The figures in Data Briefing (page 37, 2 November) are not in fact national figures for England for 1999 but come from the currently voluntary nosocomial infection national surveillance scheme (NINSS) summary report for 1997 to 1999.

LWM Arrowsmith's letter (page 25, November 16) prompted me to re-read Lidwell's original paper. The study of 8,055 patients having hip or knee operations started in 1975 and was carried out in 19 hospitals, but four of them were in Scotland and four were in Sweden.

The commonly quoted report is the one published in 1982 which looked only at deep sepsis confirmed at re-operation.

The 3.4 per cent figure related to the worst-case scenario of conventionally ventilated theatres but without prophylactic antibiotics: with antibiotics this figure became 0.8 per cent. In the various ultraclean air theatres, the equivalent figures were 1.2 per cent and 0.3 per cent respectively. So with antibiotics (as standard today) the figure for deep sepsis in both hip and knee prosthetic surgery should lie between 0.3 per cent and 0.8 per cent.

In the NINSS study, which includes both emergency and elective operations, the overall infection rate for 8,705 hip operations in 55 trusts was 2.9 per cent, but three-quarters of these infections were classified as superficial (only the skin layer was involved). If these are removed, a figure of 0.72 per cent emerges for direct comparison with Lidwell's figures quoted above. For 4,657 knees in 49 trusts an overall rate of 2.1 per cent similarly becomes 0.32 per cent. Both these rates are within Lidwell's range.

Patients undergoing hip operations in the 1970s were usually carefully selected and elective, and were fortunate that MRSA was not a problem, as infections with MRSA are additional to the number of infections with MSSA. In the NINSS study about 28 per cent of the documented hip prosthesis infections were caused by MRSA and approximately 23 per cent by MSSA while in Lidwelll's study 40 per cent of bacterial isolates were MSSA.

There is no need to put us all off the task of tackling hospitalacquired infection by making things look worse than they are.

Dr Susan GL Bragman Consultant microbiologist Infection control officer Greenwich Healthcare trust