Long term trends in the occurrence: RESULTS

23 Jan
2012

There were 2389 episodes of nosocomial blood stream in­fection between August 1, 1986 and December 31, 1996. The annual number of cases gradually rose from 184 in 1987, the first full year of the survey, to 267 in 1996. The case rate simi­larly rose from 6.0/10,000 admissions in 1986 to 8.7/10,000 admissions in 1995 (Figure 1), and abruptly rose to 11.3/10,000 admissions in 1996 (risk ratio [RR] 1.9, 95% CI 1.4 to 2.4 compared with 1986; RR 1.29, 95% CI 1.08 to 1.54 compared with 1995). The nosocomial blood stream infection rate for the entire period was 7.8/10,000 admissions. Using patient days as a denominator, the case rate increased from 4.59/10,000 patient days in 1986 to 14.31 in 1996 (RR 3.1, 95% CI 2.4 to 4.1).

Sources of infection: Table 1 presents the sources of infection for 2389 cases. Primary infection, largely resulting from infec­tion of intravascular devices, was by far the most common source, accounting for 57% of the total. Urinary tract, respira­tory tract and surgical site infections with secondary blood stream infection were a tightly clustered second group of sources, each accounting for about 10% of the total. Primary in­fection increased in frequency from 3.3/1000 admissions in 1986 to 7.5/1000 in 1996 (relative risk 2.3, 95% CI 1.6 to 3.2 compared with 1986; P<0.01). There was an abrupt increase between 1995 and 1996 (relative risk 1.4, 95% CI 1.1 to 1.8; P<0.01). While no other source of infection significantly in­creased in frequency, all secondary sources as a group in­creased from 2.5/1000 in 1986 to 3.8/1000 in 1996 (relative risk 1.5, 95% CI 1.1 to 2.1; P=0.01).


Figure 1) Annual case rate by source

Figure 1) Annual case rate by source of nosocomial blood stream infection, University of Alberta Hospital, 1986 to 1996

Microbial etiology: A total of 2582 different microbial species were identified in the 2389 cases (Table 2). Aerobic Grampositive cocci as a group were most common (61.1%). As the authors have previously reported, aerobic Gram-negative bacilli were less common, accounting for 28% of infections. Yeast and fungi infections were uncommon (6.9%), and were almost entirely caused by Candida species. Anaerobic bacteria were relatively rare causes of nosocomial blood stream infec­tion (3.1%).

Figure 2) Case rate for selected microorganisms

Figure 2) Case rate for selected microorganisms causing nosocomial bloodstream infections, University of Alberta Hospital, 1986 to 1996. 1 Risk ratio (RR) 2.56(1.52 to 4.31) compared with 1986; 2 RR 2.44 (1.45 to 4.11 compared with 1995); 3RR 2.16 (1.11 to 4.18 compared with 1995)

Only three groups of microorganisms increased in preva­lence as a cause of infection between 1986 and 1996: coagulase-negative Staphylococcus species, Enterococcus species and Candida species (Figure 2). The number and case rate for coagulase-negative Staphylococcus species increased nearly every year between 1988 (46 cases, 14.4/10,000 admis­sions) and 1996 (82 cases, 34.5/10,000 admissions) (RR 2.5 compared with 1988, 95% CI 1.5 to 4.3). The number and case rate for the Enterococcus species remained relatively stable between 1986 (5.5/1000 admissions) and 1995 (8.1/1000 ad­missions), but then both significantly increased in 1996 to 19.8/10,000 admissions (RR2.4 compared to 1986, 95% CI 1.5  to 4.1). Similarly, the number and case rate of Candidemia re­mained stable from 1986 (4.8/1000 admissions) and 1995 (5.3/1000) but then increased to 11.4/1000 in 1996, RR 2.2 compared with 1986, 95% CI 1.1 to 4.2. As previously reported, the species responsible for candidemia remained un­changed throughout the survey period. No other microbial species or group (eg, aerobic Gram-negative bacteria, anaero­bic bacteria) significantly changed in annual incidence.

TABLE 1 Sources of infection of nosocomial blood stream infection, University of Alberta Hospital, 1986 to 1996

Source

Number of
episodes of blood stream infection

(%)

Primary

1362 (57.0)

Urinary tract

248 (10.4)

Respiratory tract

237 (9.9)

Surgical site

228 (9.5)

Gastrointestinal tract

176 (7.4)

Skin/burn

102 (4.3)

Female genital tract

14 (0.6)

Other

22 (0.9)

Total

2389 (100)

There were 233 (9.8%) episodes in which more than one or­ganism was identified (polymicrobial bacteremia). Polymicro- bial cases were more likely to be secondary to skin or burn wound infection (8.6%), or of gastrointestinal source (15.1%, P=0.01), and were less likely to be primary (49.6%, P=0.01) or from a surgical site source (4.3%, P<0.01) than monomicrobial infections.
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TABLE 2 Nosocomial blood stream infection, University of Alberta Hospital, 1986 to 1996

Pathogen

Number of
isolates
(%)

Aerobic Gram-negative bacteria


Enterobacteriaceae

Escherichia coli

233 (9.0)

Klebsiella
species

146 (5.6)

Enterobacter
species

99 (3.8)

Serratia
species

39 (1.5)

Other

36

Subtotal

533 (21.5)


Non-Enterobacteriaceae

Pseduomonas
species

114 (4.4)

Acinetobacter
species

14 (0.5)

Haemophilus
species

12 (0.5)

Stenotrophomonas
species

10 (0.4)

Other

19

Subtotal

168 (6.5)

Subtotal aerobic Gram-negative bacteria

723 (28.0)

Aerobic Gram-positive bacteria

Bacilli

24 (0.9)

Cocci

Coagulase-negative staphylococcus

686 (26.6)

Staphylococcus
aureus

476 (18.5)

Enterococcus
species

235 (9.1)

Streptococcus
species

168 (6.5)

Subtotal

1573 (61.1)

Anaerobic bacteria

Bacteroides
species

48 (1.9)

Other

32

Subtotal

80 (3.1)

Other bacteria

4 (0.2)

Yeast and fungi

Candida
species

165 (6.4)

Other

13 (0.5)

Subtotal

178 (6.9)

Total

2582

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