How to compare blood tests from different laboratories

Before deciding that a result has risen or fallen, confirm the full test name on both documents. Similar abbreviations do not guarantee the same measurement. Serum iron and ferritin are different tests; free T4 and total T4 should also remain separate.

If a name is unclear, retain the original wording and ask the laboratory or clinician. Guessing a match can create a trend between unrelated results.

For each collection date, record the laboratory, test name, result, unit, reference range and any method or specimen comment. Use the sample collection date when available, rather than assuming the date the PDF was downloaded is the test date.

Mark an unreported test as “not reported.” Do not fill the gap using a value from another month. A tidy table is useful only if it remains faithful to the original documents.

A result in mg/dL cannot be directly compared with one in mmol/L. Conversions depend on the substance being measured; moving a decimal point is not a general solution. Keep the original units and have any necessary conversion checked.

Preserve inequalities too. A result of “>90” is not exactly 90, so it cannot support the same calculation as a precise value. Notes about a reporting limit belong beside the result.

Different laboratories may use different methods and ranges. Using the same laboratory can make ongoing comparisons easier, but a change of clinic may be necessary. Record the change instead of treating both reports as identical measurements.

Note preparation instructions, whether they were followed, recent illness and medication changes that your clinician asks about. If you do not remember a condition, write “unknown.” A possible influence is context to discuss, not proof of the cause of a change.

Ask: “Are these two results comparable?”, “Is this change meaningful in my situation?” and “Should the next test use particular preparation or timing?” Your clinician may need several results or other information to interpret the pattern.

Bring the original reports as well as your summary. If an app read a photograph, check dates, decimal points and units before trusting the table. Do not change medication or treatment based only on a line going up or down. The purpose of the record is to support a clinical conversation with accurate information.