How Are Blood Levels Measured?

Doctors order a blood volume test to determine the amount of blood in the body, states Cleveland Clinic. This procedure also measures the blood's amount of plasma and red cells. A blood volume test helps doctors diagnose conditions such as low or high blood volume, low or high red cell volume, high blood pressure, and heart failure.

Doctors also require a blood volume test to assess or treat orthostatic hypotension, shock, conditions related to acute blood loss and kidney dialysis, reports Cleveland Clinic. The test allows physicians to identify the cause of symptoms, circulation problems or abnormal test results. Additionally, it helps doctors determine if a person has an adequate amount of blood according to his gender, weight and height.

A blood volume test involves inserting an intravenous line into a vein in the arm to obtain blood samples and deliver a tagging agent and radioactive isotope into the body, explains Cleveland Clinic. Doctors sometimes administer certain medications intravenously.

The blood volume analysis part of the test requires injecting a tiny amount of radioiodinated serum albumin, acquiring blood samples and evaluating the samples, notes Cleveland Clinic. People who are allergic to iodine, certain dyes, shellfish or eggs typically undergo modified tests to reduce the risk of allergic reactions. Another part of the test is a hematocrit measurement, which measures the bloodstream's percentage of red blood cells.