Standard lab work screens for a wide range of health markers, from blood cell counts and organ function to blood sugar, cholesterol, and signs of infection or inflammation. Most routine panels involve a blood draw and sometimes a urine sample, and together they give your doctor a snapshot of how well your major organs and systems are working. Here’s what each common test actually measures and what the results can tell you.
Complete Blood Count (CBC)
A CBC is one of the most frequently ordered blood tests. It measures the number and size of the three main types of cells in your blood: red blood cells, white blood cells, and platelets. It also checks hemoglobin (the protein in red blood cells that carries oxygen), hematocrit (the percentage of your blood made up of red blood cells), and the average size of your red blood cells.
Each of these values tells a different story. Low red blood cells, hemoglobin, or hematocrit can point to anemia, iron deficiency, dehydration, or heart disease. White blood cell counts reflect your immune system: a high count often signals an infection or a reaction to medication, while a low count may suggest an autoimmune disorder, a bone marrow problem, or cancer. Platelet levels indicate how well your blood can clot. A version called a “CBC with differential” goes a step further, breaking your white blood cells into five subtypes so your doctor can narrow down what’s triggering an abnormal count.
Comprehensive Metabolic Panel (CMP)
A CMP measures 14 substances in your blood and is essentially a checkup for your liver, kidneys, and metabolism all at once. The panel includes:
- Glucose (blood sugar): your body’s primary energy source, and the first flag for diabetes or prediabetes.
- Calcium: essential for nerve signaling, muscle contraction, and heart rhythm.
- Electrolytes (sodium, potassium, bicarbonate, chloride): electrically charged minerals that regulate fluid balance and the acid-base balance in your body.
- Albumin and total protein: proteins made by the liver. Low levels can indicate liver disease, kidney problems, or malnutrition.
- Liver enzymes (ALP, ALT, AST): enzymes that rise when liver cells are damaged or inflamed.
- Bilirubin: a waste product from the breakdown of old red blood cells. High levels can turn skin and eyes yellow (jaundice) and suggest liver or bile duct issues.
- BUN and creatinine: waste products filtered by the kidneys. When levels climb, it usually means the kidneys aren’t filtering as efficiently as they should.
Some of these markers overlap with more targeted panels. If your CMP flags something unusual in your liver or kidneys, your doctor may order a dedicated panel for that organ.
Liver Function Tests
A dedicated liver panel measures several enzymes and proteins to evaluate how well the liver is working. ALT is the most liver-specific enzyme; when liver cells are damaged, ALT leaks into the bloodstream and levels rise. AST also rises with liver damage but can increase from muscle injury too, so it’s less specific on its own. ALP is found in both the liver and bones, so elevated ALP could point to a blocked bile duct or certain bone conditions. GGT is another enzyme that rises with liver or bile duct damage, and it’s sometimes used to help clarify whether a high ALP is coming from the liver or from bone.
Taken together, the pattern of which enzymes are elevated, and by how much, helps distinguish between different types of liver problems: inflammation, fatty liver disease, bile duct obstruction, or damage from alcohol or medications.
Kidney Function and eGFR
Creatinine is the key marker for kidney health. Your muscles produce creatinine as a normal waste product, and healthy kidneys filter it out of your blood. When kidney function declines, creatinine builds up. Your lab results will use your creatinine level (along with your age, sex, and body size) to calculate an estimated glomerular filtration rate, or eGFR, which represents the percentage of kidney function you have.
Healthy kidneys produce an eGFR of 90 or higher. The five stages of chronic kidney disease are defined by this number: stage 1 is 90 and above (mild damage but normal function), stage 2 is 60 to 89, stage 3 is 30 to 59, stage 4 is 15 to 29, and stage 5 (kidney failure) is below 15. BUN, another waste product, is often measured alongside creatinine. It can rise from dehydration or a high-protein diet, not just kidney problems, so it’s most useful when interpreted together with creatinine and eGFR.
Lipid Panel (Cholesterol)
A lipid panel measures the fats in your blood that affect cardiovascular risk. It typically includes total cholesterol, LDL (“bad” cholesterol), HDL (“good” cholesterol), and triglycerides. LDL is the number that gets the most clinical attention because it directly contributes to plaque buildup in arteries.
Target LDL levels depend on your overall cardiovascular risk. For someone at low risk, keeping LDL below 130 mg/dL is generally considered acceptable. For people at high risk due to existing heart disease or multiple risk factors, the target drops to below 70 mg/dL or even below 55 mg/dL. HDL works in the opposite direction: higher levels are protective. Triglycerides reflect how much fat from recent meals is circulating in your blood, and very high levels independently raise heart disease risk.
Blood Sugar and Diabetes Screening
Lab work screens for diabetes in two main ways: a fasting blood glucose test and an HbA1c test. Fasting glucose gives a snapshot of your blood sugar at a single point in time. HbA1c is more revealing because it reflects your average blood sugar over the past two to three months by measuring how much sugar has attached to your red blood cells.
The HbA1c thresholds are straightforward. Below 5.7% is normal. Between 5.7% and 6.4% falls in the prediabetes range, meaning blood sugar is elevated but not yet at diabetic levels. An HbA1c of 6.5% or higher on two separate tests confirms a diabetes diagnosis. Prediabetes is a particularly important finding because lifestyle changes at this stage, like increased physical activity and dietary adjustments, can often prevent progression to full diabetes.
Thyroid Panel
A thyroid panel typically measures TSH (thyroid-stimulating hormone) along with free T4, and sometimes free T3. TSH is produced by the brain to tell the thyroid gland how much hormone to make, so it acts as an early warning system. When the thyroid underperforms, the brain cranks up TSH to compensate. When the thyroid overproduces, TSH drops.
The classic pattern for an underactive thyroid (hypothyroidism) is high TSH paired with low T4 and T3. The most common cause is autoimmune thyroiditis, where the immune system attacks the thyroid gland. The reverse pattern, low or undetectable TSH with high T4 and T3, indicates an overactive thyroid (hyperthyroidism). Symptoms of the two conditions are nearly opposite: hypothyroidism causes fatigue, weight gain, and cold sensitivity, while hyperthyroidism causes weight loss, rapid heartbeat, and heat intolerance.
Inflammation Markers
C-reactive protein (CRP) is a protein your liver produces in response to inflammation anywhere in your body. It rises quickly during bacterial or viral infections, autoimmune conditions like rheumatoid arthritis and lupus, inflammatory bowel disease, and lung conditions like asthma. CRP can also be chronically elevated from obesity, smoking, exposure to environmental toxins, depression, and even insomnia. Because so many things can raise CRP, an elevated result tells your doctor that something is causing inflammation but not exactly what. It’s typically used alongside other tests and symptoms to narrow down the cause.
A high-sensitivity version of the CRP test (hs-CRP) measures very small elevations and is used specifically to assess cardiovascular risk. The erythrocyte sedimentation rate (ESR) is another inflammation marker that’s often ordered alongside CRP. While less specific, it’s useful for tracking inflammatory conditions over time.
Urinalysis
A urinalysis examines the appearance, chemical content, and microscopic makeup of your urine. It can detect problems that blood tests might miss, particularly in the urinary tract and kidneys. The chemical portion uses a dipstick to check for acidity, concentration, protein, sugar, ketones, bilirubin, blood, and signs of infection like nitrites or white blood cell byproducts.
Sugar or ketones in urine can be an early sign of diabetes. Protein in urine, beyond trace amounts, may indicate kidney damage. Nitrites or elevated white blood cells point to a urinary tract infection. If the dipstick flags anything unusual, a microscopic exam follows, looking for red blood cells (which could mean kidney disease or bladder problems), bacteria or yeast (infection), tube-shaped protein casts (kidney disorders), and crystals that may signal kidney stones.
Preparing for Lab Work
Some tests require fasting, typically for 8 to 12 hours before the blood draw. The most common fasting tests are blood glucose, a lipid panel, and a basic metabolic panel. Liver function and kidney function panels may also require fasting in some cases. Water is fine during the fasting period, but food, coffee, and other beverages can skew results. Your doctor’s office will specify which tests need fasting when they order the lab work. If you forget and eat beforehand, mention it at the lab so the results can be interpreted correctly rather than flagged as falsely abnormal.