Not everyone feels the need to seek out independent laboratory testing, but it does pay to stay on top of your blood work.

The first thing to know is that results of tests ordered by doctors are part of your medical record and you're entitled to a copy.

Sometimes an attentive patient can catch a condition before it becomes advanced, questioning test results to which the doctor may have given short shrift.

The most common blood test is the CBC, or complete blood count, measuring quantities of cells in a given sample of blood, including infection-fighting white blood cells and oxygen-ferrying hemoglobin, and the hematocrit, which measures the percentage of red blood cells. These tests can pick up infection, anemia, leukemia and other disorders.

Most doctors routinely ask the lab to measure several lipids, the technical term for blood fats. These include total cholesterol, as well as high-density lipoprotein ("good" cholesterol), low-density lipoprotein ("bad" cholesterol), and the ratio between the two, which assesses the risk of having a heart attack or stroke. Often, triglycerides are measured as well; when elevated, they're another risk for heart disease.

Glucose levels indicate diabetes risk. Levels of electrolytes, or salts, such as sodium, potassium and chloride indicate how the heart, liver and kidney are functioning. Levels of creatine, phosphorus and blood urea nitrogen -- or BUN -- can show if the kidneys are working properly.

The erythrocyte sedimentation rate -- or ESR -- measures how long it takes red blood cells to settle to the bottom of a test tube, providing a general indication of inflammation somewhere in the body. If you're at risk for a bleeding problem, either a prothrombin time or partial thromboplastin time (PTT) test will show how long it takes your blood to clot. Thyroid tests can pick up an underactive or overactive thyroid gland.

Most laboratories provide, alongside each of your results, a reference range, which is the range of normal readings; these may vary from lab to lab, depending on how the measuring is done. Labs often flag a reading that is too high or too low. But even then, you'll need to rely on your doctor in most cases to determine if the reading merits concern.