Calculate medullary index from medulla and hair diameters or interpret an entered index using broad microscopic reference observations.
Medullary Index Formula
- MI = medullary index (unitless ratio, 0 to 1)
- M = medulla diameter (µm)
- H = total hair shaft diameter (µm)
Both diameters must be measured at the same point on the hair shaft and in the same units. The result is dimensionless, so the choice of unit cancels out. Measurements are typically taken from the mid-shaft under a compound microscope at 100x to 400x.
Reference Values
Medullary index is one feature used in microscopic hair examination. FBI microscopy guidance describes human medullae as generally less than one-third of shaft diameter and animal medullae as generally greater than one-third. These broad observations overlap and are not species-specific diagnostic ranges.
Use medulla structure, cuticular scales, pigment distribution and suitable reference samples together. A ratio alone cannot establish human or animal origin, identify a species, or identify an individual.
Example
A hair sample shows a medulla diameter of 14 µm and a total shaft diameter of 65 µm.
MI = 14 / 65 = 0.215
The index is below one-third, a common observation in human hair. It does not establish origin; compare scale pattern, pigment distribution and medulla structure with suitable reference samples.
FAQ
Can the medullary index exceed 1.0?
No. The medulla sits inside the cortex, so its diameter cannot be larger than the full shaft.
Where should you measure on the hair?
Use the mid-shaft, away from the root and tip, where the medulla is most consistent.
Is MI alone enough to identify a species?
No. Use it as one of several features. Cuticular scale pattern, medulla type (continuous, fragmented, absent), and pigment distribution are all needed for a defensible identification.
