Yonago Acta medica 2014;57:159–165

Original Article

Positive Predictive Value of True Bacteremia according to the Number of Positive
Culture Sets in Adult Patients
Tsuyoshi Kitaura,* Hiroki Chikumi,*† Hiromitsu Fujiwara,‡ Kensaku Okada,* Tatsuya Hayabuchi,* Masaki
Nakamoto,*† Miyako Takata,* Akira Yamasaki,* Tadashi Igishi,* Naoto Burioka* and Eiji Shimizu**Division of Medical Oncology and Molecular Respirology, Department of Multidisciplinary Internal Medicine, School of Medicine,
Tottori University Faculty of Medicine, Yonago 683-8504, Japan, †Center for Infectious diseases, Tottori University Hospital, Yonago
683-8504, Japan and ‡Division of Clinical Laboratory, Tottori University Hospital, Yonago 683-8504, Japan

ABSTRACTthe clinical significance of culture results is challenging
because of possible culture contamination. Therefore,
methods to help differentiate true bacteremia from contamination are required.
The identification of isolated bacteria from positive
blood cultures is a helpful clue for estimating the probability of true bacteremia. This probability, which isoften expressed as the positive predictive value (PPV), isreported to vary by microorganism. For example, when
Streptococcus pneumoniae, Escherichia coli, othermembers of Enterobacteriaceae, Pseudomonas aeruginosa and Candida albicans are isolated from blood
culture, more than 90% are reported to be true bacteremia or fungemia.4–7 On the other hand, when coagulasenegative Staphylococci (CNS), Corynebacterium species
and Bacillus species are isolated, the probability of truebacteremia is reported to be low.4–10 However, most ofthese studies were performed in the 70s through the 90s.
Therefore, updated data in current patient populations
and treatment methods are needed, because the ratio andcausative bacteria of bloodstream infections might havechanged over time as a result of the increasing use of
intravascular devices and number of patients receiving
immunomodulatory treatment.11, 12
Another method for distinguishing true bacteremia
from contamination is using the number of blood culture sets (each set of blood culture consists of an aerobic
and anaerobic bottle containing different nutrients) thatgrow bacteria from among the performed culture sets.
Opportunities to adopt this methodology are increasing,
because the recently approved Clinical and Laboratory Standards Institute (CLSI) guideline recommends
more than 2 sets of blood culture should be performed
simultaneously (within minutes of one another) in patients suspected blood stream infection.13 However, it isoften difficult to interpret the results when only 1 of the

Background

Performing multiple blood culture sets
simultaneously is a standard blood culture methodology,
although it is often difficult to distinguish true bacteremia from contamination when only one of several blood
culture sets is positive. This study clarified the relationship between the number of positive blood culture sets
and clinical significance in patients with positive blood
culture.
Methods Patients aged 18 years and over with at least
1 positive blood culture were enrolled. Positive blood
culture episodes were categorized from clinical records
as true bacteremia, contamination, or unknown clinicalsignificance. The associations among episodes of truebacteremia, isolated bacteria, the number of positive
blood culture sets from among the performed sets, andthe clinical background of patients were analyzed.
Results Among a total of 407 episodes, 262, 67 and
78 were true bacteremia, contamination and unknown
clinical significance, respectively. The positive predictive values (PPVs) of 1 out of 1, 1 out of 2 and 2 out of
2 positive sets in cases of Staphylococcus aureus, were
81.3%, 50% and 100% respectively; those in cases ofcoagulase-negative Staphylococci were 20.5%, 10.8%
and 63.5%, respectively. Almost all cases of Escherichia
coli, Pseudomonas aeruginosa, Klebsiella species andCandida species were true bacteremia. The probabilityof true bacteremia was strongly associated with recent
surgery in multivariate analysis (P < 0.05).
Conclusion The probability of true bacteremia basedon the number of positive culture sets from among theperformed sets varies by microorganism. Therefore,
PPVs calculated using this method may help physicians
distinguish true bacteremia from contamination.
Key words

bacteremia; blood culture; contamination;
positive predictive value

Corresponding author: Hiroki Chikumi, MD, PhD
chikumi@grape.med.tottori-u.ac.jp
Received 2014 September 25
Accepted 2014 November 17
Abbreviations: CLSI, Clinical and Laboratory Standards Institute;
CNS, coagulase-negative Staphylococci; PPV, positive predictive
value

Bacteremia is often fatal and requires early diagnosis
and treatment with adequate antimicrobial agents.1, 2
Blood culture is regarded as the gold standard for the
diagnosis of bacteremia.3 However, accurately judging
159

T. Kitaura et al.

2 blood culture sets is positive. In such cases, the results
should be interpreted cautiously according to the identityof the isolated bacteria. In accordance with this strategy,
the PPV of positive blood culture according to the number of positive blood culture sets has been examined for
CNS.7, 14 However, data about other microorganisms are
rare.
Therefore, this study aimed to provide information
to facilitate the interpretation of positive blood culturesby analyzing the clinical significance of isolated microorganisms, the number of positive blood culture setsfrom among the performed sets, and clinical backgroundcharacteristics of patients in current clinical settings..

on the basis of the following factors as described by
Weinstein et al.:7 clinical history, physical findings, vital
signs (i.e., body temperature, blood pressure and heartrate) at the time of blood culture, white blood cell count,repeated isolation of the same bacteria at intervals morethan 24 h, time until positive blood culture result, resultsof cultures from other sites, imaging results, histopathologic findings, clinical course, response to antimicrobial
therapy, presence of a plausible source and clinical manifestations. The number of positive blood culture bottles
within a blood culture set was not used as a criterion todifferentiate true bacteremia from contamination in thepresent study, because previous studies have reported on
its inefficacy.8, 15

SUBJECTS AND METHODSPatients and blood culture
Patients aged 18 years and over with at least one positive blood culture from April 1, 2007 to March 31, 2010
at Tottori University Hospital—a 700-bed tertiary-carefacility—were evaluated. Blood was collected by thedoctors, residents, or nurses and inoculated into standard
aerobic and anaerobic bottles (BacT/ALERT FA and FN;
Biomerieux, Durham, NC). One pair of these culture
bottles was defined as 1 blood culture set. The bottles
were incubated in the BacT/ALERT 3D System (Organon
Teknika, Durham, NC) until flagged as positive or for
7 days at 37 °C. At least 2 sets of blood cultures wererecommended to be obtained simultaneously or in rapid
succession from separate vein puncture sites. However, 2
sets performed on the same day (at intervals of up to 24h) were considered contiguous. Susceptibility testing ofisolated bacteria was performed according to the CLSI
guidelines. The significance of positive blood culture
was judged retrospectively on the basis of clinical records. This study was conducted according to the Ethical Guidelines for Clinical Studies of the Ministry of
Health, Labour and Welfare, Japan, and patient consent
was not required as this was a retrospective case notereview that involved anonymized data.

Clinical background characteristics
The principal investigator reviewed the medical records
and recorded the following clinical characteristics thatwere possibly related to true bacteremia on a standardized form: demographic variables, renal function,
immunosuppressed conditions, presence of vascularcatheter, location in the intensive care unit at blood culture collected, preceding antibiotic use, recent surgery,recent trauma and nosocomial acquisition. Preceding
antibiotics use was defined as the use of any antibiotics
at the time of blood culture collection. Recent surgery ortrauma was defined as surgery or trauma within 30 days
before blood culture collection. Each episode was classified as community or nosocomial acquired according
to the guidelines of the Centers for Disease Control andPrevention.16 An episode in a patient from another hospital or nursing home was always classified as nosocomialacquired.
Statistical analysis
PPV—which is the probability that an isolated organism would be expected to be a true pathogen rather thana contaminant—was calculated as the number of truebacteremic episodes divided by the number of positive
blood culture episodes with the organism. Univariate
analysis was performed to explore the association between the collected clinical characteristics and presenceof true bacteremia. Categorical variables were assessedby the chi2 test or Fisher’s exact test. Continuous variables were analyzed using Student’s t-test. Variables
significant in univariate analysis were entered into themultivariate analysis, which was performed by logistic
regression. All analyses were performed with SPSS version 19.0 (SPSS Inc., Chicago, IL), the level of significance was set at P < 0.05.

Definitions
An episode of bacteremia or fungemia was defined as
the first positive blood culture result or a new positive
blood culture result that occurred more than 48 h after
the previous positive result unless it was clear to the investigator that the new positive culture blood was part ofthe same episode.
The clinical significance of a positive blood culture
was determined by 1 infectious disease specialist and 2
residents on the basis of clinical records. Clinical significance was categorized as true bacteremia, contamination or unknown significance. Assessments were made160

Positive culture sets in bacteremia
Table 1. Microorganisms isolated from the blood of patients suspected of bacteremia at Tottori University Hospital from April 2007 through March 2010
Microorganism (episode)Aerobic and facultative bacteria
Gram positive		
Staphylococcus aureus (80)		Cagulase-negative staphylococci (92)		
Bacillus species (30)		Enterococcus species (26)		Corynebacterium species (4)		Streptococcus pneumonia (6)		Viridans streptococci (4)		Other streptococci (6)		Other gram positive bacteria (5)Gram negative		
Escherichia coli (39)		
Klebsiella pneumonia (19)		
Klebsiella oxytoca (5)		Psudomonas aeruginosa (19)		Enterobacter cloacae (8)		Serratia marcessens (7)		Proteus mirabilis (2)		Stenotrophomonas maltophilia (2)		Acinetobacter species (5)		Citrobacter species (4)		Other gram negative bacteria (7)Anaerobic bacteria
Bacteroides fragilis (10)Fusobacterium necrophorum (2)Other gram positive bacteria (8)Other gram negative bacteria (2)Fungi
Candida albicans (5)Other Candida species (10)All Microorganisms (407)

True bacteremia (%)

Contamination (%)

Unknown (%)

67 (83.8)
20 (21.7)
14 (46.7)
15 (57.7)
0
6 (100)3 (75)
3 (50)02 (2.5)50 (54.3)
10 (33.3)
1 (3.8)2 (50)00
0
2 (40)

11 (13.8)
22 (23.9)
6 (20)
10 (38.5)
2 (50)
0
1 (25)
3 (50)
3 (60)

35 (89.7)
16 (84.2)
4 (80)
17 (89.5)
7 (87.5)5 (71.4)
2 (100)2 (100)4 (80)
4 (100)5 (71.4)

0
0
0
0
0
0
0
0
0
0
0

4 (10.3)
3 (15.8)
1 (20)
2 (10.5)
1 (12.5)
2 (28.6)
00
1 (20)
02 (28.6)

10 (100)
2 (100)7 (87.5)
1 (50)

0
0
0
0

0
0
1 (12.5)
1 (50)

5 (100)8 (80)
262 (64.3)

00
67 (16.5)

02 (20)
78 (19.2)

RESULTS
Among a total of 407 episodes, 262, 67 and 78 were truebacteremia or fungemia, contamination and unknown
clinical significance, respectively. First, we examined
the frequency of each microorganism and the relationships between their identity and clinical significance. As
shown in Table 1, the most common microorganisms
isolated from blood culture were CNS; only 21.7% (20/92)
of these episodes were true bacteremia. Nevertheless,
CNS was the third most frequent causative pathogenof true bacteremia because of its high prevalence. The
second most frequently isolated microorganism fromblood culture was Staphylococcus aureus; 83.8% (67/80)
of episodes were true bacteremia, making it the mostcommon cause of true bacteremia. All 6 Streptococcus
pneumoniae isolates were pathogens for true bacteremia.
The frequency of isolation and true bacteremia of othergram-positive bacteria were relatively low. Escherichia
coli was the third most common microorganism isolated
from blood culture and the most frequently isolated

gram-negative bacteria. Almost all episodes involving
gram-negative bacteria including Escherichia coli weretrue bacteremia and none were contamination. Anaerobic bacteria such as Bacteroides fragilis or fungi isolated
from blood culture represented true bacteremia; no episodes were contamination. Fungi isolated from blood
culture were all Candida species; Candida albicans was
frequently isolated more than other Candida species.
As mentioned above, the CLSI guidelines recommend multiple blood culture sets be performed simultaneously, but it is often difficult to determine if an isolate
is the true pathogen of bacteremia or a contaminant
when only 1 culture set is positive. Knowing the probability of true bacteremia in such situations, i.e., the PPV,is critical. Therefore, we subsequently examined therelationship between the number of positive culture sets
and the probabilities of their clinical significance regarding major isolated microorganisms (Table 2). In the caseof Staphylococcus aureus, the PPVs of a positive culturein 1 positive set out of 1 and 2 positive sets out of 2 were161

T. Kitaura et al.
Table 2. Clinical significance of each microorganism according to the number of positive blood culture sets
Microorganism (episode)
Staphylococcus aureus (80)Cagulase-negative staphylococci (92)
Bacillus species (30)Enterococcus species (26)
Escherichia coli (39)
Klebsiella species (24)Psudomonas aeruginosa (19)Candida species (15)

Number of culture sets
Positive Performed

True bacteremia

Contamination

Unknown

1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2

39
4
24
9
4
7
4
3
7
9
2
4
14
9
12
8
2
10
9
4
3
8
0
5

2
0
0
24
24
2
4
6
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0

7 (12.1%)
4 (50%)
0 (0%)
11 (25%)9 (24.3%)
2 (18.2%)
2 (20%)
2 (18.2%)
2 (22.2%)
5 (33.3%)
2 (50%)
3 (42.9%)
2 (12.5%)
2 (18.2%)
0 (0%)
3 (27.3%)
1 (33.3%)
0 (0%)
1 (10%)
0 (0%)
1 (25%)
1 (11.1%)
0 (0%)
1 (16.7%)

1
2
2
1
2
2
1
2
2
1
2
2
1
2
2
1
2
2
1
2
2
1
2
2

(67.2%)(50%)(100%)(20.5%)(10.8%)(63.6%)(40%)(27.3%)(77.8%)(60%)(50%)(57.1%)(87.5%)(81.8%)(100%)(72.7%)(66.7%)(100%)(90%)(100%)(75%)(88.9%)(0%)(83.3%)(3.4%)(0%)(0%)(54.5%)(64.9%)(18.2%)(40%)(54.5%)(0%)(6.7%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)
(0%)

Table 3. Relationship between clinical background and significance in patients with positive blood cultures
True bacteremia (n = 262)

Contamination (n = 67)

Sex						Male167 (63.7%)
39 (58.2%)
Female95 (36.3%)
28 (41.8%)
Age, years
Mean ± SD
68.1 ± 14.7
70.0 ± 14.3
Range
17–94
22–89
eGFR, mL/min/1.73m2					< 10.011 (5.3%)
2 (4.2%)
10.0–49.9
54 (26.1%)
11 (22.9%)50.0 ≤
143 (68.6%)
35 (72.9%)
No date54		
19
ImmunosuppressedDM
79 (30.2%)
12 (17.9%)
Malignant tumor
132 (50.4%)
31 (46.3%)
Corticosteroid use
36 (13.7%)
10 (14.9%)
Immunosuppressor (except corticosteroid) use
4 (1.5%)4 (6.0%)Any vascular catheter present (except arterial catheter)
183 (69.8%)
42 (62.7%)
Peripheral IV catheter present
126 (48.1%)
22 (32.8%)
CV catheter present
80 (30.5%)
22 (32.8%)
CV port system present
19 (7.3%)2 (3.0%)Location in ICU
48 (18.3%)
12 (17.9%)
Preceding antibiotics use
97 (37.0%)
33 (49.3%)
Recent surgery
54 (20.6%)
4 (6.0%)Recent trauma
9 (3.4%)2 (3.0%)
Nosocominal acquisition
204 (77.9%)
50 (74.6%)
CV, Central venous; DM, diabetes mellitus; eGFR, estimated glomerular filtration rate; ICU, Intensive care unit; IV, intravenous.
† Chi2 test.
‡ Fisher’s exact test.
§ t-test.

162

P
0.40†

0.36§
0.85†

0.046†
0.55†
0.80†
0.057‡
0.26†
0.025†
0.72†
0.16‡
0.94†
0.068†
0.010†
0.61‡
0.57†

Positive culture sets in bacteremia
Table 4. Logistic regression analysis of factors associated with true bacteremia and contamination
Factors

Odds raio

P

DM
Peripheral IV catheter present
Recent surgery

1.91
1.65
3.74

0.065
0.090
0.015

previous antibiotic use, recent trauma or nosocomialacquisition. Finally, the significant clinical factors in theunivariate analysis were entered into the multivariate
model (Table 4). However, the presence of recent surgery
was the only independent risk factor for true bacteremia(odds ratio: 3.74, P < 0.05).

DM, diabetes mellitus; IV, intravenous.

DISCUSSION
This study investigated the clinical significance of isolated microorganisms, the number of positive blood
culture sets from among the performed culture sets, andpresence of predisposing factors in order to facilitate theinterpretation of positive blood culture results. In particular, this study is the first to report the probabilities oftrue bacteremia and contamination based on the numberof positive culture sets with microorganisms besides
CNS. Our data suggest that taking into account the number of positive blood culture sets in combination with
the identity of isolated bacteria and clinical backgroundcharacteristics is helpful for distinguishing between truebacteremia and contamination.
In this study, we revealed that causative pathogensof true bacteremia in order of frequency were Staphylococcus aureus (25.6%), Escherichia coli (13.4%), CNS(7.6%), Pseudomonas aeruginosa (6.5%) and Klebsiella pneumoniae (6.1%). These frequencies have beenchanged over time. In the middle of 1970’s, the causative
pathogen of true bacteria were reported that Escherichia
coli (16.3%), Staphylococcus aureus (10%), Streptococcus pneumoniae (6.3%), Klebsiella pneumoniae (6.3%)
and Pseudomonas aeruginosa (5.3%).6 The same authors analyzed the data in 1990’s and reported the frequencies as Staphylococcus aureus (18.9%), Escherichia
coli (15%), CNS (9.2%), Klebsiella pneumoniae (6.9%)
and Enterococcus species (6.9%).7 In the beginning of
2000’s, Pien et al. reported the frequencies as Staphylococcus aureus (23.1%), Escherichia coli (12.5%), Enterococcus species (9.4%), Klebsiella pneumoniae (8.2%)
and CNS (7.7%).4 Comparing these historical data withours, the emerging increase of Staphylococcus aureus
and CNS as causative pathogen of true bacteremia is obvious. The increasing use of intravascular catheters anddevises, in the modern medical settings may contributeto this drastic change. Therefore, we need to carefullyinterpret the clinical significance of these bacteria, when
they isolated from blood cultures.
Previous studies have reported that the probability of true bacteremia or contamination depends on
the identity of the isolated bacteria, and the data of thepresent study are consistent with those of other studies. Moreover, previous studies analyzed the data fromthe middle of 1970’s to 1990’s indicate that 90–100%

81.3% and 100%, respectively. Meanwhile, the PPV in 1
positive set out of 2 was only 50%. In the case of CNS,
a representative bacteria of the normal flora of the skin,
the PPVs of 1 positive set out of 1 and 2 positive sets outof 2 were 20.5% and 63.6% respectively. However, when
1 out of 2 sets was positive, the PPV was only 10.8%
and almost all cases were contamination. In the case of
Bacillus species, which are also part of the normal floraof the skin, the PPVs of 1 positive set out of 1, 2 positive
sets out of 2 and 1 positive set out of 2 were 40%, 77.8%
and 27.3%, respectively. In the case of Enterococcus
species, the respective PPVs were 60%, 50% and 57.1%.
Regarding gram-negative bacteria such as Escherichia
coli, Klebsiella species and Pseudomonas aeruginosa as
well as fungi, no cases were considered contamination
irrespective of the number of positive culture sets, andalmost all cases represented true bacteremia.
The inclusion of clinical information facilitates theinterpretation of positive blood culture results. Therefore,
we subsequently evaluated the associations between
clinical background characteristics and the existenceof true bacteremia (Table 3). There were no significant
differences in sex, age or estimated glomerular filtration
rate between cases of true bacteremia and contamination. Regarding immunosuppressive conditions, theprevalence of diabetes was significantly higher in truebacteremia than contamination (30.2% versus 17.9%,
respectively, P < 0.05). The presence of malignancy
and corticosteroid and immunosuppressor use were notsignificantly different between groups. An intravenous
catheter was introduced in 69.8% and 62.7% of cases oftrue bacteremia and contamination, respectively, and theindwelling peripheral intravenous catheters were more
frequently observed in cases of true bacteremia than incases of contamination (48.1% versus 32.8%, respectively, P < 0.05). However, such significant differenceswere not observed with central venous catheters andcentral venous port systems. With regard to other clinical factors, the patients with true bacteremia were more
likely to have recent surgery, compared with those withcontamination (20.6% versus 6.0%, respectively, P <0.05). There was no significant difference between the 2groups with respect to location in the intensive care unit,
163

T. Kitaura et al.

of cases isolated of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa or Candida species
are true bacteremia.4, 6, 7 In the present study, no episode
was judged to be due to contamination. Therefore theprobabilities of true bacteremia of these bacteria havenot changed over time. On the other hand, the previous
studies indicate that 6–12.4% of cases of CNS were bacteremia,4, 6, 7 in the present study, the probabilities were
obviously increased to 21.7%. Thus, the results indicatethat the probability of true bacteremia based on the identity of the isolated bacteria can be changeable with thetimes and should be estimated on the bases of updated
data in current patient populations.
Although it is recommended that at least 2 setsof blood cultures taken from separate vein puncturesites be obtained,13, 18 there is little available information to aid the interpretation of the clinical significanceof positive blood cultures, especially in cases in which
only 1 of several blood culture sets is positive. To ourknowledge, CNS is the only microorganism whose PPV
has been determined on the basis of the number of positive blood culture sets from among the performed sets.
Weinstein et al. report that the PPVs of 1 and 2 positive
sets out of 2 are 2.2% and 60.0%, respectively;7 in theirstudy, 2 blood culture sets were performed within 48
h but the interval between each blood culture was not
documented. Meanwhile, Beekmann et al. report that
the PPVs of 1 and 2 positive sets out of 2 were 13% and69%, respectively;14 their data were drawn from blood
culture sets that were performed within 5 days of the index positive blood culture. In the present study, the PPVsof 1 and 2 positive sets out of 2 were 10.8% and 63.3%,
respectively, in cases involving CNS in 2 blood culturesets performed simultaneously. As the recent guideline
recommends blood cultures be obtained simultaneously
or in rapid succession (< 24 h),13 the present findings
provides consistent evidence of the PPV for CNS bacteremia.
The present study revealed that when gram-negative
bacteria such as Escherichia coli, Klebsiella species
and Pseudomonas aeruginosa or Candida species are
isolated, there is no difference in the PPV regardlessof the number of positive culture sets from among theperformed sets. Therefore, these bacteria should be considered true pathogens if isolated from at least 1 blood
culture set. In the case of Staphylococcus aureus, thepresent results highlight the importance of considering
the number of positive culture sets when interpreting theclinical significance of the findings, because the PPVof blood culture varied greatly depending on number of
positive culture sets. Therefore, when Staphylococcus
aureus is isolated from blood culture, especially if 1 outof 2 sets is positive, the clinical significance must be
judged very cautiously. Regarding other gram-positivebacteria such as Bacillus species and Enterococcus
species, the PPV of 1 positive set out of 2 was comparatively low; even 2 positive sets out of 2 does not always
indicate true bacteremia for these bacteria. The present
study shows that the PPV of blood culture based on thenumber of positive culture sets varies with respect to theisolated microorganism. Therefore, the approach utilizing the number of positive culture sets from among theperformed sets must be combined with the identity of
the isolated bacteria; thus, the probability of true bacteremia should be interpreted cautiously according to theisolated bacteria.
The present study shows that recent surgery is anindependent risk factor for true bacteremia. This result
is similar to that of Marschall et al., who report that ahistory of urogenital surgery is associated with urinarysource bacteremia in patients with Escherichia coli bacteriuria.19 In addition to the invasion of bacteria from thewound site, we speculate that immunosuppression due
to the stress of surgery is related to bacteremia. Previous experimental and clinical studies provide evidence
indicating surgery decreases both cell-mediated andhumoral immunity, affecting the host immune system.
Major surgical invasion is reported to depress the functions of monocytes and20, 21 macrophages22, 23 as well as
decrease T-lymphocyte proliferation and cytokine secretion.24 Furthermore, the decreased interleukin-2 production by T lymphocytes as a result of surgical invasion
is suggested to decrease B-cell antibody production.25
Eventually, considering clinical background characteristics such as the presence of recent surgery may help
distinguish between true bacteremia and contamination,
especially in cases in which the probability of true bacteremia is ambiguous after considering the number of
positive blood culture sets from among the performedsets.
In conclusion, when utilizing the number of positive
blood culture sets to distinguish true bacteremia fromcontamination, the variation in the probability of truebacteremia by the microorganism must be considered.
This study demonstrates that PPV calculated on the basis of the number of positive culture sets from among the
preformed sets according to the identity of the isolated
bacteria is helpful for interpreting the clinical significance of positive blood cultures.
The authors declare no conflict of interest.

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Positive culture sets in bacteremia
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