Is IP-10 a Better Biomarker? Figure 1. Concentration of IP-10 (pg/ml) in supernatants from: a) whole blood samples incubated with negative control antigen (background); b.) whole blood samples incubated with TB-specific antigens and negative control antigen, of children with active TB (n = 17), LTBI (n = 16) and a control group (n = 16). Horizontal lines indicate median values. Controls are represented by #, active TB by n, and LTBI by %. Open symbols represent unstimulated, closed symbols represent antigen-stimulated samples. doi:10.1371/journal.pone.0003901.g001 Figure 1b), however, less significant than in active TB, most likely due to the higher baseline levels. This antigen- stimulated increase in IP-10 was also noted in the control group, (median increased from 320 pg/ml to 536 pg/ml; p = 0.0234) but significantly lower than in active TB. This is largely due to one patient having a very high level (40,000 pg/ml) following antigen stimulation. Levels of IP-10 compared with IFN-c The levels of IP-10 were compared to the previously measured levels of IFN-c in the same patients using the QFG-ELISA system. In all three groups, there were significantly higher levels of IP-10 than of IFN-c following antigen stimulation (Control - IFNc median 7.25 pg/ml, range 3.5–46.5 pg/ml; IP-10 median 536 pg/ml, range: 320–40,000 pg/ml:p = 0.0001; LTBI - IFNc median 2205 pg/ml, range 60–31084 pg/ml; IP-10 median 19350 pg/ml, range 2,200–40,000 pg/ml, p = 0.0047: Active TB - median IFNc 1124.5 pg/ml, range 10–11928.5 pg/ml; IP-10 median 9544 pg/ml, range 1386–40,000 pg/ml, p = 0.0011: (Figure 2). In the group with LTBI, the levels of both IP-10 and IFN-c were higher than in patients with active TB (IP-10 p = 0.85; IFNc p = 0.05). Stimulation Index Figure 3 shows that the stimulation index (SI) of IP-10 in both active (median SI: 9.4, range: 1–125) and latent TB groups (median SI: 8.8, range: 0.32–68.7) are greater than in the control group (median SI: 1.7, range 0–125). This is significant in the active cases (p = 0.04), but not the LTBI cases (p = 0.23). There is only a slight difference in stimulation index between active and latent TB (p = 0.09). The stimulation index was also used to compare IFN-c and IP- 10 in all three groups of patients (Figure 3). There was no difference between the IP-10 and IFNc stimulation indices in the control groups, but IFN-c has a significantly higher stimulation index than IP-10 in both the active TB (median SI IFNc: 112.5, PLoS ONE | www.plosone.org 3 December 2008 | Volume 3 | Issue 12 | e3901