The ratio of polymorphonuclear leucocytes (PMN) to non-PMN cells - a novel method of assessing acute lung inflammation

Polymorphonuclear leucocyte (PMN) numbers are an indicator of the degree of acute lung inflammation. However, there is no standardized system for accurately quantifying their numbers in tissue sections. Also, the effect of lung inflation on the quantification of PMN's is usually overlooked. Lung specimens obtained from clinical biopsies are usually deflated, while inflated lung tissue is commonly used in experimental studies. We report a method, which is independent of the degree of inflation, for measuring the degree of PMN infiltration in the both inflated and non-inflated lungs. Using light microscopy, we counted the numbers of PMN and non-PMN cells in 240 fields from each of five inflated and five non-inflated lung sections and calculated a ratio of PMN: non-PMN cells (the PMN ratio). The effect on accuracy and precision of number of fields counted was investigated by randomly selecting 200, 160, 80 or 40 readings from the original 240 fields. The mean PMN ratio, its 95% confidence interval (CI) and the coefficient of variation (CV) were calculated for each of the four levels of sampling. Both CI and CV increased as the number of readings decreased. Inflated lung tissue had consistently higher values for CV compared to non-inflated lung. In practice, we recommend that for both inflated and non-inflated lungs, 80-160 fields (approximate 0.23-0.45 mm(2) Of absolute area evaluated) need to be counted to yield a PMN ratio with acceptable accuracy and precision. The PMN ratio provides a simple and objective way of quantifying the degree of acute inflammation in clinical histopathology and experimental toxicology studies involving lungs. It is suitable for use in research of lung inflammation, and as an accessory diagnostic tool and an objective descriptor for clinical histopathology.