The I:T ratio has the best sensitivity of any of the neutrophil indices.
I:T ratio = (bands+metamyelocytes+myelocytes) / (bands+metamyelocytes+myelocytes+neutrophils)
(Bands, metamyelocyte and myelocyte counts are given as part of the manual differentiation of white blood cell count from most laboratories).
Upper limits of ‘normal’:
- Day 1 – 0.16
- Day 2 – 0.14
- Day 3 onwards – 0.12
If there is an I:T ratio > 0.2 and low neutrophil count or high CRP, then infection is likely.
References: Polin, R.A., Papile, L.A., Baley, J.E., Bhutani, V.K., Carlo, W.A., Cummings, J., Kumar, P., Tan, R.C., Wang, K.S. and Watterberg, K.L., 2012. Management of neonates with suspected or proven early-onset bacterial sepsis. Pediatrics, 129(5), pp.1006-1015.