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About this tool
Work out the exact 48 and 96 hour patch test reading times from when you applied, and log each reading on the standard ICDRG scale.
The Patch Test Timer converts the moment you applied a patch into the exact calendar dates and times of every reading, using the two published schedules that matter: the ICDRG clinical sequence of application, removal and first reading at 48 hours, a second reading at 72–96 hours and an optional day 7 late reading, and the home cosmetic sequence of checks at 1, 24 and 48 hours used for hair colourant allergy alert tests. Each reading can be logged on the standard ICDRG scale from negative through doubtful, irritant and weak, strong or extreme positive. It is an informational planner, not a diagnosis.
Open Patch Test Timer on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
48 and 96 hours become weekday and clock times you can set an alarm against.
Clinical ICDRG timings and home cosmetic allergy alert timings are kept separate, with their own milestones.
Readings use the ICDRG codes a dermatologist writes down, including the irritant reaction category.
Because true allergic reactions build slowly. Patches stay on for 48 hours and are read at removal, then read again at 72–96 hours, when a genuine allergic reaction typically crescendos while an irritant one fades. Reading only at 48 hours misses a meaningful proportion of true positives, which is why the later reading is standard practice.
At least 48 hours before each colouration, and before every application rather than once. Hair colourants containing p-phenylenediamine and related dyes carry that instruction on the pack in Europe, because sensitisation can develop after products you have used uneventfully for years.
An irritant reaction is usually sharply confined to the patch outline with a glazed or scalded appearance and settles quickly once the patch comes off; an allergic reaction spreads slightly beyond the chamber, feels thickened, may show papules or vesicles, and gets stronger between the 48 and 96 hour readings. This distinction is what the ICDRG scale's separate IR code exists for.
Yes. A negative test means no reaction to what was tested, at the concentrations tested, over the days it was read — it does not cover every ingredient in a product, immediate-type reactions, or an allergy you develop later. Get facial or throat swelling, widespread hives or breathing difficulty checked as an emergency, and take persistent rashes to a dermatologist.