Leukocytes

Leukocytes, also called white blod cells (WBCs), are a population of cells in the blood responsible for immune response, inflammation, and microbe defence. These are always present in the blood, with their numbers increasing in response to infection or inflammation. They are a major part of the immune system. The normal range is 4 000 - 10000/µL, also expressed as 4 - 10 G/L or 10⁹/L.

Types

There are multiple subgroups of leukocytes, including granulocytes, lymphocytes, and monocytes. Neutrophils, eosinophils and basophils all contain granules and have a granular appearance. For this reason, they’re together called granulocytes. Monocytes and lymphocytes don’t have granular appearance and are sometimes called agranulocytes.

Recall the number and types of WBCs:

Cell type	Number
White blood cell (WBC)	4 000 – 10 000/µL
Neutrophils	60% of all WBCs
Lymphocytes	35% of all WBCs
Monocytes	5% of all WBCs
Eosinophils	3% of all WBCs
Basophils	0-1% of all WBCs

A good way to remember the order of white blood cells in decreasing order from most abundant to least abundant is the pneumonic never let monkeys eat bananas. N – L – M – E – B.

Leukocyte count (lab test)

Measurement of the number of leukocytes in the blood, a leukocyte count, is often used in the laboratory evaluation of infection or inflammation, leukaemia, and bone marrow. One can order the lab to just measure the number of leukocytes, or one can order the lab to measure the number of each type of leukocytes, called a differential count. Performing a differential gives more information than a plain leukocyte count, but the information may not always be useful.

Interpretation

Leukocytosis is a sign of inflammation. In case of leukaemia, the leukocyte count can increase beyond 50 000/µl. Other causes of leukocytosis include physical activity, stress, and use of glucocorticoids.

Leukocytopaenia is a sign of bone marrow depression, which can be due to toxic effects of medication or bone marrow malignancy. Sepsis may also cause leukocytopaenia due to increased consumption of leukocytes. A neutrophil count of < 500/µl is severe and a significant risk factor for infection.

The type of leukocyte which is elevated or decreased can be valuable in determining the cause:


Finding	Associated with
Neutrophilia	Bacterial infection, fungal infection, leukaemia, glucocorticoids
Neutropaenia	Bone marrow depression, sepsis
Lymphocytosis	Viral infections, pertussis, lymphoid malignancy
Lymphocytopaenia	Bacterial infection, fungal infection, HIV, glucocorticoids
Monocytosis	Chronic infection, myeloproliferative disorder
Monocytopaenia	Glucocorticoids
Eosinophilia	Allergic reaction, parasitic infection
Basophilia	Allergic reaction, chronic myeloid leukaemia

C-reactive protein

C-reactive protein, often abbreviated CRP, is an acute phase protein and a commonly measured laboratory parameter in the evaluation of inflammation. Its name comes from its ability to bind to the C-polysaccharide of pneumococci.

Biochemistry

CRP is a ring-shaped pentameric protein which binds to phosphocholine which is found on the cell membrane of dead cells and some bacteria. This binding activates the complement system. CRP also enhances phagocytosis of whatever CRP binds to, making it an opsonin.

CRP is synthesised in the liver.

CRP as a marker of infection or inflammation

The CRP is usually measured in the serum to evaluate acute inflammation. The reference range is <4-5 mg/L. It is also elevated after trauma or surgery, after other causes of necrosis such as myocardial infarction, and in some malignancies.

The half-life of CRP is 15-20 hours; as such, a reduction in acute inflammation is not reflected in the CRP level immediately. CRP peaks 36-50 hours after the onset of inflammation, but elevated levels of CRP can be measured after 8-12 hours after the insult.

The level of CRP correlates to some degree to the degree of inflammation and therefore the severity of the disease. An infection with CRP 200 is almost always more severe than one with 50.

The level of CRP can be used to distinguish between bacterial infection and other causes of inflammation, such as viral infections. There is no sharp cut-off, but markedly elevated levels > 100-150 is associated with bacterial infection. However, adenovirus infection can also cause a markedly elevated CRP.

Following CRP levels daily or over time can be used to monitor treatment response. If the CRP level decreases over time, one can conclude that the inflammation is decreasing and therefore that the treatment is efficacious.

CRP has a high negative predictive value for infection and inflammation, meaning that a negative result with high probability rules out severe infection and inflammation. A notable exception is some rheumatological disorders, for which erythrocyte sedimentation rate is a more sensitive marker of inflammation.

CRP as a marker of cardiovascular risk

Chronic inflammation increases the cardiovascular (CV) risk. CRP itself does not increase the risk, but it can be a marker of this risk-increasing inflammation. Newer laboratory methods allow more precise measurement of the CRP level, called high sensitivity CRP or micro CRP. This precision is unnecessary in case of evaluation for infection, but it can be useful in the determination of cardiovascular risk, as the inflammation in these otherwise healthy individuals is so low-grade that high sensitivity measurements are necessary to detect the small elevations in CRP. This chronic low-grade inflammation is sometimes called metabolic inflammation.

High sensitivity CRP (hsCRP) below 1 mg/L confers a lower CV risk, while a hsCRP > 3 confers a higher risk. In case of chronic low-grade inflammation, hsCRP levels of 3-10 are usually seen.

CRP as a marker of malignancy

CRP is elevated, usually only mildly (10-100 units) in some malignancies, most notably haematological ones.

Procalcitonin

Procalcitonin (PCT) is a prohormone for calcitonin, a calcium-regulating hormone. The level of PCT increases in case of some infections. PCT used to look very promising as a tool to distinguish between bacterial infection and other causes of inflammation (as a more specific alternative to CRP), but more recent research have dampened these hopes as it turns out to not be as specific for bacterial infections as previously thought. It's also a very expensive test. Still, it has some clinical uses.

PCT increases 3 - 4 hours after the infection, and peaks after 24 hours. The normal level is < 0,10 µg/L.

Clinical uses

For patients treated with antibiotics for infections in intensive care units, a reduction in PCT level by 80% or to < 0,5 µg/L means that discontinuation of the antibiotics is recommended
For patients in whom bacterial respiratory tract infection (like pneumonia) is suspected, a PCT level of < 0,25 µg/L means that antibiotics are not recommended, and a PCT level above that means that antibiotics may be recommended

However, using procalcitonin may also lead to overprescription of antibiotics, as many interpret an elevated PCT as a sign of bacterial infection requiring antibiotics, when there are actually many causes of falsely elevated PCT.

Interfering factors

Many conditions can cause falsely elevated PCT levels, including:

Trauma
Burn injury
Certain malignancies
Cirrhosis
Chronic kidney disease
Taking biotin supplement

Erythrocyte sedimentation rate

Erythrocyte sedimentation rate (ESR), also called the Westergren method (We), is a laboratory measurement of how quickly the erythrocytes in a sample sediment (settle on the bottom of the tube). The ESR increases in response to the acute phase reaction (inflammation), but it takes many days for the ESR to increase following an inflammation, and it takes weeks to normalise. The normal value depends on age and gender but is around < 15-30 mm/h.

It used to be the main parameter to measure when evaluating inflammation and infection, but nowadays other parameters are preferred, like CRP and leukocyte count. However, there are still some scenarios where the ESR is useful to measure.

Indications

Giant cell/temporal arteritis, polymyalgia rheumatica, rheumatoid arthritis, and Hodgkin lymphoma are conditions in which an elevated ESR is more specific and sensitive than an elevated CRP or leukocyte count.

Test procedure

Measurement of erythrocyte sedimentation rate usually requires a venous blood sample in a special black tube used only for ESR. In this tube, 4 parts blood are mixed with 1 part isotonic sodium citrate solution. The tube must then be placed vertically and some time must pass (usually 1 hour), after which the rate at which the erythrocytes have sedimented can be calculated. A regular EDTA-tube may also be used.

Pathophysiology

During inflammation and infection, the acute phase reaction is initiated, which increases the level of immunoglobulins, fibrinogen, and other positively charged acute phase proteins. Albumin is negatively charged, and its level is decreased during the acute phase reaction. As such, the albumin/globulin ratio decreases.

RBCs have negatively charged surfaces, while most plasma proteins are positively charged. Normally, the negative charges on the RBCs repel each other, but when the amount of positively charged plasma proteins increase, these charges are neutralised, making RBCs stick to each other in a characteristic stack similar to a stack of coins called a rouleau. These rouleaux sediment faster than regular RBCs.