Why is My Doctor Requesting This Blood Test?

If you’re like most people, reading your blood work results from a lab report is like trying to decipher Middle English. You can make out some terms and guess at others, but that’s where your understanding ends.

So we’ve come up with a handy primer that will explain the routine tests a physician typically orders, what they measure and why those numbers are important. Generally, people will get wellness blood work done, along with a urinalysis, during an annual physical. The results will be compared to a reference range, the upper and lower limits based on healthy people. If any of your test results fall outside the range of expected values, this may be a clue to potential disease.

“As physicians, we want to evaluate your overall health and blood work allows us to do this,” says Dr. E. Robert Schwartz, a family medicine expert with the University of Miami Health System. Blood work can also help a doctor detect a range of disorders, including cancer, diabetes, and heart disease, as well as monitor an already-diagnosed condition.

However, not all abnormal blood tests are created equal and patients should be aware of this before panicking. Older adults can experience results out of the normal range, and medications can also skew numbers. One example: The anti-coagulant Heparin can decrease platelet count.

What’s more, the out-of-range result may not be significant, as normal ranges are based on population averages, and a patient may be a bit different than the average.

“Sometimes you may be out of range a fraction of a point and that’s not something to worry about,” Schwartz said, adding that your physician is the best source for any concerns you might have.

When patients come to see Dr. Schwartz for a physical, he orders four lab tests, including a urinalysis. Here we will look at the three main types of blood tests:

A Complete Blood Count, or CBC, is perhaps the most common of blood tests. Since it measures the variety, concentration, and quality of red blood cells, white blood cells and platelets, it’s a useful screening tool for hematological abnormalities, from infections to anemia to leukemia.

A CBC will give you a count of white blood cells (which fight infection), red blood cells (which carry oxygen), hemoglobin (the red blood cell protein that carries oxygen), platelets (involved in blood clotting) and hematocrit (a ratio of red blood cells to plasma). An out-of-range in cell counts may point to a condition that needs further evaluation. A CBC also helps monitor how a medication is affecting blood cell counts and is instrumental in monitoring a blood disorder condition.

The Comprehensive Metabolic Panel, also referred to a chemistry panel, provides a broad view of your body’s chemical processes (essentially a look at your kidney and liver functioning) as well as your electrolytes, including the acidity of your blood, the amount of water in your body, and the levels of essential minerals, including calcium, magnesium, potassium, sodium and chloride. Schwartz sometimes requests Vitamin D levels as well.

This panel also measures albumin (the plasma protein that carries blood cells), bilirubin (produced when hemoglobin breaks down) and creatinine (a chemical waste product). All these important markers show how effectively your body is running. Too much bilirubin could mean jaundice, while low albumin is an indicator of liver and kidney diseases, inflammation and malnutrition. And too much creatinine can signal poor kidney function, as it’s the kidneys that remove this chemical from the body.

The CMP typically requires a fasting glucose test. High levels of glucose, which is the body’s main source of energy, can be a marker for diabetes.

The Lipid Panel is the blood work we usually talk about most. “Know your numbers” is often by healthcare campaigns — for a reason. These tests measure two types of fat in the blood,

triglycerides (fat produced in the liver) and cholesterol (quantified as HDL and LDL). For triglycerides, lower (under 150) is better. High levels can correlate with a risk of heart and blood vessel disease, as well as thyroid or liver disease. Often these numbers are high because of lifestyle factors, such as too much sugar, fat, and carbohydrates in the diet, so it’s not uncommon for overweight or obese people to have elevated levels. If you smoke, drink alcohol in excess or are physically inactive, it’s likely these numbers will be high, too. In a way, that’s the good news, as lifestyle changes can lower triglyceride levels.

Cholesterol levels are measured in three different ways. Total cholesterol, ideally less than 200 mg/dl, can serve as a warning of heart disease. People with elevated level (240 mg/dl or more) typically have twice the risk as those with normal levels.

HDL cholesterol, or high-density lipoprotein, is referred to as the good cholesterol because it takes the extra cholesterol from the blood to the liver for elimination. High levels of HDL correlate with a lower risk of cardiovascular disease. On the other end, LDL, or low-density lipoprotein, is often referred to as the “bad” cholesterol. Too much of it can build up in the arteries, forming plaque that eventually narrows and hardens them. (This is known as atherosclerosis.) High levels are also linked to heart disease.

Knowing your cholesterol/HDL ratio is important, particularly if you’re worried about your risk for heart disease. Aim for a ratio of three or below. If you’re not there, you can help lower that number by eating right (fiber-rich foods, no saturated fats and plenty of omega 3) and getting regular exercise.

Two last words of advice from Dr. Schwartz.  Unless otherwise told to do so, make sure to drink plenty of water before getting lab work done. Being dehydrated affects kidney function and may skew your test results.

And when your physician asks you to fast before getting pricked, follow instructions. Fasting requires no eating at least eight hours before a test.  “It’s better to do it fasting,” he says. “Eating before blood work, especially a big meal, will throw the numbers off.”


In Their WordsAna Veciana Suarez
Ana Veciana-Suarez, Guest Contributor

Ana is a regular contributor to the University of Miami Health System. She is a renowned journalist and author, who has worked at The Miami Herald, The Miami News and The Palm Beach Post. Visit her website at anavecianasuarez.com or follow @AnaVeciana on Twitter.