Day 4 is better than day 3 for a single patch test reading

Readings were performed on day (D) 2, D3 and D4 after application of patch tests in 88 patients. 90 patch test reactions in 49 patients were interpreted as allergic and of past or present relevance. A single D2 reading detected 58 of the allergic reactions with 32 false-negatives and 23 false-positives. A single D3 reading detected 77 allergic reactions, with 13 false-negatives and 17 false-positives. A single D4 reading detected 85 allergic reactions, with 5 false-negatives and 9 false-positives. Therefore, if only a single reading is feasible, it is better performed on D4 than on D3.