BACKGROUND: The Mantoux tuberculin skin test (TST) is widely used to diagnose latent infection with Mycobacterium tuberculosis. TST skin induration may be measured either by a transparent ruler or by a pair of callipers. We hypothesised that the type of instrument used may affect the reading. OBJECTIVE: To determine whether variability in Mantoux TST measurement is affected by the type of reading instrument. METHOD: A TST (Mantoux method) was performed among healthy adolescents. The indurations were read with among ruler and calliper by two independent readers. Limits of agreement and Kappa (kappa) scores at TST positivity cut-off points were calculated. A Bland-Altman graph was constructed. RESULTS: The 95% limits of agreement between instruments ranged from -5 mm to 3 mm. The limits of agreement between readers ranged from -5 mm to 4 mm. kappa scores between instruments were respectively 0.7 and 0.8 at 15 mm and 10 mm cut-offs. CONCLUSION: The variability between readers of TST indurations is not influenced by changing the reading instrument.