Blood test

Lore & Background

Blood tests are performed on samples obtained via venipuncture or fingerstick. Phlebotomists, laboratory practitioners, and nurses typically extract blood, though paramedics and physicians may do so in emergencies. Indwelling arterial, central venous, and peripheral venous lines can also be used. Blood flows throughout the body, providing oxygen and nutrients to tissues and carrying waste products to excretory systems, making its analysis relevant to many medical conditions.

Reader's Guide

Blood tests are the most commonly performed medical tests because the bloodstream affects or is affected by many medical conditions. They are used to assess disease, mineral content, drug effectiveness, and organ function. Typical panels include the basic metabolic panel and complete blood count. Specialized tests like arterial blood gas analysis monitor carbon dioxide and oxygen levels for pulmonary function. Blood tests also identify autoimmune diseases and Immunoglobulin E-mediated food allergies. Results should be interpreted using ranges provided by the performing laboratory. Future alternatives include saliva testing, which contains 20% of blood proteins, and microchip-based tests such as microemulsion and SIMBAS lab-on-a-chip devices.

Did You Know?

• Blood tests are the most commonly performed medical tests.
• A basic metabolic panel measures sodium, potassium, chloride, bicarbonate, blood urea nitrogen, magnesium, creatinine, glucose, and sometimes calcium.
• Saliva contains 20% of the proteins found in blood, making it a potential alternative for some blood tests.
• The SIMBAS lab-on-a-chip can diagnose diseases within 10 minutes without external tubing.

The Mechanics of Measurement

Blood pressure is the force that circulating blood exerts against vessel walls, driven primarily by the heart's pumping action. When the term is used without further specification, it refers to pressure within the brachial artery—the site most commonly used for clinical readings. The standard expression pairs systolic pressure (the peak during a single heartbeat) with diastolic pressure (the trough between beats), both recorded in millimetres of mercury above atmospheric pressure or, alternatively, in kilopascals. Two derived values carry their own clinical weight: pulse pressure, the gap between systolic and diastolic figures, and mean arterial pressure, the average across a full cardiac cycle. Healthcare professionals treat blood pressure as one of the five vital signs alongside respiratory rate, heart rate, oxygen saturation, and body temperature, making it a cornerstone of any patient assessment.

Defining Normal and Abnormal

Readings below roughly 90/60 mmHg are commonly labelled hypotension, though in practice clinicians only flag low pressure when the patient experiences symptoms. Consistently elevated readings define hypertension, a condition whose long-term presence raises the risk of stroke, heart disease, and kidney failure. There is no universally accepted diagnostic cutoff for hypotension, and both high and low extremes can arise suddenly or creep in gradually, with numerous underlying causes.

The Living Rhythm of Pressure

Blood pressure is far from a static number. It shifts from minute to minute, follows a circadian pattern peaking in early morning and evening while dipping overnight, and responds to temperature, noise, emotional stress, food, posture changes, medications, and physical activity. The loss of the normal nocturnal dip has been linked to a heightened future risk of cardiovascular disease, and emerging evidence suggests that night-time readings may actually predict cardiovascular events more powerfully than day-time ones. Because of this variability, guidelines now distinguish between office (clinic) readings, home self-measurements, and ambulatory monitoring with an automated device over a full 24-hour period. In the United Kingdom, NICE has gone so far as to recommend ambulatory measurement as the preferred diagnostic tool for hypertension. Short-term regulation is handled by baroreceptors that relay signals through the brain to the nervous and endocrine systems, while longer-term influences include cardiac output, vascular resistance, blood volume, and arterial stiffness.

Pressure Across a Lifetime

In the womb, it is the fetal heart—not the mother's—that generates the pressure needed to drive blood through the developing circulation, starting at roughly 30 mmHg in the fetal aorta at 20 weeks and climbing to about 45 mmHg by 40 weeks. Full-term newborns typically show systolic values between 65 and 95 mmHg and diastolic values between 30 and 60 mmHg. In childhood, normal ranges sit lower than adult figures and are calibrated to the child's height, with country-specific reference tables in use. Among adults, systolic pressure tends to climb from early adulthood through at least age 70, while diastolic pressure rises initially but begins its descent around age 55. Pulse pressure widens noticeably after 40, largely because arteries stiffen with age. When systolic readings exceed the normal range while diastolic remains within bounds, the condition is termed isolated systolic hypertension. Notably, this age-related rise is not universal; some isolated, unacculturated communities do not exhibit it, suggesting that lifestyle and environment play a meaningful role.