Skip to content

CBC vs CMP: the two most common blood tests, explained

· 5 min read · Vita Health Coach

The short answer

A CBC (complete blood count) counts and measures the cells in the blood: red cells, white cells and platelets. A CMP (comprehensive metabolic panel) measures substances in the liquid part of the blood, including blood sugar, minerals, and markers of kidney and liver health. Doctors often order both together because they answer different questions.

The short difference

Think of the CBC as a look at the blood cells and the CMP as a look at the body’s chemistry. The CBC tells the doctor about oxygen-carrying red cells, infection-fighting white cells and clot-forming platelets. The CMP tells the doctor about blood sugar, salts and minerals, and how the kidneys and liver are doing.

Both are common parts of a yearly checkup. They are also ordered when someone feels unwell, before a procedure, or to keep an eye on a known condition or a medicine.

Both tests usually come from the same blood draw, but the lab handles the samples differently. The CBC is done on whole blood, with the cells still in it. The CMP is done on the liquid part of the blood, called serum or plasma, after the cells are separated out.

What a CBC measures

A CBC reports several counts and measurements. Some reports add a differential, which breaks the white blood cell count into its types.

Doctors use the CBC to look for anemia, which means too few red cells or too little hemoglobin, for signs of infection or inflammation, and for problems with clotting. It is also used to check on some long-term conditions and some treatments that can affect the blood.

  • White blood cells (WBC): cells that help fight infection.
  • Red blood cells (RBC): cells that carry oxygen around the body.
  • Hemoglobin: the protein in red cells that carries oxygen.
  • Hematocrit: the share of the blood made up of red cells.
  • MCV: the average size of the red cells.
  • MCH and MCHC: how much hemoglobin is in each red cell, and how concentrated it is.
  • RDW: how much the red cells vary in size.
  • Platelets: small cell pieces that help blood clot.
  • Differential: the types of white cells, such as neutrophils and lymphocytes.

What a CMP measures

A CMP is a group of 14 tests. They fall into a few families, and the doctor usually reads each family together.

The CMP gives a broad view of how several organs are working. It is used in checkups, to look into symptoms such as tiredness or swelling, and to keep an eye on medicines that can affect the kidneys or liver.

  • Glucose: the amount of sugar in the blood.
  • Calcium: a mineral needed for bones, nerves, muscles and the heart.
  • Sodium, potassium, chloride and bicarbonate (sometimes listed as CO2): electrolytes, which are minerals that help control fluid balance and the body’s acid level.
  • BUN (blood urea nitrogen) and creatinine: waste products the kidneys clear from the blood.
  • Albumin and total protein: proteins in the blood. Albumin is made by the liver.
  • ALT, AST and ALP: enzymes linked mainly to the liver. ALP also comes from bone.
  • Bilirubin: a yellow substance made when old red cells break down, cleared by the liver.

CMP vs BMP

You may also see a BMP, or basic metabolic panel. It contains 8 of the 14 CMP tests: glucose, calcium, sodium, potassium, chloride, bicarbonate, BUN and creatinine. It leaves out the liver tests and the proteins.

A BMP is common in hospitals and emergency rooms, where doctors need a fast look at fluids, salts, blood sugar and the kidneys.

Many reports also show an eGFR next to these tests. The eGFR is an estimate of how well the kidneys filter blood. It is calculated from creatinine, age and sex rather than measured directly.

How to prepare for each test

A CBC usually does not need any preparation. You can normally eat and drink as usual.

A CMP or BMP sometimes needs fasting, because food affects glucose and can affect some other values. Fasting usually means nothing to eat or drink except water for a set number of hours. The doctor or lab will tell you if fasting is needed and for how long. Follow those instructions, and ask before skipping any medicine.

Both tests use a simple blood draw from a vein in the arm. The results often come back within a day or two.

Tell the lab or the doctor about any medicines and supplements the person takes, because some can affect certain results.

Reading CBC and CMP results

Each line on the report has a result and a reference range, the span the lab expects in most healthy people. Use the range printed on your report, because ranges differ between labs.

With so many tests in two panels, it is common to see one or two small flags in a healthy person. The doctor looks for patterns. For example, a low hemoglobin with a low MCV points in a different direction than a low hemoglobin with a high MCV. A raised creatinine with a lower eGFR is read together as a kidney question.

If the doctor wants a closer look, they may order follow-up tests. For example, iron or vitamin B12 tests may follow a CBC that shows anemia, and further liver or kidney tests may follow a CMP. These are common next steps and do not mean something serious has been found.

Compare with earlier results whenever you can. A change over time often tells more than a single number. Talk with the doctor about any result that concerns you, and call 911 for urgent symptoms such as chest pain, trouble breathing or signs of a stroke.

How Vita Health Coach helps

Vita Health Coach can read a CBC and a CMP from a photo of the report or from a connected US hospital, and saves them in that person’s own file. Every new result is explained in plain words and compared with the last one, starting with the short answer: can this wait, or is it time to see a doctor. Vita never diagnoses or prescribes. To look up a single test first, the free blood test guide at vitamedas.com/blood-test explains more than 100 of them.

Questions people ask

Is a CBC the same as a CMP?
No. A CBC looks at blood cells, while a CMP looks at blood sugar, electrolytes, and kidney and liver markers. They are often ordered together.
Do I need to fast for a CBC?
Usually not. A CMP or BMP sometimes needs fasting. Follow the instructions from the doctor or lab.
What is the difference between a CMP and a BMP?
A BMP has 8 tests: glucose, calcium, four electrolytes, BUN and creatinine. A CMP has those 8 plus albumin, total protein and four liver-related tests.
Does a CBC show cancer?
A CBC cannot diagnose cancer on its own. Some results can lead the doctor to order more tests. Ask the doctor what a specific result means for the person.

Related on vitamedas.com

Sources

Vita Health Coach is an iPhone app that keeps a parent’s hospital results in their own file and explains each new one in plain words. Free to try; paid plans in the App Store.

Vita Health Coach on the App Store

General health information, not medical advice. Vita Health Coach explains; it never diagnoses. In an emergency, call 911 or your local emergency number. Read the medical disclaimer.