Lab Tests You Must Know: CBC, CMP, LFT, and What They Mean?

Every medical student remembers the first time they looked at a patient’s lab report and felt completely lost in the numbers. Understanding what those values mean — and how to connect them to real conditions — is one of the first steps toward thinking like a doctor.

Blood tests are ordered every day, and they tell us far more than just numbers. They can point to anemia, infection, dehydration, kidney failure, or liver disease — sometimes even before symptoms appear. Among the many lab tests you’ll encounter, three are considered basic tools in clinical medicine: the Complete Blood Count (CBC), the Comprehensive Metabolic Panel (CMP), and the Liver Function Tests (LFTs).

Complete Blood Count (CBC)

The CBC gives a picture of what’s happening inside the blood — the red cells that carry oxygen, the white cells that fight infection, and the platelets that help with clotting.

• RBC count and Hemoglobin (Hb): These reflect how well the body transports oxygen. Low values mean anemia; high ones may mean polycythemia or excess erythropoietin.

• Hematocrit (Hct): The percentage of red blood cells in the blood. It falls with anemia and rises with dehydration.

• MCV (Mean Corpuscular Volume): Tells you the size of red cells. Small cells suggest iron deficiency (microcytic anemia); large cells point to B12 or folate deficiency (macrocytic anemia).

• MCH and MCHC: Indicate how much hemoglobin each red cell carries and its concentration. High MCHC is seen in hereditary spherocytosis.

• WBC count and differential: High counts mean infection or inflammation, while low counts can mean bone marrow suppression or viral infections. When the absolute neutrophil count (ANC) falls below 1500, the patient is at significant risk for infection.

• Platelet count: Low platelets increase bleeding risk; high platelets can indicate inflammation or marrow disorders.

The CBC is often the first test ordered because it’s quick, inexpensive, and gives clues about several systems at once.

Comprehensive Metabolic Panel (CMP)

The CMP looks beyond the blood cells to assess organ function and the body’s internal chemistry. It includes electrolytes, kidney markers, glucose, and proteins — helping doctors evaluate overall metabolic balance.

• Electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻): Sodium imbalances can cause confusion or seizures, while potassium changes affect the heart and muscles.

• Glucose: Essential for detecting diabetes and metabolic stress.

• BUN and Creatinine: Indicators of kidney function. High levels suggest reduced filtration or dehydration. Creatinine is particularly important because it helps estimate the glomerular filtration rate (GFR).

• Calcium: Plays roles in bone strength, muscle contraction, and nerve signaling.

• Total Protein and Albumin: Reflect nutrition and liver function. Low values may point to liver disease, kidney loss, or poor intake.

For students, the CMP is a good reminder that the body functions as one system — changes in one organ can influence many others.

Liver Function Tests (LFTs)

The liver has hundreds of roles, from detoxifying drugs to producing proteins. LFTs help detect damage to liver cells or blockage in bile flow.

• ALT (Alanine Aminotransferase): Found mostly in the liver — rises when liver cells are damaged.

• AST (Aspartate Aminotransferase): Found in the liver but also in muscles and the heart. Both AST and ALT rise in liver injury.

• AST/ALT ratio: Helps determine the cause. Ratios below 1 suggest viral or fatty liver disease, while ratios above 1.5 often point to alcoholic hepatitis.

• ALP (Alkaline Phosphatase): Rises with bile duct obstruction or bone disease.

• GGT (Gamma-Glutamyl Transferase): A sensitive marker of biliary injury that helps confirm if a high ALP is liver-related.

• Bilirubin: The pigment produced from red cell breakdown. Unconjugated bilirubin increases in hemolysis, while conjugated bilirubin rises with obstruction or cholestasis.

These enzymes and pigments tell a story — whether the problem lies in the liver cells themselves or in the bile flow that leaves the liver.

Putting it all together

The CBC, CMP, and LFT are part of almost every patient’s workup. When you understand what each value represents, you start recognizing patterns instead of isolated numbers. That’s how clinical reasoning begins, by connecting physiology to pathology.

At Brocali, we encourage students to build this understanding early. You’ll see these same tests interpreted step-by-step in our medical courses and exam preparation programs, where theory always meets clinical practice.

Learning to read labs isn’t about memorizing reference ranges — it’s about seeing what the body is trying to tell you.