318 Gut and Liver, Vol. 5, No. 3, September 2011 Table 5. Comparison of the Comet Percentages of Four Normal Indi- viduals and Four H. pylori-Negative Chronic Gastritis Patients after the Third and Fourth 25 Minutes Incubation Period Table 6. Comparison of the Comet Percentages of Gastric Cells after a Continuous 50 Minutes Incubation Period and the Comparative His- tological Diagnosis Comet percentages after seri- als of cell incubations 3rd 25 minutes incubation 4th 25 minutes incubation Case no. 31 32 33 34 35 36 37 38 16 9 21 18 15 17 16 8 5 0 2 3 0 4 0 2 Histological diagnosis Normal gastric mucosa Normal gastric mucosa Normal gastric mucosa Normal gastric mucosa H. pylori-negative chronic gastritis H. pylori-negative chronic gastritis H. pylori-negative chronic gastritis H. pylori-negative chronic gastritis Five patients with H. pylori-positive active gastritis had a mean comet percentage of 53%, two patients with H. pylori-negative active gastritis had a mean comet percentage of 62.5% and two patients with H. pylori -negative chronic gastritis had a mean comet percentage of 56.5%. The mean comet percentage was highest in the normal indi- viduals histology the lowest comet percentage was in the group of H. pylori -positive active gastritis. There was a significant difference between the comet percentage in the samples of normal histology compared to H. pylori-positive active gastritis (p=0.0006). In the seven normal individuals (Table 1), the mean comet percentage was high in the first 25 minutes comet (a mean of 52%) compared to the second 25 minutes comet (a mean of 36%). There was a significant difference in the comet percentage between the first and second 25 minutes comets (p=0.0002). In Table 2, a group of patients with the histological diagnosis of H. pylori-positive active gastritis (HPAG) the mean comet percent- ages were 41% in the first 25 minutes incubation and 29.7% in the second 25 minutes incubation. There were a significant dif- ference in the comet percentages between the normal individu- als in Table 1 and patients of H. pylori-positive active gastritis in Table 2 (p=0.0002). In Table 3, four normal individuals were compared to four patients with H. pylori-positive chronic gastritis. The mean com- et percentages of the normal individuals were 53.5% and 39.5% in the first 25 minutes and second 25 minutes, respectively, also the mean comet percentages of patients with H. pylori-positive chronic gastritis were 45% and 31.8% in the first 25 minutes in- cubation and second 25 minutes incubations, respectively. There was a significant difference in the comet percentages between the normal individuals and patients of H. pylori-positive chronic gastritis in the first 25 minutes of incubation (p=0.0004). In Table 4, four normal individuals compared to four patients Case no. Comet percentages (% comet) 39 40 41 42 43 44 45 46 47 48 49 50 51 52 72 61 76 58 69 47 52 74 68 52 49 57 65 60 Histological diagnosis Normal gastric mucosa Chronic H. pylori-negative gastritis Normal gastric mucosa H. pylori-positive active gastritis Normal gastric mucosa H. pylori-positive active gastritis Chronic H. pylori-negative gastritis Normal gastric mucosa H. pylori-negative active gastritis H. pylori-positive active gastritis H. pylori-positive active gastritis H. pylori-negative active gastritis Normal gastric mucosa H. pylori-positive active gastritis with H. pylori -negative active gastritis. The mean comet per- centages in normal individuals were 39% and 12% in the sec- ond 25 minutes and third 25 minutes incubations, respectively. While, the mean comet percentages in patients with H. pylori- negative active gastritis were 31% and 10% in the second 25 minutes incubation and third 25 minutes incubations, respec- tively. There was a high comet percentages (a mean of 39%) of normal individuals compared to the patients with H. pylori- negative active gastritis (a mean of 31%). There was a signifi- cant difference in the comet percentage between the second and third 25 minutes incubation in normal individuals (p=0.0008). In Table 5, four normal individuals compared to four patients with H. pylori -negative chronic gastritis, the mean of comet percentages of normal individuals were 16% and 2.5% in the third and fourth 25 minutes incubations, respectively. There was a high mean comet percentages of 16% of normal individuals compared to that one of 14% of the patients with H. pylori - negative chronic gastritis in the third 25 minutes incubation. The mean comet percentages of patients with negative H. pylori chronic gastritis were 14% and 1.5% in the third and fourth 25 minutes incubation, respectively. There was a significant differ- ence in the comet percentage between the third and fourth 25 minutes comet in patients with H. pylori-negative chronic gas- tritis (p=0.0014). DISCUSSION The comet assay was first reported in 1988 by Singh et al.5 and is now established as a rapid, efficient and reproducible measure of DNA damage in single cells. It has been applied ex-