The phototrichogram has 90% sensitivity for detecting androgenetic alopecia in early stages, making it the gold standard for clinical hair cycle analysis. myhairline.ai achieves 85% sensitivity without shaving by using AI fiber diameter estimation as a proxy for growth phase classification. This guide explains both methods, when each one is appropriate, and how to use them together.
This content is for informational purposes only and does not constitute medical advice.
What a Phototrichogram Measures
The phototrichogram is a two-step photographic technique developed in the 1990s to quantify the hair growth cycle in a specific scalp region. It measures three key parameters that determine hair loss severity:
- Hair density: Total number of hairs per square centimeter
- Anagen ratio: Percentage of hairs actively growing
- Telogen ratio: Percentage of hairs in the resting/shedding phase
In a healthy scalp, 85 to 90% of hairs are in anagen and 10 to 15% are in telogen at any given time. In androgenetic alopecia, this ratio shifts. As the condition progresses, the telogen percentage increases while anagen decreases. A telogen ratio above 20% in the frontal scalp is considered diagnostic for active androgenetic alopecia.
The Clinical Phototrichogram Protocol
The standard phototrichogram procedure takes two clinic visits separated by exactly 3 days.
Visit 1: Baseline Photography
The clinician selects a 1cm2 test area, typically at the frontal hairline or vertex, depending on the suspected pattern of loss. This area is shaved close to the skin surface with a surgical trimmer, leaving stubble approximately 1mm long. A clinical photograph is taken at 20x to 40x magnification using a dermatoscope or trichoscopy camera.
Visit 2: Growth Assessment (Day 3)
Three days later, the same area is photographed again at the same magnification. The clinician (or analysis software) then compares the two images:
- Hairs that grew (visible length increase from day 0 to day 3) are classified as anagen
- Hairs that did not grow (same length on both days) are classified as telogen
- New hairs visible on day 3 but not day 0 indicate early anagen re-entry
| Measurement | Healthy Scalp | Early AGA | Moderate AGA | Advanced AGA |
|---|---|---|---|---|
| Total density (hairs/cm2) | 170-230 | 140-170 | 100-140 | Below 100 |
| Anagen ratio | 85-90% | 70-85% | 55-70% | Below 55% |
| Telogen ratio | 10-15% | 15-30% | 30-45% | Above 45% |
| Vellus hair percentage | Below 10% | 10-20% | 20-40% | Above 40% |
These numbers vary by ethnicity. Caucasian hair averages 200 follicular units per cm2, Asian hair averages 170, and African hair averages 150. The phototrichogram accounts for these baselines when interpreting results.
Advantages of the Clinical Phototrichogram
The phototrichogram remains the standard for several important reasons.
Direct growth measurement: By photographing the same hairs 3 days apart, you get a definitive answer about which hairs are growing and which are not. There is no estimation or inference involved.
Miniaturization detection: Shaved hair at 1mm length reveals fiber diameter differences that are invisible in longer hair. Miniaturized vellus hairs (below 40 microns in diameter) appear visibly thinner than terminal hairs (60+ microns) even at short lengths.
Reproducibility: The method has been validated in hundreds of published studies. Inter-observer agreement is high when using standardized software for hair counting.
Sensitivity: The 90% sensitivity rate for detecting early AGA means it catches the condition before it becomes visible to the naked eye. This is critical for early intervention, when treatments like finasteride (80 to 90% halt further loss, 65% regrowth) are most effective.
Limitations of the Clinical Phototrichogram
Despite its accuracy, the phototrichogram has practical drawbacks that limit its use for routine monitoring.
Requires shaving: A 1cm2 shaved patch on the scalp is cosmetically noticeable, especially for patients with thinning hair who are trying to maximize coverage. Most patients are unwilling to shave visible areas repeatedly for serial monitoring.
Two clinic visits: The 3-day interval between photos requires two separate appointments. This doubles the cost and time commitment compared to single-visit methods.
Limited area: The phototrichogram analyzes only 1cm2 at a time. Hair loss is often heterogeneous across the scalp, so a single test area may not represent the global pattern.
Cost: At $200 to $500 per assessment (depending on the clinic and region), serial phototrichograms for quarterly monitoring become expensive. Over a year of quarterly assessments, costs reach $800 to $2,000.
Not available at home: The technique requires clinical equipment and a trained operator. Patients cannot perform phototrichograms on themselves.
How AI Approximates Phototrichogram Data
myhairline.ai uses a different approach to estimate the information that a phototrichogram provides. Instead of measuring growth directly by shaving and re-photographing, it uses AI fiber analysis from unshaved photos.
Fiber Diameter Estimation
When myhairline.ai analyzes a scalp photo, the AI identifies individual hair fibers and estimates their diameter based on pixel width, shadow characteristics, and contrast against the scalp. Thinner fibers (consistent with miniaturized, telogen-dominant hairs) are separated from thicker fibers (consistent with healthy terminal hairs in anagen).
This analysis produces an estimated terminal-to-vellus ratio that correlates with the anagen-telogen ratio measured by phototrichogram. The correlation is not perfect because fiber diameter is a proxy for growth phase rather than a direct measurement, but it achieves 85% sensitivity for detecting AGA.
Density Estimation Without Shaving
AI density analysis counts visible hairs per unit area from standard photos. This count is less precise than a phototrichogram on shaved skin (where every hair is visible at its root) but provides a useful approximation when performed under consistent lighting conditions.
| Feature | Phototrichogram | AI Home Tracking |
|---|---|---|
| Sensitivity for AGA | 90% | 85% |
| Requires shaving | Yes | No |
| Clinic visit needed | Yes (2 visits) | No |
| Cost per assessment | $200-500 | Free |
| Assessment frequency | Quarterly (practical limit) | Weekly |
| Growth phase measurement | Direct | Estimated from fiber diameter |
| Area analyzed | 1cm2 | Multiple regions |
| Serial tracking | Expensive | Built into platform |
When to Use Each Method
Choose the Phototrichogram When:
- You need a definitive diagnosis distinguishing AGA from other types of hair loss (telogen effluvium, alopecia areata, cicatricial alopecia)
- Your hair loss is very early stage and not yet visible in standard photos
- You need clinical-grade documentation for insurance pre-authorization of treatment
- Your dermatologist wants to measure treatment response with clinical precision
- You are considering a hair transplant and need pre-surgical density mapping (to calculate whether your donor area can support the needed grafts, with the safe extraction limit being 45% of the donor area)
Choose AI Home Tracking When:
- You want frequent monitoring without clinic visits
- You are tracking treatment response over time (finasteride, minoxidil at 40 to 60% regrowth rate, PRP at $500 to $2,000 per session)
- You want to compare density across multiple scalp regions simultaneously
- Cost is a consideration for ongoing monitoring
- You prefer not to shave any area of your scalp
Using Both Methods Together
The strongest monitoring approach combines periodic clinical phototrichograms with continuous AI home tracking. The phototrichogram provides high-accuracy calibration points, while AI tracking fills the gaps between clinic visits with weekly data.
Recommended combined protocol:
- Get a baseline phototrichogram at your dermatologist
- Start weekly AI tracking on myhairline.ai on the same day
- Continue weekly home tracking for 12 to 16 weeks
- Return for a follow-up phototrichogram
- Compare the clinical results with your AI trend data
This approach gives you the accuracy of clinical measurement at key decision points plus the granularity of continuous home monitoring. Your dermatologist gets both the clinical data they trust and the between-visit trend data that adds context.
For a detailed comparison of clinical versus AI methods, see comparing trichoscopy and AI hair analysis. And for understanding what the anagen-telogen numbers mean for your specific situation, the guide to understanding the anagen-telogen ratio breaks down the clinical significance.
Getting Started With AI Tracking
You do not need a clinic visit to start building your tracking timeline. Begin with a free AI density analysis that runs entirely in your browser with no downloads, accounts, or data transmitted to servers.
Get your baseline reading at myhairline.ai/analyze.
This article is for informational purposes only and does not constitute medical advice. The phototrichogram should be performed by a qualified dermatologist or trichologist. Consult a specialist for diagnosis and treatment recommendations.