Evaluation of three methods for culturing long intravascular catheters

Journal

of Hospital

Infection

Evaluation

14, 183-191

of three methods for culturing intravascular catheters

K. G. Kristinsson, Department

(1989)

Isobel

A. Burnett

and R. C. Spencer

of Bacteriology, Royal Hallamshire Shefield SlO 2_3%, UK Accepted for publication

ion

25 April

Hospital,

Glossop Road,

1989

Summary: During a one-year period, 236 intravascular catheter tips were investigated by culturing the outside by rolling the tips on the surface of blood agar plates, the inside by flushing with nutrient broth, and finally by culture after ultrasonication. Clinical information was collected prospectively by visiting the ward and symptoms and signs of catheter infection were correlated with the results of the catheter cultures. Coagulase-negative staphylococci were the organisms that were most frequently isolated from the catheters. The results of culture showed a clear bimodal distribution, and counts of 100 colony-forming units (cfus) and culture from the inside of the catheters were the best predictors of infection. Almost all infected catheters were colonized both on the inside and outside. Keywords: Intravenous negative staphylococcus.

catheters;

catheter-associated

infections;

coagulase-

Introduction

Central venous catheters play an essential role in the management of many acutely ill patients. However, their frequent use has meant a significant increase in the number of hospital-acquired bacteraemias (Eykyn, 1984). The most frequent organism causing catheter-associated bacteraemia is the coagulase-negative staphylococcus (Christensen et al., 1982, Weightman et al., 19SS), and because this organism is a common blood culture contaminant the diagnosis of bacteraemia can be difficult, and acutely ill patients may have other reasons for their pyrexia. A comparison of quantitative blood cultures taken through a peripheral vein and through the catheter itself can help to resolve this difficulty (Weightman et al., 1988), but such a system may be difficult to set up in many hospitals. The diagnosis is therefore retrospective in most cases, and is based upon semiquantitative or quantitative cultures of the catheter and the clinical response after removal. Correspondence Box 1465, 121 019X-6701/X9/070183

to: Dr. Reykjavik,

Karl. G. Iceland.

Kristinsson,

Department

+09 $03.00/O

of Microbiology,

University

of Iceland,

0 1989 The Hosp~ral

183

lnfemon

P.O.

Society

184

K. G. Kristinsson

et al.

Most laboratories use the method developed by Maki, Weise & Sarafin (1977) for culturing catheters, despite their study being based mainly upon the culture of short peripheral cannulas. By this method, the outside of the catheters is cultured semiquantitatively. Linares et al. (1984) studied 135 subclavian catheters used for total parenteral nutrition and concluded that hub colonisation was the primary source of catheter-related sepsis, and of the 20 cases with catheter-related sepsis two were negative by the method of Maki et al. (1977). This led them to suggest that culturing the inner surface of long catheters may be necessary. Since there is no consensus as to which method to use for the culturing of long intravascular lines nor upon the number of organisms considered significant, we undertook a prospective study with the aim of correlating three culture methods with symptoms and signs of catheter infection. Materials

and

methods

All catheter tips from long intravascular lines received by our department during a twelve-month period were investigated. Information about the patient was obtained by visiting the ward, discussions with medical and/or nursing staff and examination of the case notes. The reason for catheter removal was recorded, i.e. no longer needed, blocked or considered infected. If the catheter was considered likely to be infected, but there were other possible causes for the pyrexia, it was classified as probably infected. In cases where catheter infection could not be excluded, but was considered unlikely, it was classified as probably not infected. Catheter sepsis was defined as bacteraemia where the same organism was isolated from the peripheral venous and also the intravascular line blood cultures. Catheter cultures Long intravascular catheters were received in sterile universal containers and processed as follows. (1). Semiquantitative culture of the catheter surface as described by Maki et al. (1977). Briefly, using sterile forceps, the catheter was rolled or smeared back and forth across the surface of a blood agar plate at least four times using a slight downward pressure. (2). Quantitative culture of the inside of the catheters. Using the same forceps the catheter was removed from the blood agar plate and a sterile Pasteur pipette inserted into one end. The other end was dipped under the surface of 5 ml of nutrient broth which was flushed through the catheter three times. A 1 ml aliquot of that broth was then mixed with 10 ml of molten nutrient agar and poured into a petri-dish. (3). Culture after ultrasonication. After flushing, the catheter tip was put into 10 ml of phosphate buffered saline (PBS) and ultrasonicated for 3 min (12 m peak to peak, MSE ultrasonicator). A 1 ml aliquot of this solution was then mixed with molten nutrient agar and poured into a petri-dish.

Culturing

intravascular

catheters

185

After 24 to 48 h incubation at 37°C in CO, the colonies were counted on all the plates. The extent of growth was classed as one of the following: no growth (N.G.); 1 to 15 colonies; 16 to 50 colonies; 51 to 99 colonies; 100 to 999 colonies and 2 1000 colonies. The organisms were identified by routine laboratory methods. Statistical methods Culture results were contingency tables.

compared

with

the

chi-square

test using

2X2

Results

Clinical findings The total number of intravascular catheters received during this period was 236. The average patient age was 53 years with a median age of 56 (range 2 to 92). Most of the catheters were labelled as central venous pressure lines (136). There were also subclavian catheters (44), long lines (26), Hickman catheters (17) and others (13, including Swan-Ganz catheters). Blood agar roll cultures were performed on 229 catheters, culture of the inside by flushing on 222 and culture after ultrasonication on 219. For 209 of the catheters it was possible to obtain relevant clinical information about the patient, of which 31 catheters (15%) were considered infected, 16 (8%) probably infected, 20 (9”/0) probably not infected and 142 (68%) not infected. Information about the major underlying disease was noted for 191 of the cases. Most of these patients had had a recent operation on the gastrointestinal tract (42), although renal failure accounted for an almost equal number of cases (40), many of whom were being haemodialysed through a subclavian catheter. Twenty two had undergone operation on the cranium w-hile 18 were being treated for head trauma.

Culture j&dings By far the most frequent organisms to be isolated from the catheters were coagulase-negative staphylococci: from 63% of 52 infected/probably infected lines and 63% of 118 not infected/probably not infected lines (Table I). Mixed cultures were significantly more common in lines that were not or probably not infected (x2= 22.38, DF= 1, p 100 cfus except one case (no. 4) where the patient was receiving antistaphylococcal antibiotics intravenously at the time. Fourteen patients were judged to have exit-site infections. The most frequent organism isolated in these cases was again the coagulase-negative

186

K. G. Kristinsson

Table

I. Microorganisms

isolated

from

Microorganisms

Infected (n= 36)

Coagulase-negative staphylococci Staphylococcus aureus Coryneforms Enterobacteriaceae Enterococci Candida albicans Viridans streptococci Micrococci Bacillus spp. *Numbers

the catheter

in parenthesis

number

with

surfaces

according

Clinical

classification

Probably infected (n= 16)

20(6)* 7(l) 4(2) 3(l) l(l) l(O) indicate

et al.

Probably not infected (n = 22)

13(l) -

classification

Not

infected (n = 96)

62(28) 5(4) 7(6) 3(2) 10)

12(7) l(1)

w WI

3(O) mixed

to clinical

l(l)

2%) 2w

7?7) 7(7) 4(4)

-

cultures.

staphylococcus (8), followed by coryneforms, Staphylococcus aureus and coliforms (2 each). None of the patients had tunnel infections. Figure 1 shows the results of culture for each method used. Colony counts are clearly bimodally distributed with most catheters having either < 15 cfus or > 100 cfus. This bimodal distribution was most apparent for results obtained from flushing the inside of the catheters, only 8 catheters out of 222 having 16 to 100 cfus. The inside of the catheters was significantly more often sterile than the outside (138 of 222 vs. 107 of 229, x2= 10.8, DF=l, p
Table

II.

Results

of culture

inpatients

with catheter-related method of culture

sepsis according

Colony

counts

to catheter

type

(cfus)

Case No.

Catheter

1. 2. 3. 4. 5. 6.

Hickman Hickman Hickman Hickman CVP line CVP line

S. S. S. S. S. S.

epidermidis epidermidis epidermidis aureus aureus haemolyticus

100-999 > 1000 > 1000 51-99 100-999 100-999

100-999 > 1000 > 1000 l-15 > 1000 100-999

100-999 > 1000 > 1000 51-99 >lOOO 1 O&999

I:

Hickman

S. Candida simulans albicans

> 1000

> 1000

> 1000

type

Microorganism

Blood

agar

roll

Flushing

Sonication

and

Culturing

intravascular

catheters

187

130120IIO-

-

IOOgo; ,” 5 % 22

EO-

Blood

agar

roll

Flushing

Sonlcotlon

706050-

”

403020IOO-

Figure

1. Number

of catheters

rwith

a given

colony

count

according

to culture

method.

correlation between growth from two of the different culture methods. The figures in parentheses represent the number of catheters considered infected or probably infected in each case. There was good correlation between results from both types of culture, except in some cases where the line was not considered infected, and cultures from the outside produced false positive results. Table III. parenthesis

Correlation represent

between cfus on the inside and on the outside of catheters. the number of these catheters considered infected or probably

Blood agar roll (cfus)

Flushing No growth

No

growth l-15 16-50 51-99 100-999 > 1000 Total

87 (1) 15 6 (1) G3) 117

(5)

l-15

16-50

(cfus) 51-99

z 5 (1) 5 (1) 4 1 23 (2)

100-999

> 1000

-

-

1 -(l)

I 1 (1) 5 (5) 15 (10) 21 (16)

4 (1) : (1)

Figures infected

i

(2)

3 (2)

1: (10) 8 (5) 24 (16)

Total 90 20 :; 36 25 193

in

K. G. Kristinsson

188

et al.

1 IZOI IO100 gom & g B z

80-

z”

50-

Blood

agar

roll

Flushing

7060-

403020IOo-

Figure 2. Correlation of clinical impression with the culture catheter surface and flushing of the inside. The shaded catheters considered infected or probably infected.

results for blood areas represent

agar roll of the the number of

Correlation of clinical j&dings and culture results Figure 2 shows the results of culture by blood agar roll or flushing for those catheters on which clinical information was available and correlates the two. The shaded areas represent catheter cultures considered infected or probably infected including exit-site infections. Using a threshold of d 15 cfus, catheters cultured by blood agar roll had a 99% likelihood of being free from infection, whilst those with > 15 cfus had only a 46% likelihood of being associated with infection. By increasing the threshold to 100 cfus, the respective figures were 96% and 56%. Similarly, catheters in which the inside had been cultured by flushing had a 9.5% likelihood of predicting absence of infection with a threshold of 15 cfus, and a 66% chance of predicting infection; the respective figures for a threshold of 100 cfus being 93% and 71%. Finally, the correlation between the different culture methods and colony counts, in cases of exit-site infections, is shown in Table IV. Catheters with < 100 cfus were from patients that had been on antibiotics.

Culturing Table

IV.

Colony

Results

of catheter

intravascular cultures No.

Count (cfus)

Blood

growth 1-15 16-50 51-99 100-999 I=- 1000 Total

catheter

was

blocked

in patients

of catheters agar

No

*One

catheters

roll

with by culture

189 exit-site

infections

method* Flushing 3

2 0 6 5 14 and

could

not

:, 0 3 6 13

be flushed

Discussion

Most studies on intravascular catheter cultures have been on catheters that have been carefully removed by the investigators, after skin disinfection (Maki et al., 1977; Cleri, Corrado & Seligman, 1980; Adam et al., 1982; Moyer, Edwards & Fairley, 1983; Collignon et al., 1986; Jones et al., 1986), but in our experience these precautions are not always taken and it often takes several hours for the catheters to reach the laboratories. A threshold level of 15 cfus/catheter may therefore be too low. The results of our study in a routine diagnostic laboratory showed a clear bimodal distribution of culture results, with 96% of the cultures of the inside and 88% of cultures of the outside of catheters having < 15 and > 100 cfus, the difference between the two sites probably being due to contamination of the outside of the catheters when they are pulled through the skin. Culture of the inside of the catheter, which does not get contaminated in this way, should therefore be a better way of distinguishing true colonization from infection. When the culture results were reviewed in the light of the available clinical information, this supposition was supported; whether a threshold of 15 cfus or 100 cfus were used. The predictive value of a positive culture being associated with infection was 46% (outside culture) and 66% (inside) for a threshold of 15 cfus, but 56% and 71%, respectively, for 100 cfus. The corresponding figures for a negative culture being associated with absence of infection were 99% and 94%, respectively, for a threshold of 15 cfus, but 96% and 93%, respectively, for 100 cfus. A threshold value of 100 cfus was therefore considerably better at predicting the presence of infection without being significantly inferior at predicting absence of infection. The only situation in which culture of the inside of the catheter by flushing proved inferior to culture of the outside in predicting infection was, as might be expected, in the case of exit-site infections. None of the catheters showed significant growth on culture after ultrasonication, in cases where other cultures failed to show significant growth. Therefore, ultrasonication of catheters is not needed to remove

190

K. G. Kristinsson

et al.

adherent bacteria before culture. Since most of the catheters that were thought likely to be infected, but had insignificant growth, were from patients receiving antibiotics, culture results should be interpreted in the light of current antibiotic use. The 8 catheters from patients with catheter-related sepsis all had > 100 cfus by all cultures methods, except one which was infected with S. aweus from a patient receiving antistaphylococcal antibiotics. Coagulase-negative staphylococci were by far the most frequent organisms isolated, colonising 63% of infected lines, 63% of lines not giving rise to symptolms and 63% of lines associated with sepsis. Other studies have similarly found that these organisms cause between 25% and 75% of line infections (Maki et al., 1977; Moyer et al., 1983; Collignon et al., 1986; Jones et al., 1986). Mixed cultures were significantly less common in our study from catheters causing symptoms of infection than from other catheters (p < 0.001). The pathogenesis of long-intravascular-catheter colonization is not clear. Some authors maintain that colonization starts at the exit site and spreads along the outside of the catheters (Snydman et al., 1982) while others (Linares et al., 198.5, Weightman et al., 1988) consider hub colonization the initial event with subsequent colonization of the inside. This present study did not look specifically at this issue, but it is noteworthy that virtually all heavily colonized catheters were colonized both on the inside and the outside. This has implications for treatment, for, if antibiotic treatment alone is to be attempted, the antibiotics must be left inside the catheter as antibiotic locks, for a sufficient length of time to eradicate colonizing bacteria, in addition to achieving adequate systemic concentrations. We thank catheters

the ML’S0 and the ward

staff staff

in the Department for their cooperation.

of Bacteriology

for

help

in processing

the

References Adam,

R. D., Edwards, L. D., Becker, C. C. & Schrom, H. M. (1982). Semiquantitative cultures and routine tip cultures on umbilical catheters. Journal of Pediatrics 100, 123-126. culture of intravenous Cleri, D. J., Corrado, M. L. & Seligman, S. J. (1980). Q uantitative catheters and-other intravascular inserts. journal of Infectious Diseases 141, 781-786. Christensen, G. D., Bisno, A. L., Parisi, J. T., McLaughlin, B., Hester, M. G. & Luther, R. W. (1982). Nosocomial septicemia due to multiply antibiotic-resistant Staphylococcus epidermidis. Annals of Internal Medicine 96, l-10. Collignon, P. J., Soni, N., Pearson, I. Y., Woods, W. P., Munro, R. & Sorrell, T. C. (1986). Is semiquantitative culture of central vein catheter tips useful in the diagnosis of catheter associated bacteremia? Journal of Clinical Microbiology 24, 532-535. Eykyn, S. J. (1984). Infection and intravenous catheters. journal of Antimicrobial Chemotherapy 14, 203-208. Jones, P. G., Hopfer, R. L., Elting, L., Jackson, J. A., Fainstein, V. & Bodey, G. P. (1986). Semiquantitative cultures of intravascular catheters from cancer patients. Diagn. Microbial. Infect. Dis. 4, 299-306.

Culturing

intravascular

catheters

191

Linares, J., Sitges-Serra, A., Garau, J., Perez, J. L. & Martin, R. (1985). Pathogenesis of catheter sepsis: a prosective study with quantitative and semiquantitative cultures of catheter hub and segments. Journal of Clinical Microbiology 21, 357-360. Maki, D. G., Weise, C. E. & Sarafin, H. W. (1977). A semiquantitative culture method for identifying intravenous-catheter-related infection. New England Journal of Medicine 296, 1305-l 309. Moyer, M. A., Edwards, L. D. & Fairley, L. (1983). Comparative culture methods on 101 intravenous catheters. Archives of Internal Medicine 143, 66-69. Snydman, D. R., Gorbea, H. F., Pober, B. R., Majka, J. A., Murray, S. A. & Perry, L. K. (1982). Predictive value of surveillance skin cultures in total-parenteral-nutritionrelated infection. Lancet ii, 138.51388. Weightman, N. C., Simpson, E. M., Speller, D. C. E., Mott, M. G. & Oakhill, A. (1988). Bacteraemia related to indwelling central venous catheters: prevention, diagnosis and treatment. European Journal of Clinical Microbiology 7, 125-129.