🧪 Lab Values · Lesson 3 of 5 · 6 min
Troponin, BNP, Liver Enzymes and Lactate
⏲️ A client with chest pain has a normal troponin an hour after symptoms began. Reassuring? No. Troponin takes hours to rise, which is why the exam asks about the repeat draw, not the first one.
💡 Key idea
Troponin: serial draws, peaks 12 to 24 hours. BNP over 100: heart failure. Ammonia up: encephalopathy, lactulose. Lactate of 4 or more: sepsis bundle. High TSH means hypothyroid.
🧠 Why it matters
TROPONIN is the most specific marker of myocardial injury: it begins rising within a few hours of injury, peaks around 12 to 24 hours and stays elevated for about a week or more, so serial levels are drawn; CK-MB rises and falls faster and helps detect reinfarction. BNP (B-type natriuretic peptide) rises when the ventricles stretch; above about 100 pg/mL supports a diagnosis of heart failure, and higher levels track severity. LIVER enzymes AST and ALT rise with hepatocellular injury (hepatitis, alcohol, acetaminophen toxicity, statins); ALKALINE PHOSPHATASE and GGT rise with biliary obstruction; normal total BILIRUBIN is up to about 1.2 mg/dL, and visible jaundice usually appears above about 2 to 3 mg/dL; AMMONIA rises in liver failure and causes hepatic encephalopathy, treated with lactulose to produce 2 to 3 soft stools a day; a low ALBUMIN and a prolonged PT reflect lost synthetic function. AMYLASE and LIPASE rise in pancreatitis, and lipase is the more specific. LACTATE above 2 mmol/L indicates tissue hypoperfusion; 4 or more in suspected sepsis triggers the fluid bolus. LIPIDS: total cholesterol under 200, LDL under 100, HDL above 40 for men and 50 for women, triglycerides under 150. TSH about 0.4 to 4.0 mU/L: high TSH means hypothyroidism (the pituitary is shouting), low TSH means hyperthyroidism.
🏥 In the real world
🧠 A client with cirrhosis becomes confused and flaps his hands when he holds them out (asterixis). The ammonia is 142. The nurse gives the ordered lactulose and titrates it to two or three soft stools a day, watching for the dehydration and hypokalemia that overtreatment causes. Confusion in a cirrhotic client is ammonia until proven otherwise.
📌 Takeaways
- Troponin rises in hours and stays up for days: repeat it
- Lipase beats amylase for pancreatitis; ammonia explains confusion in liver failure
- TSH and thyroid hormone move in opposite directions
📖 Terms in this lesson
Troponin: The most specific blood marker of heart muscle injury, drawn in series because it rises over hours.
BNP: A hormone released when the ventricles stretch; above about 100 pg/mL supports heart failure.
Lactate: A marker of poor tissue perfusion; above 2 mmol/L is concerning and 4 or more in sepsis triggers the fluid bolus.
TSH: The pituitary hormone that is high in hypothyroidism and low in hyperthyroidism.
Ammonia (NH3): Normal about 15 to 45 mcg/dL; it climbs in liver failure and causes hepatic encephalopathy (confusion, asterixis), treated with lactulose titrated to two or three soft stools a day, and GI bleeding or a high-protein diet raise it.
AST (aspartate aminotransferase): Normal about 10 to 40 U/L; it leaks from injured liver cells in hepatitis, alcohol and acetaminophen toxicity, and from heart and skeletal muscle too, so it is less liver-specific than ALT.
ALT (alanine aminotransferase): Normal about 7 to 40 U/L (upper limits vary by lab); it is the more liver-specific transaminase, rising with hepatitis, fatty liver, statins and acetaminophen overdose, often into the thousands in acute hepatitis.
Alkaline phosphatase (ALP): Normal about 30 to 120 U/L; it rises with biliary obstruction (with GGT and bilirubin), bone growth or disease such as Paget's and bone metastases, and normally in growing children and late pregnancy.
Total bilirubin: Normal about 0.1 to 1.2 mg/dL; jaundice becomes visible above about 2 to 3, and a rising level means hemolysis, liver damage or a blocked bile duct, with clay-colored stools and dark urine in obstruction.
Lipase and amylase: Lipase about 0 to 160 U/L and amylase about 30 to 110 U/L; both rise in acute pancreatitis within hours, and lipase is the more specific and stays up longer, so it is the one the exam trusts.
CK-MB: The heart-specific creatine kinase, normally about 0 to 3 ng/mL or under 5% of total CK; it rises 4 to 6 hours after infarction, peaks at 12 to 24 hours and normalizes in 2 to 3 days, so it flags reinfarction.
Total cholesterol: Desirable under 200 mg/dL, borderline 200 to 239, high 240 or above; it is drawn after a 9 to 12 hour fast with the full lipid panel, and the statin decision rests more on LDL and overall risk.
LDL cholesterol: The artery-clogging cholesterol, optimal under 100 mg/dL (under 70 for clients with known heart disease or diabetes), near optimal 100 to 129, high 160 or above; statins, exercise and less saturated fat lower it.
HDL cholesterol: The protective cholesterol, which should be above 40 mg/dL in men and above 50 in women; below 40 raises heart risk, and exercise, weight loss and quitting smoking raise it.
Triglycerides: Normal under 150 mg/dL, borderline 150 to 199, high 200 to 499 and very high at 500 or above, where pancreatitis becomes a risk; alcohol, sugar and uncontrolled diabetes drive them up.
Lactulose: A sugar laxative that traps ammonia in the gut for hepatic encephalopathy and treats constipation; titrate to two or three soft stools a day, watch for dehydration and low potassium from diarrhea, and judge success by a clearer mind and falling ammonia.
✅ Test yourself
A client arrives 1 hour after the onset of chest pain and the initial troponin is normal. What should the nurse anticipate?
- A single CK-MB as a replacement test
- No further cardiac testing
- Serial troponin levels
- Discharge because an MI is ruled out
Answer: C · Serial troponin levels
Troponin may not be elevated in the first hours; serial levels over the next several hours are needed to rule infarction in or out.
A client with heart failure has a BNP of 950 pg/mL. How should the nurse interpret this value?
- Mild dehydration from the diuretic
- Liver dysfunction from hepatic congestion
- Worsening heart failure
- Normal; no further action is needed
Answer: C · Worsening heart failure
BNP rises in proportion to ventricular wall stress; a level this high reflects significant ventricular stretch and worsening failure.
A client's TSH is 12 mU/L (elevated). Which condition does this suggest?
- Hypothyroidism
- Diabetes insipidus
- Hyperthyroidism
- Cushing's syndrome
Answer: A · Hypothyroidism
A high TSH means the pituitary is stimulating an underactive thyroid: primary hypothyroidism.
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