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Estimating the infection rate in mothers following caesarean section
Journal of Hospital Infection (1987) 10, 138-144
Estimating
the infection rate in mothers caesarean section
J. R. Thompson*, *Dept
R. M. Hutton7
following
and B. R. Moir-Bussyl
qf Ophthalmology, Laboratory,
Leicester Royal Infirmary, TArea Public Coventry, $Glenfield Hospital, Leicester, UK
Accepted for publication
Accepted for publication
Health
16 October 1986
Summary: Some hospitals send comparatively few swabs to their laboratories for examination, consequently any measure of infection that depends on routinely analysed swabs is likely to underestimate the actual level of infection. In a survey of routine wound swabs from mothers following caesarean section, it was found that the average infection rate in 3 1 hospitals was 6%, with considerable variation between the hospitals. Further consideration of the data as described in this paper shows that the actual infection rate was about 14%, with much less variation between hospitals.
Introduction
Moir-Bussy, Hutton & Thompson (1984) reported the results of a survey of infection in women following caesarean section. This survey covered all of the caesarean sections performed in each of a sample of 31 hospitals over a 3 month period and gave an overall infection rate of 6%. In this type of survey the observed infection rate depends critically on the definition of infection that is chosen; a fact that may account, in part, for the large variation in the results of such studies. Previous surveys have reported infection rates varying from the 1.3% of Kettering & Walter (1977) to the 16% found by Hawrlshyn, Bernstein & Rapsin (1981), though little work has been done on the differences between hospitals. In general, the clinical consequences of wound infection in mothers following caesarian section are not severe. In the survey of Moir-Bussy et al., (1984) none of the patients died of infection during the trial and less than 1% of patients were recorded as suffering bacteraemia due to the organism isolated from their wound. Patients with infections suffered pyrexias over 38°C during 7.1% of the postoperative period as opposed to Correspondence to: Dr J. R. Thompson, Department of Opthalmology, L&ester Royal Infirmary, P.O. Box 65, L&ester, LE2 7LX. 0195-6701,'87/050l38+07
Clinical
Sciences Building,
Q ,987 The Hospttal
803 00/O
138
Infectmn
Soaety
Estimating
infection
139
rates
3.4% for those not infected. Qn average they were in hospital for two extra days. In that survey the definition of infection was that the wound be inflamed in appearance and yield a positive culture. Thus the definition of infection required that four criteria be fulfilled: (a) the wound had to appear to be inflamed on at least 1 day between the operation and the patient’s discharge from hospital; (b) a swab must have been taken from the wound and sent for examination; (c) the organism had to survive the period of transportation to the laboratory; (d) a positive culture had to result from the material on the swab. Failure to meet any one of these criteria meant that the wound would not be classified as infected. As a result of strict adherence to this definition it was possible that a wound might break down completely and yet, if no swab were sent for examination, the wound would not be classified as infected. While one can be sure that this definition did not often classify a wound as infected when it was not, it seems likely that some cases of genuine infection were overlooked. In a separate survey of the laboratory services of the 31 hospitals little difference was found either in their facilities or their laboratory practice and so the two main reasons for any underestimation of the infection rate in some of the hospitals would appear to be, (a) that the staff failed to classify the wound as inflamed when it was so, (b) that a swab was not sent to the laboratory. Ideally one would seek to reduce the effects of these sources of underestimation by training the staff and insisting that a swab be sent for examination whenever a wound appears inflamed. Unfortunately this is not always a practical option in an observational study. Our survey of infection following caesarean section, like many other hospital-based surveys, was designed to observe existing practices. Consequently compensation for any underestimation must be made using only the data that were actually available. The analysis that follows looks again at the data collected by Moir-Bussy et al., (1984) and attempts to measure the degree of underestimation in the observed infection rate. Variation
in the observed
infection
rates
Table I shovvs the results obtained from each of the 31 hospitals and it is from these data that we wish to derive some estimate of the national level of infection as well as considering the differences between hospitals. The entire sample of 2368 cases contains 141 that met the survey’s criteria and were classified as infected, a rate of just under 6%. Those criteria required that, to be called infected, the wound had to show both inflammation or sepsis and a positive bacteriological culture in material taken from the wound. One graphical way of investigating the differences between hospitals is to show them individually on a probability plot of their
140
J. R. Thompson Table Hospital
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
I.
Vmiation
No. of cases 41 97 137 45 68 101 50 85 95 43 42 109 124 60 83 98 98 147 58 44 39 11 53 27 56 iii 71 60 93 191
et al.
in the observed infection No. with clinically mflammed wounds 6 11 27 10 4 25 6 26 26 16 7 15 20 20 17 14 iii 11 10 1.5 2 6 5 : 12 1 2; 22
standardized deviations from the national expected normal deviates against:
rates No. with measured
bacterlologically infected wounds -3
; 2 1 1 14 4 1 6 1 4 7 10 1 14 2 4 5 2 9 8 1 2 3 3 5 3 1 0 16 5
average rate, that is by plotting
j=l,.
. . 31
where p, and nJ are the proportions infected and the total number of cases in each hospital and p is the overall proportion infected, i.e., O-06. If the variation between hospitals was due to random effects rather than a systematic difference then this plot would produce a straight line. The actual plot is shown as Figure 1 and illustrates very clearly how the hospitals divide into two distinct groups. The larger group consists of 26 hospitals that appear to have very little infection, their average rate is, in fact, only 4%. On the other hand the smaller group of five hospitals has an average infection rate of 17%.
Estimating
infection
141
rates
X 30 x 15 x 20 x 21
kt”
x6
XX
Figure 1. A probability plot of the standardized rates, diagnosed the numbers of the five hospitals with the highest values.
Figure 2. A probability indicates the position
plot of the standardized of a standardized normal.
climcal
bacteriologically,
inflammation
rates.
showing
The
line
142
J. R. Thompson
et al.
There are two possible explanations for the observed division of the hospitals into two groups. It is possible, if unlikely, that five of the hospitals do actually have an infection rate over four times as great as the other 26, or it may be that the majority are underestimating their true infection rates for one of the reasons suggested in the Introduction. Figure 2 shows a probability plot of the standardized inflammation rates, defined as localized erythema and induration, in the same way that Figure 1 shows the standardized infection rates. It will be observed that there are no longer any major divisions within the group of 31 hospitals. If the variation between hospitals were just random then the points would lie close to the straight line shown as part of the plot. Thus, in this case, there is still more variation between hospitals than one would expect by chance, however this variation is considerably less than was observed in the plot of infection rates. Thus despite its subjective element, the observed inflammation rate is much more uniform than the measured infection. Improving
the estimate
of infection
rate
While it is the case that swabs are sometimes sent to the laboratory even when there is no sign of inflammation, our main interest lies in the chance that an inflamed wound has a swab taken. Moir-Bussy et al., (1984) found that the rate at which inflamed wounds had swabs sent to the laboratory averaged at 42%, but that this figure showed considerable variation between hospitals. Figure 3 shows a standardized plot of the rates with which hospitals sent swabs from inflamed wounds. This plot is revealing, for once again the hospitals divide into two groups and the five hospitals with the worst infection rate all appear in the top seven for sending swabs to the laboratory. This information is also open to various interpretations. It could be, for example, that more swabs are sent because there is more infection or that there appears to be more infection because more swabs are sent. To investigate this further Figure 4 looks at the proportion of swabs sent that turned out to be positive against the proportion of inflamed wounds that had a swab sent. One hospital sent no swabs at all, thus ensuring a zero infection rate, and this hospital is excluded from the plot. an observation confirmed by its Figure 4 shows no clear pattern; unweighted correlation which is only 0.05. It thus appears that the proportion of positives does not depend on the rate at which swabs are sent to the laboratories. If one averages over all of the hospitals it is found that 8 1% of swabs show some positive culture. It would appear that the main reason for the observed differences in infection between hospitals is that they have developed different practices for sending swabs. If swabs had been sent for every inflamed case then there would not have been such major differences between hospitals and the observed infection rates would have been much higher.
Estimating
infection
rates
143
X I
-2
I
I
2
4
D
Figure
3.
-4 probability
t.0
plot
of the rates at which
xxx
xx
xx
x
inflamed
wounds
xxxx
X
have swabs sent for
X X
0.6
X
X X X
X
x
X
X X
X
X X x 0.2
X
I 0.2
-
I
I
I
0.0
I.0
I
0.4 Proportm
Figure 4. The proportion of positive wound inflamed wounds that have a swab sent.
0.6 sent
swabs
plotted
against
the proportion
of
144
J. R. Thompson
et al.
Since 81 oio of swabs sent gave a positive result and this does not appear to depend on the proportion sent, a better indication of the true level of infection would be obtained by taking 81% of the inflammation rate. The overall inflammation rate in this survey was 18% suggesting a revised estimate of the national infection rate of about 14%. Conclusions The analysis of these data is highly dependent on the assumptions that one chooses to make. However it would appear that: (a) infection was underestimated in most hospitals because they did not send sufficient swabs for examination, (b) the national infection rate in mothers following caesarean section is probably about 14% as opposed to the measured rate of 6%, rate, even with its subjective element, is a better Cc)the inflammation indicator of the relative levels of infection in any two hospitals than the measured infection rate. References Hawrlshyn, P. A., Bernstein, P. & Rapsin, F. R. (1981). Risk factors associated with caesarean section. Amercian Journal of Obstetrzcs and Gynaecology 139, 294-298. Kettering, H. A. & Walter, D. F. (1977). Complications of caesarean section. Transactions of the Pacific Coast Obstetrics and Gynaecology Society 44. 29. Moir-Bussy, B. R., Hutton, R. M. & Thompson, J. R. (1984). Wound infection after caesarean section. Journal of Hospital Infection 5, 359-370.
Estimating
the infection rate in mothers caesarean section
J. R. Thompson*, *Dept
R. M. Hutton7
following
and B. R. Moir-Bussyl
qf Ophthalmology, Laboratory,
Leicester Royal Infirmary, TArea Public Coventry, $Glenfield Hospital, Leicester, UK
Accepted for publication
Accepted for publication
Health
16 October 1986
Summary: Some hospitals send comparatively few swabs to their laboratories for examination, consequently any measure of infection that depends on routinely analysed swabs is likely to underestimate the actual level of infection. In a survey of routine wound swabs from mothers following caesarean section, it was found that the average infection rate in 3 1 hospitals was 6%, with considerable variation between the hospitals. Further consideration of the data as described in this paper shows that the actual infection rate was about 14%, with much less variation between hospitals.
Introduction
Moir-Bussy, Hutton & Thompson (1984) reported the results of a survey of infection in women following caesarean section. This survey covered all of the caesarean sections performed in each of a sample of 31 hospitals over a 3 month period and gave an overall infection rate of 6%. In this type of survey the observed infection rate depends critically on the definition of infection that is chosen; a fact that may account, in part, for the large variation in the results of such studies. Previous surveys have reported infection rates varying from the 1.3% of Kettering & Walter (1977) to the 16% found by Hawrlshyn, Bernstein & Rapsin (1981), though little work has been done on the differences between hospitals. In general, the clinical consequences of wound infection in mothers following caesarian section are not severe. In the survey of Moir-Bussy et al., (1984) none of the patients died of infection during the trial and less than 1% of patients were recorded as suffering bacteraemia due to the organism isolated from their wound. Patients with infections suffered pyrexias over 38°C during 7.1% of the postoperative period as opposed to Correspondence to: Dr J. R. Thompson, Department of Opthalmology, L&ester Royal Infirmary, P.O. Box 65, L&ester, LE2 7LX. 0195-6701,'87/050l38+07
Clinical
Sciences Building,
Q ,987 The Hospttal
803 00/O
138
Infectmn
Soaety
Estimating
infection
139
rates
3.4% for those not infected. Qn average they were in hospital for two extra days. In that survey the definition of infection was that the wound be inflamed in appearance and yield a positive culture. Thus the definition of infection required that four criteria be fulfilled: (a) the wound had to appear to be inflamed on at least 1 day between the operation and the patient’s discharge from hospital; (b) a swab must have been taken from the wound and sent for examination; (c) the organism had to survive the period of transportation to the laboratory; (d) a positive culture had to result from the material on the swab. Failure to meet any one of these criteria meant that the wound would not be classified as infected. As a result of strict adherence to this definition it was possible that a wound might break down completely and yet, if no swab were sent for examination, the wound would not be classified as infected. While one can be sure that this definition did not often classify a wound as infected when it was not, it seems likely that some cases of genuine infection were overlooked. In a separate survey of the laboratory services of the 31 hospitals little difference was found either in their facilities or their laboratory practice and so the two main reasons for any underestimation of the infection rate in some of the hospitals would appear to be, (a) that the staff failed to classify the wound as inflamed when it was so, (b) that a swab was not sent to the laboratory. Ideally one would seek to reduce the effects of these sources of underestimation by training the staff and insisting that a swab be sent for examination whenever a wound appears inflamed. Unfortunately this is not always a practical option in an observational study. Our survey of infection following caesarean section, like many other hospital-based surveys, was designed to observe existing practices. Consequently compensation for any underestimation must be made using only the data that were actually available. The analysis that follows looks again at the data collected by Moir-Bussy et al., (1984) and attempts to measure the degree of underestimation in the observed infection rate. Variation
in the observed
infection
rates
Table I shovvs the results obtained from each of the 31 hospitals and it is from these data that we wish to derive some estimate of the national level of infection as well as considering the differences between hospitals. The entire sample of 2368 cases contains 141 that met the survey’s criteria and were classified as infected, a rate of just under 6%. Those criteria required that, to be called infected, the wound had to show both inflammation or sepsis and a positive bacteriological culture in material taken from the wound. One graphical way of investigating the differences between hospitals is to show them individually on a probability plot of their
140
J. R. Thompson Table Hospital
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
I.
Vmiation
No. of cases 41 97 137 45 68 101 50 85 95 43 42 109 124 60 83 98 98 147 58 44 39 11 53 27 56 iii 71 60 93 191
et al.
in the observed infection No. with clinically mflammed wounds 6 11 27 10 4 25 6 26 26 16 7 15 20 20 17 14 iii 11 10 1.5 2 6 5 : 12 1 2; 22
standardized deviations from the national expected normal deviates against:
rates No. with measured
bacterlologically infected wounds -3
; 2 1 1 14 4 1 6 1 4 7 10 1 14 2 4 5 2 9 8 1 2 3 3 5 3 1 0 16 5
average rate, that is by plotting
j=l,.
. . 31
where p, and nJ are the proportions infected and the total number of cases in each hospital and p is the overall proportion infected, i.e., O-06. If the variation between hospitals was due to random effects rather than a systematic difference then this plot would produce a straight line. The actual plot is shown as Figure 1 and illustrates very clearly how the hospitals divide into two distinct groups. The larger group consists of 26 hospitals that appear to have very little infection, their average rate is, in fact, only 4%. On the other hand the smaller group of five hospitals has an average infection rate of 17%.
Estimating
infection
141
rates
X 30 x 15 x 20 x 21
kt”
x6
XX
Figure 1. A probability plot of the standardized rates, diagnosed the numbers of the five hospitals with the highest values.
Figure 2. A probability indicates the position
plot of the standardized of a standardized normal.
climcal
bacteriologically,
inflammation
rates.
showing
The
line
142
J. R. Thompson
et al.
There are two possible explanations for the observed division of the hospitals into two groups. It is possible, if unlikely, that five of the hospitals do actually have an infection rate over four times as great as the other 26, or it may be that the majority are underestimating their true infection rates for one of the reasons suggested in the Introduction. Figure 2 shows a probability plot of the standardized inflammation rates, defined as localized erythema and induration, in the same way that Figure 1 shows the standardized infection rates. It will be observed that there are no longer any major divisions within the group of 31 hospitals. If the variation between hospitals were just random then the points would lie close to the straight line shown as part of the plot. Thus, in this case, there is still more variation between hospitals than one would expect by chance, however this variation is considerably less than was observed in the plot of infection rates. Thus despite its subjective element, the observed inflammation rate is much more uniform than the measured infection. Improving
the estimate
of infection
rate
While it is the case that swabs are sometimes sent to the laboratory even when there is no sign of inflammation, our main interest lies in the chance that an inflamed wound has a swab taken. Moir-Bussy et al., (1984) found that the rate at which inflamed wounds had swabs sent to the laboratory averaged at 42%, but that this figure showed considerable variation between hospitals. Figure 3 shows a standardized plot of the rates with which hospitals sent swabs from inflamed wounds. This plot is revealing, for once again the hospitals divide into two groups and the five hospitals with the worst infection rate all appear in the top seven for sending swabs to the laboratory. This information is also open to various interpretations. It could be, for example, that more swabs are sent because there is more infection or that there appears to be more infection because more swabs are sent. To investigate this further Figure 4 looks at the proportion of swabs sent that turned out to be positive against the proportion of inflamed wounds that had a swab sent. One hospital sent no swabs at all, thus ensuring a zero infection rate, and this hospital is excluded from the plot. an observation confirmed by its Figure 4 shows no clear pattern; unweighted correlation which is only 0.05. It thus appears that the proportion of positives does not depend on the rate at which swabs are sent to the laboratories. If one averages over all of the hospitals it is found that 8 1% of swabs show some positive culture. It would appear that the main reason for the observed differences in infection between hospitals is that they have developed different practices for sending swabs. If swabs had been sent for every inflamed case then there would not have been such major differences between hospitals and the observed infection rates would have been much higher.
Estimating
infection
rates
143
X I
-2
I
I
2
4
D
Figure
3.
-4 probability
t.0
plot
of the rates at which
xxx
xx
xx
x
inflamed
wounds
xxxx
X
have swabs sent for
X X
0.6
X
X X X
X
x
X
X X
X
X X x 0.2
X
I 0.2
-
I
I
I
0.0
I.0
I
0.4 Proportm
Figure 4. The proportion of positive wound inflamed wounds that have a swab sent.
0.6 sent
swabs
plotted
against
the proportion
of
144
J. R. Thompson
et al.
Since 81 oio of swabs sent gave a positive result and this does not appear to depend on the proportion sent, a better indication of the true level of infection would be obtained by taking 81% of the inflammation rate. The overall inflammation rate in this survey was 18% suggesting a revised estimate of the national infection rate of about 14%. Conclusions The analysis of these data is highly dependent on the assumptions that one chooses to make. However it would appear that: (a) infection was underestimated in most hospitals because they did not send sufficient swabs for examination, (b) the national infection rate in mothers following caesarean section is probably about 14% as opposed to the measured rate of 6%, rate, even with its subjective element, is a better Cc)the inflammation indicator of the relative levels of infection in any two hospitals than the measured infection rate. References Hawrlshyn, P. A., Bernstein, P. & Rapsin, F. R. (1981). Risk factors associated with caesarean section. Amercian Journal of Obstetrzcs and Gynaecology 139, 294-298. Kettering, H. A. & Walter, D. F. (1977). Complications of caesarean section. Transactions of the Pacific Coast Obstetrics and Gynaecology Society 44. 29. Moir-Bussy, B. R., Hutton, R. M. & Thompson, J. R. (1984). Wound infection after caesarean section. Journal of Hospital Infection 5, 359-370.