Metabolic Panel Explained: CMP vs BMP Results

CMP vs BMP: what is actually included

A basic metabolic panel measures sodium, potassium, chloride, bicarbonate, blood urea nitrogen, creatinine, and glucose. A comprehensive metabolic panel adds albumin, total protein, bilirubin, alkaline phosphatase, ALT, and AST. The BMP is often used in urgent care and hospital settings; the CMP is common in primary care because it also screens the liver.

The extra liver and protein values in a CMP matter when symptoms point to the liver, when you take a medication that can affect it, or when a screening test is being ordered. If your report is a BMP, the absence of liver enzymes does not mean they were normal; it means they were not tested. Check which panel was ordered before you interpret a missing value as reassuring.

Some laboratories report calculated values such as the anion gap or eGFR alongside the panel. These are derived from the measured electrolytes and creatinine. They are useful summaries, but they inherit any error in the underlying numbers, so they should not be read in isolation from the rest of the report.

Electrolytes: sodium, potassium, chloride, and bicarbonate

Sodium is the main regulator of fluid balance. A low sodium is common and often caused by medications, heart or kidney disease, or drinking too much water; a high sodium usually reflects dehydration or a problem with water regulation. Symptoms such as confusion, seizures, or severe weakness with a very low sodium are a medical emergency.

Potassium affects the heart's electrical rhythm. Both low and high potassium can cause palpitations, weakness, and dangerous arrhythmias. A mildly abnormal potassium is often repeated because a difficult blood draw, a tight tourniquet, or a delayed sample can raise it artificially. A potassium above 6.0 mmol/L, or any potassium abnormality with symptoms or kidney disease, needs prompt review.

Chloride and bicarbonate usually move with sodium and acid-base status. A low bicarbonate can suggest metabolic acidosis, which has many causes including kidney disease, diarrhoea, and diabetes. A high bicarbonate can suggest vomiting or other causes of metabolic alkalosis. These values are rarely interpreted alone; they are read as a pattern with the rest of the panel.

Kidney markers: BUN, creatinine, and eGFR

Blood urea nitrogen is produced when protein is broken down and is cleared by the kidneys. It rises with dehydration, a high-protein diet, gastrointestinal bleeding, and kidney dysfunction. Creatinine is a muscle breakdown product that is filtered almost entirely by the kidneys, so it is a more specific marker of filtration. BUN and creatinine are read together: a high BUN with a normal creatinine often points to dehydration, while both rising suggests reduced kidney function.

eGFR is calculated from creatinine, age, and sex. It estimates how much blood the kidneys filter per minute. A value above 90 is generally normal, 60-89 is mildly reduced, and values below 60 that persist for three months define chronic kidney disease. eGFR is less reliable in people with very high or very low muscle mass, in pregnancy, and during acute illness.

A single low eGFR is not a diagnosis. It should be repeated, checked with a urine albumin-to-creatinine ratio, and interpreted alongside blood pressure, diabetes, and medications. The kidney function guide explains CKD stages in more detail, and the eGFR explainer covers the calculation and its limits.

Glucose and liver enzymes

A fasting glucose between 70 and 99 mg/dL is normal; 100-125 mg/dL is impaired fasting glucose; 126 mg/dL or higher on two occasions suggests diabetes. A random glucose can be normal in diabetes and high in acute stress, so the context matters. HbA1c gives a three-month average and is usually needed to confirm a diagnosis; the HbA1c guide explains the cut-offs.

ALT and AST are liver enzymes. ALT is more specific to the liver; AST is also found in muscle and red cells. Mild elevations are common and can follow exercise, alcohol, fatty liver, medications, or a viral illness. The pattern matters: a very high ALT with a modest AST suggests viral or drug-induced liver injury; an AST higher than ALT can suggest alcohol-related liver disease or muscle injury.

Alkaline phosphatase and bilirubin add context. A raised alkaline phosphatase with a raised bilirubin suggests bile duct obstruction; an isolated alkaline phosphatase rise can come from bone. Albumin reflects nutrition and liver synthetic function. None of these values diagnoses liver disease on its own, but the combination tells your doctor whether to repeat, image, or refer.

When a metabolic panel needs follow-up

Mild abnormalities in an otherwise well person are often repeated. The values that need prompt attention are a very low or very high sodium, a potassium above 6.0 or below 3.0 mmol/L, a bicarbonate below 18 mmol/L, a creatinine that has risen sharply, or any abnormality with confusion, palpitations, severe weakness, or reduced urine output.

For chronic conditions, the trend matters more than the single value. A creatinine that has been stable for years is different from one that has risen 30% in three months. A potassium that drifts upward after a new blood pressure medication is a reason to review the medication, not just the number. Bring your previous reports to the appointment so the trend is visible.

An AI interpreter can translate the panel and flag values outside your report's ranges, but it cannot know why a value changed. Use it to prepare questions, then let a clinician connect the numbers to your history and medications. The blood count and lab numbers guide is a useful companion.

How to prepare and what to bring

Fasting is the main preparation issue. Glucose and lipids need 8-12 hours without calories; water and usual medications are fine unless your doctor says otherwise. Electrolytes and kidney markers do not require fasting, but if they are part of a combined panel, follow the fasting instruction for the whole test.

Bring a list of medications, especially diuretics, ACE inhibitors, ARBs, potassium supplements, and NSAIDs, because they change electrolytes and kidney markers. Tell the phlebotomist if you have been vomiting, had diarrhoea, or changed your fluid intake, since dehydration can raise BUN and creatinine without kidney disease.

For repeat testing, compare the new panel with the old one. A creatinine rise of 0.3 mg/dL or more, a potassium change of 0.5 mmol/L, or a bicarbonate fall of 4 mmol/L is more meaningful than a single value just outside the range. The thyroid panel guide covers the next common panel.

A single panel is a snapshot, not a movie. Kidney function can fluctuate with hydration, a high-protein meal, or a hard workout, and liver enzymes can rise after exercise. If a value is mildly abnormal, repeating it under similar conditions is often more informative than acting on the first result. Ask your clinician whether the test should be repeated fasting, after hydration, or at a different time of day.