Lab Test Comparisons

Last updated: September 14, 2026

Many lab tests come in easily confused pairs — ALT and AST, LDL and HDL, TSH and T4. Each comparison below explains what the two tests measure, how they differ, and why doctors often order them together.

Why Paired Tests Are Read Together

Related tests often measure two sides of the same process: iron in storage versus iron in circulation, or a hormone versus the signal that controls it. On their own, either number can be ambiguous. Together, the pattern narrows down what may be going on.

Each guide includes a side-by-side table, normal ranges for both tests, and answers to the most common questions — including whether one can be abnormal while the other stays normal.

All Comparisons

LDL vs HDL Cholesterol

Both are cholesterol carriers, but LDL delivers cholesterol to arteries while HDL removes it — why one is called “bad” and the other “good”.

A1C vs Fasting Glucose

A snapshot versus a three-month average — how the two main blood sugar tests differ and when doctors use each.

ALT vs AST

Two liver enzymes that rise for different reasons — and why the ratio between them matters.

BUN vs Creatinine

Two kidney markers that respond differently to hydration, diet, and kidney function — and what the BUN-to-creatinine ratio shows.

TSH vs T4

The signal hormone versus the thyroid hormone itself — why TSH usually moves first and what the combination reveals.

Ferritin vs Iron

Stored iron versus circulating iron — why ferritin can be low while serum iron still looks normal.

CRP vs ESR

Two inflammation markers on different clocks — CRP rises and falls quickly, ESR changes slowly.

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