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What a band is, and what it is not
A band is a range of values that a guideline body or laboratory standard has given a name: normal, elevated, stage 1, deficient. This tool places your number in the band the publishing body defines, and it tells you which body and when that definition took effect. It does not average different sources, it does not fill a gap between two bands with a guess, and where the source publishes no band containing your number it says so. The biomarker library holds the same ledgers with every row and its source visible.
A band is not a diagnosis. Most of these definitions require the result to be repeated, to be read alongside another test, or to persist for a period of time before they mean anything clinical: KDIGO requires a kidney abnormality such as an eGFR under 60 to be present for at least three months before it is chronic kidney disease; the ADA requires a second measurement to confirm a diabetes-range A1C unless symptoms are clear; the AUA requires two early-morning testosterone draws on separate occasions. The result links the marker's full reference page, and the band's own page where one exists.
Why your lab's range may differ
The reference interval printed on your report is set by your laboratory, and ferritin intervals in particular are typically wide and drawn from a general population. The bands here come from the body that publishes them, and the two can disagree. A ferritin of 40 µg/L often prints as normal; WHO's 2020 ferritin guideline counts it as iron deficiency in an adult with infection or inflammation, where its threshold is below 70 µg/L, while its threshold for a healthy adult is below 15 µg/L. A TSH of 4.5 mIU/L is above 0.45 to 4.12, the interval the 2012 AACE and ATA hypothyroidism guideline uses when a laboratory prints none, yet below the 7.49 mIU/L 97.5th percentile that Surks and Hollowell (2007) found in NHANES III participants 80 and over without thyroid disease. Where a marker has that kind of split, its reference page explains it.
Check the unit before anything else. Hemoglobin can be reported in g/dL or g/L, glucose in mg/dL or mmol/L. The test menu shows the unit each marker expects; a number entered in another unit can land in the wrong band, and the tool cannot tell.