What parameter does a complete blood count (CBC) measure?

A complete blood count (CBC) is a comprehensive blood test that provides critical information about various components of the blood. It measures not only red blood cells and platelets but also includes white blood cells, hemoglobin, and hematocrit levels. This extensive range of measurements enables healthcare providers to evaluate a patient's overall health, detect a wide array of disorders such as anemia, infection, and many other diseases, and monitor changes in health status over time.

The specific parameters included in a CBC are essential for diagnosing conditions and guiding treatment decisions. For instance, the red blood cell count helps assess oxygen-carrying capacity, while the white blood cell count is crucial for evaluating immune function. Hemoglobin measures the protein that carries oxygen, and hematocrit reflects the proportion of blood volume that is occupied by red blood cells. Platelet counts are vital for assessing blood clotting ability.

While the other options mention specific components measured in a CBC, they do not encompass the full scope of the test, which includes all these vital parameters. This comprehensive measurement approach is what makes option D the most accurate representation of what a complete blood count evaluates.