CBC Test Explained: What a Complete Blood Count Measures

What is a CBC test?

A Complete Blood Count, or CBC, is a blood test that measures several types of cells circulating in your blood. It commonly reports red blood cells, white blood cells, platelets, haemoglobin and related indices.

Doctors may use a CBC as part of a routine health assessment or when evaluating symptoms such as tiredness, fever, weakness, bruising or other concerns. The test itself does not establish a diagnosis on its own.

What does a CBC measure?

Red-cell measurements help describe the number and characteristics of cells that carry oxygen. White-cell measurements describe cells involved in immune responses. Platelets are involved in normal blood clotting.

Laboratories may report additional indices alongside these counts. Reference intervals can vary by laboratory, age, sex and clinical context, so results should be interpreted with your clinician.

Preparation and sample

A CBC usually uses a small venous blood sample. Fasting is not generally required for a CBC alone, although another test ordered at the same time may have different preparation requirements. Always follow the instructions attached to your actual booking or provided by your clinician.