiretroviral or other prescription
drugs, drinking a lot of alcohol, being obese, having insulin resistance or
diabetes, or having high blood lipid levels.
Alkaline phosphatase
and total bilirubin are two other liver tests that may be ordered. Elevated
levels of either may be a sign of liver damage or another condition and so
results are interpreted along with other liver function test results. One HIV
drug, atazanavir (Reyataz), causes a
type of rise in bilirubin levels which is usually harmless.
Vitamin D: is not routinely measured but 25-hydroxyvitamin D, the form measured in
a blood test, may be, as part of monitoring bone health. Extremely low levels
of the vitamin may be associated with bones that do not mineralise (or harden)
while more-than-mild deficiency may be linked to osteoporosis (low levels of
calcium in the bone, which leads to weakness and fractures). Deficiency has
recently been found in a large proportion of people with HIV from diverse
geographical regions.3 There is no evidence as yet that
supplementing mildly depleted levels of vitamin D is of benefit, but as it’s a
cheap supplement, doctors may prescribe it.
Calcium: levels may also be related to bone health, but are only useful
diagnostically in cases of severe bone disease. Low calcium may indicate kidney
failure or a number of other conditions.