Inconsistent correlation between aerobic bacterial surface and air counts in operating rooms with ultra clean laminar air flows: proposal of a new bacteriological standard for surface contamination

The relationship between surface contamination (cfus/m(2)/h) with particles carrying aerobic bacteria and corresponding air contamination rates (cfus/ m(3)) was evaluated in operating rooms (OR) equipped with ultra clean verti cal or horizontal laminar airflow (LAF). For the evaluation we collected da ta during strictly standardized sham operations using non-woven disposable or cotton clothing. Air contamination in the wound and instrument areas (Ca sella slit sampler) was related to the surface contamination rate (settle p lates) in the same areas and in addition, on the patient chest. Typically t he mean surface counts were 20-70 cfus/m(2)/h and the air counts 1-2 cfus/m (3) in disposable clothing experiments, whilst the use of cotton clothing r esulted in higher counts of 100-200 cfus/m(2)/h (wound P > 0.05, patient P> 0.05, instruments P<0.01) and 4 cfus/m(3) (P<0.02-0.001). In the vertical L AF, taking both disposable and cotton clothing operations together, the sur face and air contamination rates (surface/air ratio SAR) were highly correl ated (P=0.02-0.004) and the ratio varied between 18:1 and 50:1 with a mean for wound air of 36:1. Using only disposable clothing in the vertical LAF, the number of significant correlations was reduced. With cotton clothing ex periments in vertical LAF and in the horizontal LAF using disposable clothi ng, no significant correlation between surface and air contamination was fo und. The wide variation of SAR values and the inconsistent relationship bet ween surface and air counts indicates that measurement of OR air contaminat ion represents an unhelpful method for assessment of surgical site contamin ation in LAF units. We propose instead that colony counts on sedimentation plates is a clinically more relevant indicator of bacterial OR contaminatio n in LAF units. In addition to the current bacteriological standard for ult ra clean OR air of (<10 cfus/m(3)) we suggest a corresponding standard for the surface contamination rate of <350 cfus/m(2)/h.