A blood test is a laboratory investigation in which a sample of blood is collected and analysed to obtain information about a person's health or disease. Blood contains cells, proteins, electrolytes, hormones, nutrients, waste products, antibodies, clotting factors and many other substances, making it a valuable source of information about numerous biological processes.
A sample may be collected from a vein, usually in the arm, through venepuncture, or from a finger or other site using a smaller capillary sample for certain tests. The required sample depends on the analysis being performed. Blood testing can assess individual substances, groups of measurements or complex panels designed to investigate particular organs or clinical problems. Results are interpreted using appropriate reference ranges, clinical thresholds and the person's symptoms and circumstances. A blood test is therefore not a single investigation but a broad category encompassing many different laboratory analyses.
Different blood tests examine different components of blood. A full blood count measures quantities and characteristics of red blood cells, white blood cells and platelets and can provide evidence relevant to anaemia, infection, inflammation, blood disorders and other conditions. Biochemistry tests can measure electrolytes, glucose, kidney markers, liver enzymes and proteins. Lipid testing assesses substances such as cholesterol and triglycerides. Hormone tests measure endocrine signals such as thyroid hormones or reproductive hormones.
Immunological testing can detect antibodies or other immune markers, while microbiological testing can sometimes identify organisms or components of infectious agents. Coagulation tests assess blood-clotting pathways. Blood groups and compatibility testing are used for transfusion and other clinical purposes. Each test has its own methodology, accuracy, preparation requirements and interpretation.
Blood collection usually involves cleaning the skin, inserting a sterile needle into a vein and drawing blood into one or more labelled tubes. Different tubes contain different additives or anticoagulants depending on the analysis required. Serum is the liquid portion of blood obtained after clotting, whereas plasma is the liquid component collected with anticoagulant and still contains clotting factors. Whole blood can be required for tests that examine blood cells or certain other characteristics. After collection, samples are transported to a laboratory where they may undergo centrifugation, automated analysis, microscopy, immunological assays, molecular testing or other procedures. Correct patient identification, sample labelling, collection technique, transport and storage are important because errors at these stages can produce unreliable results.
Preparation for a blood test varies according to the specific investigation. Some tests require fasting because recent food intake can temporarily alter certain measurements, although many routine blood tests do not require fasting. Certain medicines, supplements, strenuous exercise, dehydration and the timing of collection can influence specific results. Patients should not routinely stop prescribed medicines unless instructed to do so by an appropriate healthcare professional.
Blood volume requirements also vary, and modern collection systems usually require only relatively small amounts for individual analyses. Venepuncture can cause brief discomfort, bruising, light-headedness or, less commonly, bleeding or infection at the puncture site. People who are prone to fainting can tell the healthcare professional beforehand so appropriate precautions can be taken.
Blood tests can contribute to diagnosis, screening, monitoring and treatment decisions. They may detect anaemia, abnormalities in kidney or liver function, disturbances of electrolytes, changes in blood glucose, inflammation, abnormal cholesterol levels, immune responses, hormone disturbances and evidence relevant to infection. They can also monitor the effects or toxicity of medicines and track chronic diseases over time.
Some tests are highly specific to particular clinical questions, while others provide broad information that helps identify which body system may require further investigation. A blood result rarely provides a complete diagnosis by itself. An abnormal result may have several possible causes, and a normal result may not exclude every possible disease. Clinicians therefore interpret blood tests in relation to symptoms, examination, medical history and other investigations.
Blood tests use different analytical technologies. Automated haematology analysers count and characterise blood cells, while clinical chemistry analysers quantify molecules using chemical or enzymatic reactions. Immunoassays detect antibodies, antigens, hormones or other molecules through specific molecular interactions. Molecular tests can detect DNA or RNA associated with genetic conditions or infectious organisms. Microscopy can identify abnormal cell morphology or blood parasites in selected circumstances. Mass spectrometry and chromatography can provide highly specific measurements of chemicals and metabolites. Modern laboratories use quality-control systems and calibrated instruments to ensure analytical reliability. Nevertheless, laboratory accuracy depends on both the measurement system and the quality of the specimen.
Blood-test results are generally reported using numerical values, units and reference intervals or decision thresholds. A reference interval is not necessarily a boundary between healthy and diseased states. Approximately most values in a selected reference population fall within a conventional reference interval, meaning some healthy people naturally have values outside it. Conversely, disease can occur with a value inside the reference interval.
Results must therefore be interpreted according to age, sex-related biological factors, pregnancy status, medications, clinical circumstances and the laboratory's own methods. Trends can be especially informative; a modest change may be clinically important if it represents a substantial departure from a person's previous baseline. Conversely, an isolated abnormal result may sometimes be transient or caused by factors unrelated to disease.
Blood testing has evolved from relatively simple observations of blood appearance and cell morphology to highly automated, molecularly sophisticated laboratory medicine. The development of microscopy, clinical chemistry, immunology, molecular diagnostics and automated analysers has greatly increased the range of information obtainable from small blood samples. Blood tests are now integral to routine primary care, emergency medicine, hospital medicine, surgery, oncology, genetics and monitoring of long-term conditions.
Their major strength is that blood circulates throughout the body and reflects processes occurring in multiple organs, but this same breadth means that abnormal findings are often non-specific. The clinical usefulness of a blood test therefore depends on asking an appropriate question and interpreting the result within the broader clinical picture.
