Understanding Commonly Ordered Lab Tests

Ever wonder what medical lab tests are run when you get your blood drawn?

Why do they collect several tubes or blood? What does the colored top of each tube mean? And why is a urine sample collected? Do I really need all these tests?

Although it may seem that the lot of blood is collected when you get your blood drawn, it usually is only about 2 tablespoons total. This amount of blood is divided, into several different tubes with variously colored rubber tops. Even though it seems like a maze of colors and tubes, there are essentially only a few major types of blood (and urine) tests.

The most common two types of blood and urine tests are:

  1. a complete metabolic panel, and
  2. a complete blood count
  3. a complete urinalysis
  4. coagulation tests

This article will focus on a basic understanding of these three commonly ordered tests.

Complete Metabolic Panel (the “gray top” test tube)

A complete metabolic panel, or CMP for short, checks for several basic chemical components in your blood.

The first set of these components are what’s called your “electrolytes”. Some of the most commonly checked electrolytes are sodium, potassium, and others.


Sodium is basically salt. And although it may seem strange to think of it, most of the liquid in our bodies can be considered “saltwater”. This is called “saline”. The most commonly used type of IV fluid is saline. This contains sodium, which can become abnormally high or low in certain disease states such as dehydration. Thus, saline helps restore a normal sodium balance/level in your bloodstream. Sodium is used for almost every metabolic process in the body. Hence it is the most commonly checked electrolyte.


Another very commonly checked electrolyte is potassium. Potassium is used for by the body for many purposes, such as muscular strength/contraction. When potassium levels get low, muscles make 10 to response sluggishly and are weaker than usual. Potassium will become abnormal in various disease states as well.

Kidney Function

Another part of the complete metabolic panel checks your kidney function.

The two tests used to check kidney function are called a BUN (blood urea nitrogen) and aced creatinine.

Both of these blood chemicals are essentially waste products that are eliminated by the kidney into the urine. They can also reflect dehydration. Hence if the kidneys are not functioning well, the BUN and creatinine will go up.

Liver Function

Most complete metabolic panels also include tests having to do with liver function. The liver is an extremely important organ that is involved in nearly all aspects of metabolism.

Certain disease states, such as alcoholism or hepatitis, will cause abnormal liver function tests. Certain diseases or infections of the gallbladder, which is an organ that is connected to the liver, may also be detected by checking these liver function tests.

Hence we can see that the complete metabolic panel includes electrolytes, kidney function, and liver function.

Complete Blood Count (the “purple top” test tube)

The complete blood count, or “CBC”, basically tests the red blood cells in the white blood cells and cells that have to do with blood coagulation or clotting.


When the red blood cells become less than normal this is called anemia. Red blood cells function to carry oxygen to the various body tissues. If there are not enough red blood cells, less oxygen can be carried, and you will feel weak and fatigued easily.

If you are very anemic (very low red blood cell count) then you can get short of breath, especially with exertion. The most common to red blood cell counts are the hemoglobin, and the hematocrit. The hemoglobin molecule is what actually carries the oxygen to all the tissues of the body. The hematocrit is related to the hemoglobin.

White Blood Cell Count

The CBC also checks the white blood cell count in addition to the red blood cell count. The major purpose of white blood cells is to fight infection.

Hence, if the white blood cell count is high, this usually indicates that the body is fighting an infection. Simple infection such as strep throat or even the common cold, can raise the white blood cell count.

White blood cells are broken into their various different components, and these various components fight different types of infections such as bacterial infections or viral infections. These components are usually measured within the complete blood count.

Platelet Count

Another blood count included in the CBC is the platelet count. Platelets are involved in blood clotting. When you get a cut and it forms a scab, this is because the platelets have gone to the area of injury and started to form a tiny blood clot which eventually becomes the scab.

However, if platelets form a clot inside of the body, this is usually not good. Common illnesses such as strokes and heart attacks are usually caused by blood clots inside the body.

First stroke the blood clot is in the blood vessels of the brain.

For a heart attack, the blood clot is in the arteries that feed the heart muscle.

Sometimes larger blood clots can form in the legs or even in the lungs.

If the platelet count is very high, the blood tends to clot work coagulate more readily. If the platelet count is too low, the blood may be to “thinned out” and you will be prone to easy bleeding or bruising.

The Urinalysis

The urinalysis, commonly called the “UA”, tests the urine and is related to kidney function.

The kidneys are like two big filters that filter your blood and produce urine. Hence some of the metabolic waste products, based on nitrogen, or filtered out of the blood by the kidneys and flow into the urine.

The UA can also give your physician an indication of how well hydrated or dehydrated you may be. Have you ever have noticed that your urine tends to be darker if you’ve been out in the heat and not drinking enough fluids, that is to say, dehydrated?

The reason the urine is darker when you are dehydrated is because your kidneys are trying to conserve water and keep that in your bloodstream, rather than losing this water in the urine. Hence the urine becomes more concentrated when you are dehydrated and therefore it looks darker.

It is important to drink plenty of fluids throughout the day to stay well hydrated. All your organs function better when you are hydrated, and don’t function as well when you are dehydrated.

Various infections, such as urinary tract infections, can also be detected in the urine. As a matter of fact, white blood cells will usually increase in the urine if there is a urinary tract infection. Just as in the complete blood count, the white blood cells have gone into the urine/kidneys to try to fight the infection.

Coagulation tests

As discussed above, when you get a small cut in your body forms a scab, this is known as “coagulation”, which is basically the formation of the blood clot. This is a good thing when you have a cut because it generally stops the bleeding.

There are several tests that determine how effectively or ineffectively your blood coagulates. The three most common coagulation-related tests are known as the PT, PTT, and INR.

These tests are all related to each other and give your physician important information related to how your blood coagulates, forms clots. As mentioned above, blood clots inside the body may indicate a higher risk for strokes or heart attacks.


So in summary, the most commonly ordered laboratory tests are the complete metabolic panel (CMP), the complete blood count (CBC), the urinalysis (UA), and coagulation tests.

The CMP tests electrolytes and kidney function. The CBC checks for anemia (low red blood cell counts), and white blood cells( that fight infection), and the UA checks kidney function and hydration status.

This is of course a very basic summary of these common laboratory tests. They all are involved with your metabolism, and hence are good screening tools that can sometimes pick up underlying medical disorders that may not be apparent at first. Thus, they help your doctor make sure that your overall metabolism, particularly kidney and liver metabolism, is “in tune”

There are many other blood tests that your physician can order, such as thyroid tests, heart enzyme tests, various hormone levels, and others. Checking these tests, in addition to the basic four tests mentioned above, gives the physician an overall picture of your health.

In this age of the Covid pandemic, several other tests have captured a lot of attention. (See the Covid update from June 15, 2020 for more information on Covid testing.)

LGBTQ.Clinic can order laboratory tests through our professional physicians group, Guha Medical, utilizing the most current, high-tech techniques by a company called LabCorp, that specializes in laboratory testing.

By S. John Guha, MD – copyright 2020, Guha Medical - LGBTQ.Clinic

Keep in the Loop

Implore user action with this attractive form.

Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.