3.4 Tear Sphingolipid concentration assessed by LC-MS/MS
Pooled tear specimen of the normal and VKC group were analysed by mass spectrometry to quantify the sphingolipids on the ocular surface.Based on standards available and sensitivity of detection, 11 different sphingolipids were estimated by LC-MS/MS analysis. Amongst these, S1P (d17:1), S1P (d17:0) (both p<0.01) and S1P (d20:1) (p <0.001) sphingolipids were significantly decreased in the tear of the total VKC patients when compared to the control. Further, S1P (d20: 0) and S1P (D17:1 DMA adduct) were raised more than two-fold in VKC though not statistically significant (Figure 3A) .
The difference in the levels of these sphingolipids was also noted within the VKC subgroups when classified as NR-VKC and R-VKC(Figure 3B) . Looking at characteristic changes in R-VKC, specific sphingolipids were found to be significantly low as compared to control namely, S1P (d17:1) by 4-fold, (p<0.001),S1P (d17:0) by 3 folds (p<0.001), S1P (d20:1) by nearly 2.5-fold (p<0.01) and C1P (d18:1/ 2:0) by nearly 2-fold (p<0.05). All these SL were also relatively lower in R-VKC in comparison to NR-VKC of which S1P (d17:0) and S1P (d20:1) were significantly lower (p<0.001). In addition, significant lowering of Cer(d18:1/17:0) and C1P(d18:1/18:0) (p<0.001 and p<0.01 respectively) was observed. Both Cer(d18:1/17:0) and C1P(d18:1/1/8:0) showed an increase by nearly 1.5 fold in NR-VKC compared to control but were not statistically significant.