Clinical Chemistry Flashcards

Using the Friedewald equation, what is the LDL cholesterol if total cholesterol is 240, HDL 40, and triglycerides 150 mg/dL?

Answer: 170 mg/dL

LDL = 240 − 40 − (150/5) = 170 mg/dL.

Above what triglyceride level is the Friedewald LDL calculation considered invalid?

Answer: 400 mg/dL

Above 400 mg/dL, TG/5 no longer gives a reliable VLDL estimate, so LDL should be measured directly.

Low TSH with low free T4 is most consistent with:

Answer: Secondary (pituitary) hypothyroidism

An inappropriately low TSH with low T4 means the pituitary is not stimulating the thyroid.

The overnight dexamethasone suppression test is mainly used to screen for:

Answer: Cushing syndrome

Normal people suppress morning cortisol after dexamethasone, but autonomous hypercortisolism does not suppress.

When should the trough level be drawn for therapeutic drug monitoring?

Answer: Immediately before the next dose

The trough is the lowest concentration, which occurs just before the next dose.

For an accurate serum digoxin level, blood should be drawn at least how long after an oral dose?

Answer: 6–8 hours

Digoxin needs about 6–8 hours to distribute into tissue, and earlier samples give falsely high results.

A patient with hemolytic anemia has a hemoglobin A1c that seems too low for their glucose levels. The reason is:

Answer: Shortened red cell lifespan reduces glycation time

Younger red cells have less time to be glycated, so A1c falsely underestimates average glycemia.