The Phototrichogram Method: How Growth Rate Gets Measured Scientifically
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The phototrichogram is how clinical trials quantify hair growth. Understanding the method explains why hair loss trials report the outcomes they do — and how to read those numbers meaningfully.
Most of what is known about treatment-induced changes in hair density comes from phototrichogram data in clinical trials. The method is not glamorous, but it is the quantitative foundation on which efficacy claims rest — understanding it allows clinicians and patients to evaluate those claims with appropriate precision.
Method overview
A small reference area (typically 1.8–2.5 cm²) is selected in the target zone — vertex for AGA trials, frontal for hairline studies. The area is tattooed or permanently marked with a reference grid to ensure consistent repositioning across photography sessions. Baseline macrophotography is performed at high magnification under standardised lighting.
The area is then clipped or shaved. After two to three days of regrowth, the same area is re-photographed. Hairs that have grown are in anagen; hairs not yet visible above the surface are in telogen. Image analysis software counts the hairs, measures shaft diameters, and calculates the anagen:telogen ratio. Repeat photography at defined intervals (typically 2, 4, 6, and 12 months in treatment trials) allows calculation of growth rate and density changes.
TrichoScan
The TrichoScan system (Hoffmann, 2001) digitised and automated much of the phototrichogram analysis, using dermoscopy imaging combined with image recognition software to count follicle units, classify hairs, and measure density automatically. TrichoScan has become a standard research tool in hair loss trial design and is available in specialist centres.
What the trial numbers mean
When a finasteride trial reports “+18 hairs per cm² versus -17 in placebo” — that 35-hair difference per cm² is a phototrichogram endpoint. It represents actual hair count in the reference area, not subjective assessment. Understanding that these numbers come from a carefully bounded, standardised, macrophotographed zone helps contextualise why trial results may look more dramatic than what patients observe in their mirror — the mirror integrates the whole scalp; the trial endpoint measures a cm².
Clinical Q&A
What is a phototrichogram?
A phototrichogram is a serial photography technique used to measure hair density and growth rate. A defined area of scalp is photographed, then the area is shaved and re-photographed two to three days later. Hairs that have grown back are in anagen; hairs still absent are in telogen. Repeat photography over time quantifies growth rate.
What endpoints does phototrichogram measure?
Primary endpoints include: terminal and vellus hair counts per cm², anagen:telogen ratio, hair shaft diameter distribution, and cumulative hair growth (density × rate). The pivotal finasteride and minoxidil trials used phototrichogram-derived hair count as their primary efficacy endpoint.
Is phototrichogram available in routine clinical practice?
Not routinely — it requires specialised equipment (macrophotography with marked scalp reference points) and is primarily used in clinical research settings and specialist hair clinics. Trichoscopy (dermoscopy) with follicle counting is more widely available for routine clinical assessment.
References & further reading
- Hoffmann R. TrichoScan: combining epiluminescence microscopy with digital image analysis for the measurement of hair growth in vivo. European Journal of Dermatology, 2001.
- Price VH, Menefee E, Strauss PC. Changes in hair weight and hair count in men with androgenetic alopecia, after application of 5% and 2% topical minoxidil, placebo, or no treatment. Journal of the American Academy of Dermatology, 1999.
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Medical disclaimer: This article is educational and does not constitute clinical advice, diagnosis, or treatment recommendations. Consult a licensed physician or dermatologist before starting, stopping, or changing any medication or treatment for hair loss.