Guide · 7 min read
CBC vs CMP vs BMP
Three abbreviations account for most of the blood drawn in a hospital: CBC, BMP and CMP. They are ordered so routinely that nobody explains them, and the acronyms are unhelpfully similar - two of them differ by a single letter and share most of their contents.
This guide sets out exactly what each panel contains, what question each one is built to answer, and how they relate to each other. Composition is standard across laboratories; the reference intervals attached to each analyte are not, so treat every number you meet as belonging to the laboratory that produced it.
The short version
The CBC counts and characterises blood cells. The BMP and CMP measure dissolved chemistry in serum or plasma. The CMP is the BMP plus liver analytes - that single fact resolves most of the confusion between the two.
They are frequently ordered together because they answer non-overlapping questions. A CBC will not tell you the kidneys are failing and a BMP will not tell you the patient is anaemic.
| Panel | Typical analytes | What it is asking |
|---|---|---|
| CBC | 8-10 (plus 5 more with a differential) | How many cells of each kind are there, and what do the red cells look like? |
| BMP | 8 | Electrolytes, acid-base, kidney function and glucose |
| CMP | 14 | Everything in the BMP plus liver enzymes, bilirubin and proteins |
The BMP: eight analytes, three questions
The basic metabolic panel is a set of eight: sodium, potassium, chloride, bicarbonate (often printed as CO₂ or total CO₂ content), blood urea nitrogen, creatinine, glucose and total calcium. Some systems label it Chem-7 (the same list without calcium) or SMA-7.
Those eight cluster into three questions. The first four describe fluid, electrolyte and acid-base status - and because they are reported together, they let you calculate the anion gap, which is derived arithmetic rather than a separate measurement. BUN and creatinine describe how the kidneys are filtering, usually alongside a calculated eGFR. Glucose stands on its own, and calcium rides along because it is cheap to add and clinically noisy when it moves.
The BMP is the panel most likely to be repeated daily on an inpatient, which makes it the one where the trend matters most. A creatinine that has doubled inside its reference interval is a more interesting finding than one that has sat flagged and unchanged for a year.
- Sodium, potassium, chloride, bicarbonate - fluid, electrolytes and acid-base.
- BUN and creatinine - renal filtration, usually with a reported eGFR.
- Glucose - carbohydrate metabolism at the moment of the draw.
- Total calcium - reported as total, so it moves with albumin as well as with calcium itself.
The CMP: the BMP plus the liver
The comprehensive metabolic panel is the same eight analytes with six more added: albumin, total protein, alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase and total bilirubin. Fourteen in all.
Those six are the standard liver screen. Two are transaminases that leak from injured hepatocytes; alkaline phosphatase and bilirubin lean toward the biliary side; albumin and total protein describe synthetic function and, indirectly, nutrition and fluid status. Read as a group, their pattern is more informative than any one of them - which is why they are ordered as a set rather than individually.
Albumin earns its place twice over. It appears in the liver group, and it is also the reason total calcium has to be read carefully: roughly half of circulating calcium is protein-bound, so a low albumin drags total calcium down without the physiologically active fraction having moved at all. Where that distinction matters, ionised calcium is measured directly rather than inferred.
Choosing between the panels is mostly a question of whether the liver is part of the question. Daily electrolyte monitoring does not need six extra analytes; a new presentation with abdominal pain, jaundice or a medication list full of hepatically cleared drugs does.
The CBC: cells rather than chemistry
The complete blood count is a different kind of test. Rather than measuring dissolved substances, it counts cells and describes their physical characteristics. The core report gives white blood cell count, red blood cell count, haemoglobin, haematocrit and platelet count, plus the red cell indices - mean corpuscular volume, mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration and red cell distribution width.
The indices are what make the CBC more than a set of counts. MCV describes average red cell size and is the first branch point in classifying anaemia into microcytic, normocytic and macrocytic groups. RDW describes how much cell size varies within the sample, which distinguishes a uniform population from a mixed one. Neither requires a separate specimen; the analyzer derives them from the same measurement.
"With differential" adds the white cell breakdown - neutrophils, lymphocytes, monocytes, eosinophils and basophils - reported as percentages, as absolute counts, or both. The absolute neutrophil count is the number that governs neutropenic precautions, and it is a calculation from the total white count and the neutrophil percentage rather than a directly measured value.
- Counts - white cells, red cells, platelets.
- Red cell mass - haemoglobin and haematocrit, which move together and are often confused for each other.
- Indices - MCV, MCH, MCHC and RDW, which describe the red cells rather than count them.
- Differential - the five white cell lines, as percentages and as absolute counts.
Why they are ordered together, and what they leave out
A CBC and a CMP together cover cell lines, electrolytes, renal function, glucose and the liver in two tubes. That breadth is why the pair is a common admission order, and it is also why so many results come back flagged: the more analytes reported, the higher the chance that at least one falls outside its interval by ordinary statistical variation.
What the pair does not cover is worth knowing. Thyroid function, coagulation, lipids, cardiac markers, iron studies and inflammatory markers are all separate orders. Neither panel says anything about clotting, and neither includes a lipid profile - a fact that surprises patients who assume a "full blood test" checked their cholesterol.
Panel composition also drifts between institutions. Some laboratories add magnesium or phosphate to a routine chemistry, some report an anion gap automatically and some do not, some fold eGFR into the BMP as a matter of course. The names are conventional, not contractual: read the analyte list on the report rather than assuming it from the acronym.
Frequently asked questions
What is the difference between a BMP and a CMP?
The CMP is the BMP plus six liver analytes: albumin, total protein, alkaline phosphatase, ALT, AST and total bilirubin. The BMP has eight analytes (sodium, potassium, chloride, bicarbonate, BUN, creatinine, glucose, calcium) and the CMP has fourteen. Everything in a BMP is in a CMP.
Does a CBC include electrolytes?
No. A CBC counts and characterises blood cells - white cells, red cells, platelets and the red cell indices. Sodium, potassium, chloride and bicarbonate are chemistry analytes and come from a BMP or CMP, which is why the two panels are so often ordered together.
What does 'CBC with differential' add?
It breaks the white blood cell count into its five lines - neutrophils, lymphocytes, monocytes, eosinophils and basophils - reported as percentages, absolute counts, or both. The absolute neutrophil count is derived from the total white count and the neutrophil percentage rather than measured separately.
Is cholesterol included in a CMP?
No. Lipids are a separate order. A comprehensive metabolic panel covers electrolytes, kidney function, glucose and the liver; total cholesterol, LDL, HDL and triglycerides come from a lipid panel, which usually has its own collection conditions.
Why is calcium on a metabolic panel read alongside albumin?
Because the reported value is total calcium, and roughly half of circulating calcium is bound to protein - mostly albumin. A low albumin lowers total calcium without changing the physiologically active ionised fraction. Where the distinction matters, ionised calcium is measured directly.