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Volume 10, Number 8—August 2004

Research

Antimicrobial Drug Use and Methicillin-resistant Staphylococcus aureus, Aberdeen, 1996–2000

Dominique L. Monnet*, Fiona M. MacKenzie†Comments to Author , José María López-Lozano‡, Arielle Beyaert§, Máximo Camacho§, Rachel Wilson†, David Stuart†, and Ian M. Gould†
Author affiliations: *Statens Serum Institut, Copenhagen, Denmark; †Aberdeen Royal Infirmary, Aberdeen, Scotland; ‡Hospital Vega Baja, Orihuela (Alicante), Spain; §University of Murcia, Murcia, Spain

Main Article

Table 4

Estimated multivariate Polynomial Distributed Lag (PDL) model for monthly %MRSA (R2=0.902)a

Explaining variable Lag (mo.) Direct effectb
Indirect effectc
Sum of both effectsd
Coeff T-stat p Coeff Coeffe T-stat p
%MRSA
1
0.420
3.96
0.0003




Macrolide use








Each month
1
0.083



0.083
4.02
0.0003

2
0.055


0.035
0.090
5.34
<0.0001

3
0.027


0.038
0.065
6.02
<0.0001

4



0.027
0.027
3.16
0.003
Overall
1–3
0.165
4.02
0.0003





2–4



0.100




1–4




0.265


Third-generation cephalosporin use








Each month
4
0.116



0.116
2.75
0.009

5
0.087


0.049
0.136
3.27
0.002

6
0.058


0.057
0.115
3.70
0.0007

7
0.029


0.048
0.077
3.91
0.0004

8



0.032
0.032
2.75
0.009
Overall
4–7
0.290
2.75
0.009





5–8



0.186




4–8




0.476


Fluoroquinolone use








Each month
4
0.170



0.170
3.43
0.002

5
0.085


0.071
0.156
3.37
0.002

6



0.066
0.066
2.31
0.03
Overall
4–5
0.255
3.43
0.002





5–6



0.137




4–6




0.392


Constant –36.7 –4.42 0.0001

aMRSA, methicillin-resistant Staphylococcus aureus.
bPast %MRSA as well as past use of these three antimicrobial drug classes had direct effects on %MRSA. These direct effects diminished the longer the lag time.
cBecause every increase in %MRSA by the value 1 was followed the next month by a significant increase in %MRSA by the value 0.420, use of the three antimicrobial drug classes also had indirect effects on the %MRSA. As 0.420 is <1, these indirect effects necessarily vanished over time. As an example, decreasing indirect effects are only presented for a few months. There were substantial indirect effects of macrolide use up to month 8 (final coefficient for sum of both effects = 0.284), of third-generation cephalosporin use up to month 12 (final coefficient for sum of both effects = 0.499), and of fluoroquinolone use up to month 11 (final coefficient for sum of both effects = 0.440).
dEach month, the total effect of each class of antimicrobial on the %MRSA resulted from the sum of the direct and indirect effects.
eThe estimated coefficients indicate the values by which the %MRSA would increase in response to an increase in 1 DDD per 1,000 patient-days for each of the three significant antimicrobial classes, when all other variables remain constant. Since the average figure for monthly patient-days at Aberdeen Royal Infirmary is 22,800, 10 DDD per 1,000 patient-days correspond to approximately 230 DDD per month or thirty 7- to 8-day antimicrobial courses. For example, an increase in macrolide use by 10 DDD per 1,000 patient-days on a certain month, or 30 more patients treated with a macrolide as compared with the previous month, would lead to a direct increase in %MRSA by 0.83, 1 month later, by 0.55, 2 months later and by 0.27, 3 months later. The total direct effect would therefore be evident after 3 months, amounting to an increase in %MRSA by the value 1.65. Additionally, %MRSA indirectly attributable to macrolide use would increase by the value 0.35 (i.e., 0.83 x 0.42) after 2 months and by 0.38 (i.e.. [0.83 x 0.42] + [0.55 x 0.42]) after 3 months. From the 4th month onwards, there would be no direct effect of macrolide use on the %MRSA, only ever-decreasing indirect effects that would practically disappear after 8 months (decreasing effects in months 5 to 8 not shown).

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