Blood test
Laboratory analysis performed on a blood sample
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Through Blood, Vein and Health care
Extraction
A venipuncture is useful as it is a minimally invasive way to obtain cells and extracellular fluid (plasma) from the body for analysis. Blood flows throughout the body, acting as a medium that provides oxygen and nutrients to tissues and carries waste products back to the excretory systems for disposal. Consequently, the state of the bloodstream affects or is affected by, many medical conditions. For these reasons, blood tests are the most commonly performed medical tests.
If only a few drops of blood are needed, a fingerstick is performed instead of a venipuncture.
Indwelling arterial, central venous and peripheral venous lines can also be used to draw blood.
Phlebotomists, laboratory practitioners and nurses are those in charge of extracting blood from a patient. This can be done in-clinic, in-office, or through mobile (home) phlebotomists. However, in special circumstances, and/or emergency situations, paramedics and physicians extract the blood. Also, respiratory therapists are trained to extract arterial blood to examine arterial blood gases.
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Types of tests
Biochemical analysis
A basic metabolic panel measures sodium, potassium, chloride, bicarbonate, blood urea nitrogen (BUN), magnesium, creatinine, glucose, and sometimes calcium. Tests that focus on cholesterol levels can determine LDL and HDL cholesterol levels, as well as triglyceride levels.
Some tests, such as those that measure glucose or a lipid profile, require fasting (or no food consumption) eight to twelve hours prior to the drawing of the blood sample.
For the majority of tests, blood is usually obtained from the patient's vein. Other specialized tests, such as the arterial blood gas test, require blood extracted from an artery. Blood gas analysis of arterial blood is primarily used to monitor carbon dioxide and oxygen levels related to pulmonary function, but is also used to measure blood pH and bicarbonate levels for certain metabolic conditions.
While the regular glucose test is taken at a certain point in time, the glucose tolerance test involves repeated testing to determine the rate at which glucose is processed by the body.
Specimen storage and stability
Blood specimens for complete blood count (CBC) analysis should ideally be analysed as soon as possible after collection to minimise pre-analytical changes. When delays are unavoidable, EDTA-anticoagulated blood may be stored under refrigerated conditions (2–8 °C), although the stability of individual CBC parameters varies. Haemoglobin concentration and red blood cell and white blood cell counts are generally more stable during refrigerated storage than platelet-related parameters and some red blood cell indices. Laboratories should follow validated storage recommendations and consider potential analytical changes when interpreting delayed specimens.
Blood tests are also used to identify autoimmune diseases and Immunoglobulin E-mediated food allergies (see also Radioallergosorbent test).
Normal ranges
Blood tests results should always be interpreted using the ranges provided by the laboratory that performed the test. Example ranges are shown below.
Common abbreviations
Upon completion of a blood test analysis, patients may receive a report with blood test abbreviations. Examples of common blood test abbreviations are shown below.
Molecular profiles
- Protein electrophoresis (general technique—not a specific test)
- Western blot (general technique—not a specific test)
- Liver function tests
- Polymerase chain reaction (DNA). DNA profiling is today possible with even very small quantities of blood: this is commonly used in forensic science, but is now also part of the diagnostic process of many disorders.
- Northern blot (RNA)
- Sexually transmitted diseases
Cellular evaluation
- Full blood count – Routine laboratory test of blood cells
- Hematocrit – Volume percentage of red blood cells in blood
- Mean corpuscular volume (MCV) – Average volume of a red blood cell, which sometimes helps in diagnosis
- Mean corpuscular hemoglobin concentration (MCHC) – Measure of hemoglobin concentration in red blood cells
- Erythrocyte sedimentation rate (ESR) – Physiological quantity
- Cross-matching. Determination of blood type for blood transfusion or transplants
- Blood cultures are commonly taken if infection is suspected. Positive cultures and resulting sensitivity results are often useful in guiding medical treatment.
Future alternatives
Saliva tests
In 2008, scientists announced that the more cost effective saliva testing could eventually replace some blood tests, as saliva contains 20% of the proteins found in blood. Saliva testing may not be appropriate or available for all markers. For example, lipid levels cannot be measured with saliva testing.
Microemulsion
In February 2011, Canadian researchers at the University of Calgary's Schulich School of Engineering announced a microchip for blood tests. Dubbed a microemulsion, a droplet of blood captured inside a layer of another substance. It can control the exact size and spacing of the droplets. The new test could improve the efficiency, accuracy, and speed of laboratory tests while also doing it cheaply.
SIMBAS
In March 2011, a team of researchers from UC Berkeley, DCU and University of Valparaíso have developed lab-on-a-chip that can diagnose diseases within 10 minutes without the use of external tubing and extra components. It is called Self-powered Integrated Microfluidic Blood Analysis System (SIMBAS). It uses tiny trenches to separate blood cells from plasma (99 percent of blood cells were captured during experiments). Researchers used plastic components, to reduce manufacturing costs.