Common Blood Tests: An Overview

Blood tests are one of the most commonly used diagnostic tools in healthcare, providing insight into a patient’s physiological status, organ function, inflammatory response, and metabolic balance. From identifying infection and electrolyte disturbances to recognising acute kidney injury or cardiac injury, pathology results often help guide clinical decision-making.

Understanding what common blood tests measure is an important foundation for interpreting results in context. While some abnormalities may be mildly altered with little immediate significance, others require urgent review and intervention. This article provides a practical overview of commonly requested blood tests and what abnormal results may indicate.

What You Need to Know

Blood tests can provide valuable clinical information, but knowing what a result actually represents is just as important as recognising whether it falls outside the reference range. A raised white cell count may suggest infection, but it can also occur with inflammation, physiological stress, or corticosteroid use. A low haemoglobin may indicate chronic anaemia, or it may signal acute blood loss depending on the clinical situation.

Common blood tests include:

• Full Blood Examination (FBE) – assesses red blood cells, white blood cells, and platelets

• Urea, Electrolytes and Creatinine (UEC/U&Es) – evaluates kidney function, hydration, and electrolyte balance

• Liver Function Tests (LFTs) – assesses liver function and potential hepatobiliary abnormalities

• Coagulation studies – such as INR and APTT, used to assess clotting function

• C-reactive protein (CRP) – a non-specific marker of inflammation

• Blood glucose level (BGL) – assesses glucose regulation and metabolic status

• Lactate – may indicate impaired tissue perfusion or physiological stress

• Troponin – a cardiac biomarker associated with myocardial injury

• Blood cultures – used when bloodstream infection is suspected

• Arterial or venous blood gases (ABG/VBG) – provides information about oxygenation, ventilation, acid-base balance, and metabolic status

Some abnormalities are immediately concerning. A critically elevated potassium, profoundly low glucose, or significantly raised lactate may indicate urgent clinical deterioration and require prompt action. Other results are less meaningful in isolation and become more useful when interpreted alongside symptoms, observations, medical history, and previous pathology.

When reviewing blood results, useful questions include:

• What does this test actually measure?

• Is this result mildly abnormal or critically outside the expected range?

• Does this fit with the patient’s clinical presentation?

• Does this require repeat testing, escalation, or urgent intervention?

Understanding the purpose of common pathology tests helps build clinical reasoning and supports safer decision-making.

Image: different blood tests require specific colour-coded collection tubes, as each contains different additives that help preserve the sample and ensure accurate laboratory analysis.

Beyond the Basics

Full Blood Examination (FBE)

A full blood examination provides information about the cellular components of blood, including red blood cells, white blood cells, and platelets. It is commonly used to assess infection, anaemia, inflammation, bleeding risk, and broader haematological concerns.

Haemoglobin (Hb)
Haemoglobin is the oxygen-carrying protein found within red blood cells. Low haemoglobin may suggest anaemia, acute blood loss, chronic disease, nutritional deficiency, or other underlying conditions. Reduced haemoglobin can impair oxygen delivery to tissues, potentially contributing to fatigue, dizziness, pallor, tachycardia, or shortness of breath.

Elevated haemoglobin may occur with dehydration, chronic hypoxia, or certain haematological disorders.

White Cell Count (WCC)
White blood cells play a key role in immune function. An elevated white cell count may indicate infection, inflammation, physiological stress, steroid use, or haematological disease. A low white cell count may increase infection risk and can occur with viral illness, immunosuppression, chemotherapy, or bone marrow suppression.

Neutrophils
Neutrophils are a specific type of white blood cell involved in the body’s response to bacterial infection. Raised neutrophils commonly suggest bacterial infection, inflammation, stress, or corticosteroid use. Low neutrophil levels can increase vulnerability to infection and may require closer monitoring.

Platelets
Platelets are essential for clot formation. Low platelet levels may increase bleeding risk, while elevated levels can sometimes occur in response to inflammation, infection, iron deficiency, or other conditions.

Urea, Electrolytes, and Renal Function (U&Es)

Urea and electrolyte studies provide information about fluid balance, kidney function, and electrolyte regulation. These tests are particularly relevant in acutely unwell patients, those receiving intravenous therapy, patients with renal impairment, and individuals at risk of fluid or electrolyte disturbances.

Sodium (Na+)
Sodium plays a major role in fluid balance and neurological function. Abnormal sodium levels can contribute to confusion, seizures, altered conscious state, weakness, or fluid imbalance.

Low sodium may occur with fluid overload, syndrome of inappropriate antidiuretic hormone secretion (SIADH), diuretic use, or various medical conditions. High sodium often suggests dehydration or impaired fluid regulation.

Potassium (K+)
Potassium is critical for cardiac, neuromuscular, and cellular function. Even moderate abnormalities can be clinically significant.

Low potassium may contribute to weakness, muscle cramps, arrhythmias, or ECG changes. High potassium is particularly concerning, as severe hyperkalaemia can cause life-threatening cardiac arrhythmias.

Urea
Urea is a waste product produced from protein metabolism. Elevated levels may occur with dehydration, renal impairment, gastrointestinal bleeding, or increased catabolism.

Creatinine
Creatinine is a commonly used marker of kidney function. Rising creatinine may indicate worsening renal function, acute kidney injury, or chronic kidney disease.

Estimated Glomerular Filtration Rate (eGFR)
eGFR provides an estimate of how effectively the kidneys are filtering blood. Reduced eGFR suggests impaired kidney function and should be interpreted in conjunction with creatinine, urine output, and clinical context.

Liver Function Tests (LFTs)

Liver function tests assess markers associated with liver injury, bile flow, and protein synthesis. They can help identify hepatic dysfunction, biliary obstruction, medication effects, or broader systemic illness.

Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST)
These enzymes are released when liver cells are damaged. Elevated levels may occur with hepatitis, medication toxicity, alcohol-related liver injury, or other hepatic conditions.

Alkaline Phosphatase (ALP)
ALP may rise in biliary obstruction or liver disease, but can also increase with bone-related conditions, so interpretation depends on the wider clinical picture.

Bilirubin
Bilirubin is produced during red blood cell breakdown. Elevated bilirubin may cause jaundice and can suggest liver dysfunction, biliary obstruction, or increased red blood cell destruction.

Albumin
Albumin is a protein produced by the liver and contributes to fluid balance and transport functions. Low albumin may occur with chronic illness, malnutrition, liver dysfunction, renal loss, or systemic inflammation.

Coagulation Studies (Coags)

Coagulation studies assess the body’s clotting function and are particularly relevant in patients receiving anticoagulants, those with liver disease, active bleeding, or pre-operative assessment requirements.

INR (International Normalised Ratio)
INR is commonly used to assess clotting tendency, particularly in patients receiving warfarin. A high INR may indicate increased bleeding risk.

APTT (Activated Partial Thromboplastin Time)
APTT evaluates part of the clotting cascade and is commonly monitored in patients receiving unfractionated heparin. Prolonged clotting times may indicate anticoagulant effects, clotting factor deficiencies, or other abnormalities.

Other Common Blood Tests

C-Reactive Protein (CRP)
CRP is an inflammatory marker that often rises in infection, inflammation, or tissue injury. While elevated CRP may support suspicion of infection or inflammatory illness, it is non-specific and must be interpreted alongside the broader clinical picture.

Blood Glucose
Blood glucose testing is essential in patients with diabetes, suspected hypoglycaemia, acute illness, or metabolic disturbances. Both low and high glucose levels can cause significant clinical deterioration if not recognised and managed.

Lactate
Lactate can rise when tissues are not receiving adequate oxygen or perfusion, such as in sepsis, shock, or critical illness. An elevated lactate may indicate physiological stress or deterioration and often warrants close review.

Troponin
Troponin is a cardiac biomarker associated with myocardial injury. Elevated troponin may suggest myocardial injury, including acute coronary syndrome, but can also rise in other conditions such as renal failure, sepsis, or significant physiological stress.

In Practice

Before the Blood Test: Getting It Right

Accurate pathology results begin long before the sample reaches the laboratory. Blood collection may seem straightforward, but errors during collection, handling, or transport can affect result accuracy, delay treatment, and lead to unnecessary repeat testing.

A practical approach includes:

1. Confirm the blood test request
   Review the pathology request form or electronic order to confirm exactly which tests have been requested. Different investigations require specific collection tubes and handling requirements. Confirm patient identity according to local policy before specimen collection, usually before obtaining the specimen, after labelling, and before sending specimen to pathology.

2. Check the correct collection tubes
   Different blood tests require specific colour-coded collection tubes containing different additives. Most facilities provide a pathology collection guide, specimen manual, or electronic reference outlining which tube is required for each test. Using the wrong tube may result in sample rejection or inaccurate results.

3. Follow the correct order of draw
   When collecting multiple specimens, the sequence matters. Additives from one tube can contaminate subsequent samples and alter pathology results, so local policy should always be followed.

4. Collect and label the specimen correctly
   Correct patient identification, accurate labelling, and appropriate collection technique are essential. Inadequate sample volume (the required fill volume is usually printed on the specimen tube label), haemolysis (destruction of red blood cells), or collection errors can compromise results..

5. Handle and transport the sample appropriately
   Some specimens require prompt transport, temperature control (such as being placed on ice), or gentle inversion after collection to mix additives correctly. Delays or incorrect handling may result in clotting, sample degradation, or inaccurate biochemical results.

Interpreting Results in Context

Once results are available, they become another important source of clinical information that can support clinical reasoning and decision-making. Pathology findings may help identify potential deterioration, guide monitoring, help prioritise care, and inform when escalation is required. For example, a critically elevated potassium, significantly raised lactate, or rapidly worsening renal function may indicate the need for urgent review and escalation.

While pathology results contribute to the broader clinical picture, nurses do not use these findings to make formal medical diagnoses. Instead, pathology results are interpreted alongside clinical assessment findings to recognise concerns, support escalation, and inform ongoing patient care.