Lab Tests Reference Library

ALT is an enzyme found predominantly in liver cells (hepatocytes). When liver cells are damaged or inflamed, ALT leaks into the bloodstream, causing elevated serum levels. ALT is the most liver-specific enzyme test and is the primary marker of hepatocellular injury.

AST is an enzyme found in liver, heart muscle, skeletal muscle, kidneys, brain, and red blood cells. Elevated AST indicates tissue damage, most often in the liver or heart. AST is less liver-specific than ALT but is valuable when interpreted alongside ALT and other tests.

ALP is an enzyme found in the liver (bile ducts), bone, intestine, kidneys, and placenta. Elevated ALP indicates either liver cholestasis (bile duct obstruction) or increased bone turnover. Distinguishing between these sources requires GGT (elevated in liver disease but not bone disease) and other tests.

GGT is an enzyme found primarily in the liver, pancreas, and kidneys. It is the most sensitive marker of liver disease and alcohol consumption. GGT is elevated in virtually all liver disorders and is used to determine whether elevated ALP is of liver or bone origin.

Bilirubin is the breakdown product of heme from red blood cells. Total bilirubin consists of unconjugated (indirect) bilirubin and conjugated (direct) bilirubin. Elevated bilirubin causes jaundice (yellowing of skin and eyes). The type of bilirubin elevated helps identify the cause.

Albumin is the most abundant protein in blood plasma, produced exclusively by the liver. It maintains oncotic pressure (keeping fluid in blood vessels), transports hormones, fatty acids, and drugs, and acts as a buffer. Albumin is a marker of liver synthetic function and nutritional status.

Creatinine is a waste product from normal muscle metabolism, produced at a steady rate and excreted almost entirely by the kidneys. Because its production is relatively constant, creatinine is an excellent marker of kidney filtration function. It is always interpreted alongside eGFR and compared to the patient's baseline.

BUN measures the amount of nitrogen in urea, a waste product formed in the liver from protein metabolism. It is excreted by the kidneys. BUN is used alongside creatinine to assess kidney function and to distinguish kidney causes of azotemia (elevated waste products) from non-kidney causes.

eGFR estimates the volume of blood filtered by the kidneys per minute, calculated from serum creatinine, age, sex, and race. It is the best single measure of overall kidney function and is used to stage chronic kidney disease (CKD) from G1 (>90 mL/min, normal) to G5 (<15 mL/min, kidney failure).

Uric acid is the final breakdown product of purine metabolism. Purines are found in nucleic acids (DNA, RNA) and are abundant in red meat, organ meats, shellfish, and beer. Elevated uric acid (hyperuricemia) can crystallize in joints causing gout and in the kidneys causing uric acid stones.

Sodium is the primary extracellular cation and the main determinant of blood osmolality (tonicity). It regulates fluid balance, blood pressure, and nerve and muscle function. Sodium is tightly regulated by the kidneys under the influence of aldosterone, ADH (vasopressin), and ANP.

Potassium is the primary intracellular cation and is essential for maintaining the resting membrane potential of cells, particularly in cardiac muscle and nerves. Even small deviations from the normal range can cause life-threatening cardiac arrhythmias. Potassium is tightly regulated by the kidneys and aldosterone.

Chloride is the major extracellular anion and works with sodium to maintain osmotic balance and fluid distribution. It plays a key role in acid-base balance and is exchanged for bicarbonate in red blood cells. Chloride abnormalities are rarely isolated and usually accompany sodium and acid-base disorders.

Fasting blood glucose (FBG) measures blood sugar after at least 8 hours without food or caloric drink. It is the primary test for diagnosing diabetes mellitus, prediabetes, and hypoglycemia. Together with HbA1c, it forms the cornerstone of diabetes screening and management.

HbA1c (glycated hemoglobin) measures the percentage of hemoglobin that has glucose permanently attached to it, reflecting average blood glucose over the past 2–3 months. It is the gold standard test for long-term glycemic control in diabetes management and is used for both diagnosis and treatment monitoring.

Total cholesterol is the sum of all cholesterol in the blood including LDL, HDL, VLDL, and IDL fractions. While it provides a general cardiovascular risk overview, total cholesterol alone is an insufficient cardiovascular risk marker — it does not distinguish between protective HDL and atherogenic LDL cholesterol.

LDL (low-density lipoprotein) cholesterol is the primary carrier of cholesterol in the blood and the main driver of atherosclerotic plaque formation. LDL particles infiltrate arterial walls, become oxidized, and trigger inflammation that leads to narrowing and hardening of arteries (atherosclerosis), increasing the risk of heart attack and stroke.

HDL (high-density lipoprotein) cholesterol transports excess cholesterol from peripheral tissues back to the liver for excretion (reverse cholesterol transport). It is called "good cholesterol" because higher levels are associated with lower cardiovascular risk. HDL also has anti-inflammatory and antioxidant properties.

Triglycerides are the main form of fat stored in the body and are a major source of energy. They are carried in the blood by VLDL and chylomicrons. Elevated triglycerides (hypertriglyceridemia) are associated with pancreatitis risk, insulin resistance, metabolic syndrome, and cardiovascular disease when combined with low HDL.

Troponin (I or T) is a cardiac muscle protein released into the blood when heart muscle cells are damaged or die. High-sensitivity troponin (hs-cTn) assays can detect very small amounts, enabling earlier diagnosis of acute myocardial infarction (MI/heart attack). It is the most specific and sensitive biomarker for myocardial injury.

CK-MB is an isoenzyme of creatine kinase found predominantly in cardiac muscle. It was the standard cardiac biomarker before troponin became widely available. CK-MB rises 4–6 hours after MI, peaks at 12–24 hours, and normalizes within 48–72 hours — faster than troponin. Its shorter window of positivity is used to diagnose re-infarction.