Basic Blood Tests

Why to they have to draw so much blood?

This is one of the questions most frequently asked by patients and family members. This section answers the question and explains basic laboratory tests.

Health or illness of many of the body's important systems is reflected in the blood stream. Because changes occur rapidly in critical illness, blood is sampled frequently in order to make adjustments to these changes.

Since repeated needle sticks are uncomfortable for anyone, the ICU team may draw the patient's blood through tubes (catheters) inserted into arteries or veins. Not all patients have or require these tubes. However, if they are being used, these tubes may stay in place for several days so that the number of needle sticks is reduced dramatically.

Blood being drawn through tubing (called a catheter).

Laboratory tests have been so improved recently that today less blood needs to be drawn than in the past, often less than a teaspoon. Newer technologies now allow many blood tests to be performed right at the bedside. This is called point of care testing. Some newer methods don't remove blood at all. Instead, blood is tested right in the blood stream or a small amount of blood is withdrawn, tested and then returned to the patient.

Many tests can be performed on the blood. Only the most common tests are described here.

Arterial Blood Gas (ABG)

One of the hallmarks of critical illness or injury is an abnormality in the body's ability to take in (inhale) enough oxygen and to give off (exhale) enough carbon dioxide. The amount of oxygen and carbon dioxide in the blood stream can be directly measured from a small blood sample drawn from an artery. This is usually done from a small artery at the wrist or in the upper part of the leg.

In addition this test provides information about the acid/base balance of the body. The body's systems function best within a certain narrow acid/base range. Too much acid or too much base may cause serious problems. The doctor and nurse may follow this closely when the patient is critically ill or injured.

Basic Blood Chemistry

The basic blood chemistry is a common blood test performed on many ICU patients. The test provides information regarding the patient's electrolyte balance (potassium and sodium) and measures the kidney function.

Complete Blood Count (CBC)

The CBC provides information about the numbers of red blood cells, the amount of hemoglobin in the blood, number and type of white blood cells and the number of platelets. Anemia is the term that describes a patient's condition when the red blood cells are low. Hemoglobin is the substance in the red blood cells that carries oxygen to the body. The white blood cells are the cells that help fight infection in the body. Platelets are involved in the body's clotting system. By studying the blood cells under a microscope much can be learned about health and illness.

Heart (Cardiac) Enzymes

Heart enzymes are proteins found in the heart muscle cells. When the heart muscle is damaged (such as heart attack) certain enzymes or proteins leak out of the cells from the heart muscle into the blood stream. These enzymes may be measured over time (usually 3-4 times in 24 hours) and can help make the diagnosis of heart attack.

Lactic Acid

Elevated lactic acid levels may indicate that the body is not receiving enough oxygen or that it cannot use the available oxygen. Poor lung function, poor heart function, very low blood levels, very low blood pressure, or poisons can be associated with elevated lactic acid levels. This is a very dangerous situation and a persistently elevated lactic acid level is usually a bad sign.

Other Blood Tests

Many other blood tests are performed. A sick liver or pancreas can be diagnosed and followed with blood tests. Foreign substances such as drugs and poisons can be directly measured in the blood stream. Studies of the blood's ability to clot may be performed. Hormone levels can be measured. The list of other blood tests available is extensive. Speak with your doctor or nurse if you have specific questions.