IP-10 release assays in the diagnosis of tuberculosis infection: current status and future directions

Figures & data

Figure 1. IP-10 induction through adaptive immune responses.

(A) T cells recognize Mycobacterium tuberculosis-specific peptides on the MHC molecules on the surface of the APCs, and this interaction activates the T cells and also the APCs. (B) T cells and the APCs secrete a series of proinflammatory cytokines, which further activates the APCs and leads to (C) IP-10 secretion from activated APCs.

APC: Antigen-presenting cell; IFN: Interferon; TCR: T-cell receptor.

Table 1. Selected studies assessing the performance of IP-10 in patients with confirmed or clinically assessed active tuberculosis.

Study (year)	Country	Study subjects	Results	Ref.
Ruhwald et al. (2008)	Switzerland and Denmark	80 confirmed TB patients	Similar positivity rate between IP-10 test (74%) and QFT-IT (81%) Increased when combining tests (88%)	[52]
Aabye et al. (2010)	Tanzania	92 confirmed TB patients	Similar positivity rates between the IP-10 test (75%) and the QFT-IT (74%). Increased when combining tests (86%)	[55]
Kabeer et al. (2010)	India	175 confirmed TB patients	Similar positivity rate between IP-10 test (91%) and QFT-IT (87%) and increased when combining tests (93%)	[54]
Kabeer et al. (2011)	India	17 confirmed TB patients	Similar positivity rate between IP-10 test (94%) and QFT-IT (100%) Combination of tests not reported	[51]
Ruhwald et al. (2011)	Italy, Spain, Greece, Denmark, Sweden, Finland (nine centers)	168 confirmed TB patients	Equal positivity rate between IP-10 test (79%) and QFT-IT (79%) Increased when combining tests compared with either test alone (87%)	[53]

QFT-IT: QuantiFERON^®-TB Gold In tube; TB: Tuberculosis.

Table 2. Selected studies assessing the specificity of IP-10 in unexposed healthy controls.

Study (year)	Country	Study subjects	Results	Ref.
Healthy controls
Ruhwald et al. (2008)	Denmark	86 high-school students with no history of exposure to TB	No positive results by either test	[52]
Kabeer et al. (2010)	India	50 healthy controls with no close family contact with TB and a negative TST and QFT-IT result	No positive results by either test, but inclusion criteria was a QFT-IT negative result	[73]
Ruhwald et al. (2011)	Denmark and Italy	101 healthy students with no history of exposure to TB, or prior TB disease or treatment	Similar negativity rate between IP-10 test (95%) and QFT-IT (99%)	[53]
Frahm et al. (2011)	USA	26 healthy controls with a negative TST and QFT-IT result	96% IP-10 test negative and 100% QFT-IT negative, but inclusion criteria was a QFT-IT negative result	[20]
Kabeer et al. (2010)	India	100 controls with no symptoms indicative of TB and no history of a close family contact with TB, but from high endemic setting	More negative results by the QFT-IT (52%) than by the IP-10 test (48%)	[54]
Diseased controls
Ruhwald et al. (2011)	Italy, Spain, Greece, Denmark, Sweden, Finland (nine centers)	175 patients initially suspected of TB but finally diagnosed with another disease	More positive results by IP-10 test (35%) than by QFT-IT (27%). Increased AOR of a positive result by either test with the presence of risk factors for exposure (born in a TB endemic country, prior TB)	[53]

AOR: Adjusted odds ratio; QFT-IT: QuantiFERON^®-TB Gold In tube; TB: Tuberculosis; TST: Tuberculin skin test.

Table 3. Selected studies assessing the performance of IP-10 in children.

Study (year)	Country	Study subjects	Results	Ref.
Yassin et al. (2011)	Ethiopia	22 active TB, 136 probable TB, 131 unlikely TB, 335 healthy high risk and 156 low risk of TB infection	Positivity rate was 81% for IP-10, 72% for QFT-IT and 79% for TST. High levels of IP-10 were found in children with active TB and close contact with TB. Up to 37% indeterminate QFT-IT. No effect of HIV on IP-10 responses	[61]
Lighter et al. (2009)	USA	Seven active TB, 12 with high risk, 87 moderate risk and 35 low risk of TB infection	Comparable positivity rates between biomarkers in risk groups. Negative effect of young age on mitogen-induced levels of IFN-γ but not IP-10. IP-10 responses after antigen stimulation correlated with exposure risk	[63]
Petrucci et al. (2008)	Nepal and Brazil	259 children were included	Positive correlation between IP-10 release and QFT-IT and TST results	[120]
Ruhwald et al. (2008)	Nigeria	59 high risk, 38 moderate risk and 23 low risk of TB infection	Comparable proportion of IP-10 test and QFT-IT positivity rates in high-risk (both 71%), medium-risk (8 and 21%, respectively) and low-risk (13 and 26%, respectively) groups. Association with young age and negative IP-10 test and TST results, but not QFT-IT results	[64]
Alsleben et al. (2011)	Germany	11 active TB, 22 high risk of TB infection and 14 with NTM infection	Positivity rates in TB patients was similar (91%) for the IP-10 test and the QFT- IT. No false-positive results in children with NTM infection	[62]

NTM: Non-tuberculous mycobacteria; QFT-IT: QuantiFERON^®-TB Gold In tube; TST: Tuberculin skin test.

Table 4. Selected studies assessing the performance of IP-10 in immunosuppressed patients with active tuberculosis or tuberculosis exposure.

Study (year)	Country	Study subjects	Results	Ref.
HIV-positive patients
Goletti et al. (2010)	India	28 confirmed TB with HIV coinfection (39% with CD4 cell count <200 cells/µl)	Borderline more positive by the IP-10 test (86%) than QFT-IT (61%). Negative trend of decreasing positivity rate with decreasing CD4 cells count for the QFT-IT, but not for the IP-10 test	[56]
Aabye et al. (2010)	Tanzania	65 confirmed TB patients with HIV coinfection (median CD4 cell count: 272 cells/µl)	Lower positivity rate in HIV-infected than uninfected individuals. Equal positivity rate of the IP-10 test (63%) and the QFT-IT (63%). Increased when combining tests (71%) compared with either test alone. Negative trend of decreasing positivity rate with decreasing CD4 cell count for the QFT-IT, but not for the IP-10 test	[55]
Kabeer et al. (2010)	India	50 confirmed TB patients with HIV coinfection (median CD4 cell count 86 cells/µl)	More IP-10 test positive (86%) than QFT-IT positive (70%)	[73]
Kabeer et al. (2011)	India	170 HIV-infected individuals with no signs of or history of TB infection (22% with CD4 cell count <200 cells/µl)	More IP-10 test positive (45%) than QFT-IT positive (38%). Increased when combining tests (48%) compared with the QFT, but not with the IP-10 test	[10]
Patients receiving immunosuppressive treatment
Chen et al. (2011)	Taiwan	56 TB patients, 24 TST-positive using QFT	IP-10 responses were higher in TST positive and in patients with active TB compared with patients with negative TST	[78]

QFT-IT: QuantiFERON^®-TB Gold In tube; RA: Rheumatoid arthritis; TB: Tuberculosis; TST: Tuberculin skin test.

Table 5. Overview of assays used to detect IP-10 and potential cutoffs linked to the assays.

IP-10 assay	Assay range^†(pg/ml)	Studies using this assay	Dilution factor^‡	Cutoff (pg/ml)	Validation study (year)
Luminex^® human IP-10 kit (Biosource/Invitrogen, UK)	5–18,000^§	[17,20,22,52,53,55,64]	×4, ×2, ×8, ×10	673 pg/ml (455 pg/ml also suggested) Ruhwald et al. (2008) [52]	Ruhwald et al. (2011) [53]; Aabye et al. (2010) [55]; Frahm et al. (2011) [20]; Ruhwald et al. (2008) [64]
ELISA DuoSet^® (R&D systems, MN, USA)	5–500	[21,50,51,56,78,121]	×10, ×5, ×2	698 pg/ml Goletti et al. (2010) [56]
ELISA IP-10 construction kit (Antigenix America, NY, USA)	Unknown	[61,120]	×2	3128 pg/ml Yassin et al. (2011) [61]
Luminex LincoPlex^®, (Linco Research, MO, USA)	3.1–10,000^§	[19,25,63]	×4, ×2	732 pg/ml^¶ Lighter et al. (2009) [63]
ELISA OptEIA™, (BD Biosystems, NJ, USA)	5–500	[10,54,70,73]	×20	300 pg/ml Kabeer et al. (2010) [73]; Kabeer et al. (2009) [70]	Kabeer et al. (2011) [10]

^†The range of the assay in undiluted sample.

^‡Factor in bold was used to set the cutoff.

^§Varies depending on the batch of kit and number of analytes included in the multiplex.

^¶Cutoff set in children.

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