tidepool Report: reference ranges Page 1 of 1

One in twenty

The common blood panels, and what the range printed next to each result means. With a printer that runs panels on made-up healthy people and counts how often they get flagged.

General information about how lab reports work. Not advice about anyone's results: a report's own range, and whoever ordered the test, are the ones to go by.

01What is in a panel

A routine blood draw usually runs one or more panels: bundles of tests ordered together because they look at the same part of the body. Tick the ones to put on the requisition. The rest of the page uses the tick marks.

Panel contents follow MedlinePlus's pages; labs vary a little in what they include. A CBC is sometimes run with a "differential" that splits white cells into five types, left out here.

02Where the range comes from

For most tests, the range is not a line drawn at "healthy" and "sick". A lab measures a group of healthy people, often at least 120 of them, and keeps the middle 95% of their results. The lowest 2.5% and the highest 2.5% fall outside, even though those people were picked for being healthy.

So, by design, about one healthy person in twenty lands outside the range on any single test. That is why many labs say "reference range" rather than "normal range".

95% is a convention, not a law of biology. A wider range flags fewer healthy people, and also lets more real problems through unflagged. The 90% and 99% options are here to show that trade, not because labs commonly use them.

03The printer

Each result below is a draw from a healthy group, shown as where it sits against the range, not in real units. The bar is the range; * is the result. Results past either end get flagged H or L.


  

  
The simulation treats every test as independent. Real tests are not: hemoglobin and hematocrit, for example, move together, which tends to pull the real share of flagged people down a bit. Treat the numbers as the shape of the effect, not a forecast.


  
04Tests judged by cutoffs instead

  
A few tests are not read against the middle 95% of healthy people. Cholesterol and A1c are compared with cutoffs set by clinical guidelines, chosen from what predicts later disease. MedlinePlus, for example, lists an A1c below 5.7% as normal, 5.7% to 6.4% as prediabetes, and 6.5% or higher as diabetes.

  
Plenty of people who feel fine sit past those cutoffs, so the one-in-twenty arithmetic does not apply to them. That is why the printer leaves the lipid panel and A1c out of its count, even when they are ticked.


  
05Why two labs print different ranges

  

    
ReasonWhat it means

    
      
Reference groupEach lab's range comes from the healthy people it (or the test's maker) measured. Ranges can be split by age and sex, and children often get their own.

      
MethodDifferent machines and chemistry can read the same sample a little differently.

      
UnitsGlucose, for example, can be reported in mg/dL or mmol/L. The same blood gives very different-looking numbers.

    
  

  
So the range printed on a report is the one that goes with that result. A range copied from somewhere else can be off.


  
06What a flag is, and is not

  

  
07Already out there

  
  
Those explain each test well. This page tries something narrower: making the 95% convention visible by running it on made-up healthy people.


  
08Where this is rough

  

  
09Sources