Interpretive criteria and quality control guidelines for lomefloxacin and meropenem in susceptibility tests of Haemophilus influenzae using Haemophilus test medium

DIAGN MICROBIOL INFECT DIS 1992;15:145-150

145

Interpretive Criteria and Quality Control Guidelines for Lomefloxacin and Meropenem in Susceptibility Tests of Haemophilus influenzae Using Haemophilus Test Medium Michael A. Pfaller, Ronald N. Jones, John A. Washington, Franklin P. Koontz, E. Hugh Gerlach, and Meridith E. Erwin

Lomefloxacin and meropenem were tested in a multilaboratory study to establish susceptibility testing interpretive criteria and quality control (QC) guidelines for Haemophilus influenzae using Haemophilus test medium (HTM). Interpretive criteria were established by using triplicate testing of 102 representative H. influenzae strains. Only a susceptible category was proposed for lomefloxacin (>-22 mm and <-2 izg/ml) and

meropenem (>-13 mm and <-4 i~g/ml) due to the lack of resistant isolates. QC range for H. Influenzae ATCC 49247 were established using multiple HTM agar and broth base lots, three disk lots for each drug, and a number of test replicates consistent with the National Committee for Clinical Laboratory Standards M23-T guideline.

INTRODUCTION

stimulated the investigation of the clinical activity of several n e w e r broad s p e c t r u m anfimicrobial agents for the treatment of serious systemic H. influenzae infections which in turn has led to a n e e d for in vitro susceptibility test p r o c e d u r e s a n d interpretive criteria that are predictive of therapeutic outcome (Doern et al., 1990 and 1991; Jorgensen et al., 1987, 1998a and b, and 1990; M e n d e l m a n et al., 1990a a n d b). The National Committee for Clinical Laboratory Standards (NCCLS) has d e v e l o p e d a m e t h o d for testing H. influenzae a n d other fastidious organisms (NCCLS, 1990a and b) based on the use of Haemophilus test m e d i u m (HTM) described by Jorgensen et al. (1987). In order for susceptibility testing with H T M to be widely accepted, reliable quality control (QC) p e r f o r m a n c e criteria m u s t be d e v e l o p e d for all antimicrobial agents tested on this n e w m e d i u m . Based on the recent w o r k of D o e r n et al. (1991), H T M lots can n o w be applied to p r o d u c e QC criteria for n e w c o m p o u n d s using the guidelines of the NCCLS (1989) M23-T document. This d o c u m e n t (NCCLS M23T) and the r e c o m m e n d a t i o n s for H T M lot quality f o u n d in NCCLS (1990b) M7-A2 should avoid those

Resistance of Haemophilus influenzae to conventional antimicrobial agents, such as ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole, has increased over the past two decades (Campos et al., 1989; Doern et al., 1986 and 1988; Jorgensen et al., 1988a; Powell et al., 1987). The recent report of Mendelman et al. (1990b) describing the cefuroxime treatm e n t failure of H. influenzae meningitis associated with altered penicillin-binding proteins serves to underscore these concerns. These observations have From the University of Iowa College of Medicine (R.N.J., M.A.P., F.P.K., M.E.E.), Hospitals and Clinics (F.P.K.), and the Veterans Affairs Medical Center (R.N.J., M.A.P.), Iowa City, Iowa; the Cleveland Clinic Foundation (J.A.W.), Cleveland, Ohio; and St. Francis Regional Medical Center (E.H.G.), Wichita, Kansas, USA. Address reprint requests to Dr. R. N. Jones, Department of Pathology, University of Iowa Hospitals and Clinics, Iowa City, IA 52242, USA. Received 3 January 1991; revised and accepted 1 April 1991. © 1992 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/92/$5.00

146

interpretive and growth quality problems observed by Mendelman et al. (1990a). In the present multilaboratory evaluation, the newer antimicrobial agents with significant anti-Haemophilus activity were studied using the NCCLS M23T (1989) guidelines and the statistical recommendations published by Barry et al. (1989) and Gavan et al. (1981). The drugs studied were the fluoroquinolone, lomefloxacin (Chin et al., 1988; Jones et al., 1988), and the carbapenem, meropenem (Jones et al., 1989a; Jones and Gardiner, 1989; Powell et al., 1989; Slaney et al., 1989).

MATERIALS AND METHODS

M.A. Pfaller et al.

Accumedia (Baltimore, MD; lot 8909-131), and Oxoid (London, England; lot 20642912). The media were prepared as recommended by the manufacturer and dispersed into microdilution trays. Each of the five laboratories participating in the study was assigned a specific lot as well as Difco lot 776242, which was used as the lot common to all study sites. Current HTM agar base lots were obtained from BBL (lot J1DUXG), Difco (lots 784399 and 780220), and Accumedia (lots 9003-151 and 8912-113). The media were prepared as recommended by the manufacturer. Each of the five participating laboratories was assigned a specific lot as well as Difco lot 784399, which was used as the lot common to all study sites.

Antimicrobial Agents

Susceptibility Testing Methods

Lomefloxacin was obtained from G.D. Searle and Company (Skokie, IL, USA). Meropenem was provided by Stuart Pharmaceuticals (Wilmington, DE, USA). Ampicillin 10-p,g disks for QC of the HTM lots and lomefloxacin and meropenem 10-M,g disks for regression comparisons were manufactured by Difco Laboratories (Detroit, MI, USA). Diffusion test disk lots for the QC studies were also obtained from BBL Microbiology Systems (Cockeysville, MD, USA) and Difco Laboratories. Three lots of disks for each drug were evaluated as follows: lomefloxacin 10-~g disks from BBL (lot 811560) and Difco Laboratories (lots 691349 and 691316), and meropenem 10-~g disks from BBL (lot 909585) and Difco (lots 691324A and 691324B).

The susceptibility tests were performed by the broth microdilution and disk diffusion methods recently recommended by the NCCLS (1990a and b). All tests were performed either in HTM broth base or on HTM agar base (Doern et al., 1991; NCCLS, 1990a and b). Incubation was carried out at 35°C in ambient air (broth microdilution tests) or 5% CO2 (disk diffusion tests), and results were read at 20-24 hr. All strains tested in the regression-interpretive criteria studies were processed in triplicate in one of the participating laboratories (R.N.J.) by each susceptibility testing method. The mean zone diameter rounded to the nearest whole millimeter and the model log2 dilution step were used to calculate regression statistics. In nearly all cases, this represented a single minimum inhibitory concentration (MIC) value from all three tests or two identical MIC results with the remaining end point at an adjacent dilution step. QC trials generated a minimum of 60 zones and 20 MICs per drug per organism pair at each participating facility using their unique HTM agar and broth base lots. Each participant also used a common lot of HTM agar and HTM broth base. Thus, the study sites produced 75 zone diameters and 25 MICs for each drug determined on the common HTM agar (Difco lot 784399) and in the broth (Difco lot 776242) base lots. The study design followed that was described in detail previously (Jones et al., 1989b).

Organisms Two groups of organisms were tested, one for the determination of lomefloxacin and meropenem tests interpretive criteria and the second for the QC trials. The H. influenzae QC organism (ATCC 49247) currently recommended by the NCCLS (1990a and b) was provided to participants for replicate testing in the QC trials. Additional nonfastidious control strains included Staphylococcus aureus ATCC 29213, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Enterococcus faecalis ATCC 29212. A total of 102 H. influenzae clinical isolates from several geographic areas that possessed various level of susceptibility to ampicillin and other 13-1actam agents were used to determine susceptibility break points. These included strains resistant to ampicillin by ~-lactamase production (26 isolates) as well as by nonenzymatic mechanisms (18 isolates).

Media Current HTM broth base lots were obtained from BBL (lot DIDTOU), Difco (lots 776242 and 783714),

Statistical Methods Lomefloxacin and meropenem MICs were compared to their zone diameters around commercially prepared 10-M,g disks. Where possible, suggested susceptible break points conformed to those of similar agents published in NCCLS (1990a and b) or international documents (Doern et al., 1990 and 1991). The control ampicillin regression was consistent with that previously published (Doern et al., 1990). The

HTM Tests for Lomefoxacin and Meropenem

147

regression equation was y = 17.5-0.30 x with a correlation coefficient of r = 0.91. The statistical criteria for selecting QC limits for broth microdilution and disk diffusion susceptibility tests were those described by Barry et al. (1989) and Gavan et al. (1981), respectively.

0.25 F

LOMEFLOXACIN (10 ug) 0.12 F I

I 1

0.06

RESULTS

0.03-

Interpretive Criteria for Lomefloxacin and Meropenem Figures 1 and 2 and Table 1 present the results of the lomefloxacin (Figure 1) and meropenem (Figure 2) interpretive criteria studies. By HTM broth base dilution tests, both lomefloxacin and meropenem were very active against all strains of H. influenzae with MIC90s of <0.06 ~g/ml. Direct comparative lomefloxacin MICs with meropenem (data not shown) revealed that lomefloxacin was equal to or more potent than meropenem against ampicillin-resistant strains. All 10-~g lomefloxacin disk zone diameters were ---25 mm (Figure 1), and all 10-~g meropenem disk zone diameters were ->13 mm (Figure 2). The regression-line scattergrams demonstrate poor correlation between the MIC and zone diameter for both lomefloxacin (Figure 1, r = 0.15) and meropenem (Figure 2, r = 0.33). This is contrasted in each case with the regression line obtained from previous studies of numerous bacterial species in which a reasonable correlation between MIC and zone diameter was obtained for both lomefloxacin (r = 0.83, Figure 1) and meropenem (r = 0.70, Figure 2) (Jones et al., 1988 and 1989a). As with other previously studied fluoroquinolones (ciprofloxacin, ofloxacin) and carbapenems (imipenem), no documented resistant H. influenzae isolates have been observed clinically. Therefore, we suggest (Table 1) only susceptible criteria for both lomefloxacin (_>22 mm, ~2 ~g/rnl) and meropenem (->13 mm, -<4 ~g/ml) to be consistent with the interpretive break points of comparable agents or other pathogens (Jones et al., 1989a; Doern et al., 1990; NCCLS 1990a and b). No interpretive errors were identified using those test strains that had a very high incidence of fB-lactam (ampicillin) resistance.

Broth Microdilution Test Quality Control Table 2 lists the lomefloxacin and meropenem MIC results from the five collaborating laboratories testing the H. influenzae ATCC 49247 strain (NCCLS 1990b). The H. influenzae ATCC 49247 lomefloxacin MIC mode was established at 0.06 ~,g/ml leading to a suggested QC range of 0.03-0.12 ~g/ml. The H. influenzae ATCC 49247 meropenem MIC mode was

2

16211031

313613

1 1 2

o o1

i!

~13-- 5 3 2 1 - ~ - - ~

1

L ~

20

25

30

35

Lomefloxacin

Zone

40 Diameter

i

j

45

50

(mm)

FIGURE 1 Lomefloxacin scattergram comparing 10-~g disk zone diameters to MICs. The solid diagonal line is the calculated regression line from this study. The broken diagonal regression line was taken from Jones et al. (1988) for 830 strains representing numerous bacterial species. The vertical line is the proposed susceptible zone interpretive criteria. 0.12 ~g/ml leading to a suggested QC range of 0.060.25 FLg/ml. The recommended MIC ranges conform to the statistical methods suggested by Barry et al. (1989) with 96.8% of meropenem results and 100% of lomefloxacin results included within the proposed MIC guidelines.

Disk Diffusion Test Quality Control

2

The zone diameters from the five-laboratory study of 10-~g lomefloxacin disks (three lots, 300 total rep-

MEROPENEM (10 ug)

0.5 0.25

~_

-- _ -~

0.12 i

1 ~

1 1

~ - _

1 1 1

A

-~ o.o5 L_

1

--~---1-~--,~76583

0.03

1

11

1

1 4 3 3 4 5 2 ~'F-~2~L

0.015 L

21111

0.008 -

1

0.004

1

-

6

1

10

15 Meropenem

20

25 Zone

Diameter

1

30

35

40

(ram)

FIGURE 2 Meropenem scattergram comparing 10-~g disk zone diameters with MICs. The solid diagonal line is the regression line for the Haemophilus influenzae strains (this study) and the broken diagonal regression line was plotted from a study of numerous bacterial species (Jones et al., 1989a). The vertical line is the proposed susceptible zone interpretive criteria.

148

M.A. Pfaller et al.

TABLE 1

Interpretive Criteria for Lomefloxacin and M e r o p e n e m Disk Diffusion Susceptibility Tests with Haemophilus influenzae Including the F r e q u e n c y (%) of Errors Zone-Diameter Criteria (ram)

% of Errors a

Antimicrobial Agent

Susceptible

Intermediate

Resistant

Very Major

Major

Minor

Lomefloxacin Meropenem Ampicillin c

/>22 (~<2)b />13 (~<4) t>25 (~<1)

--22-24 (2)

--~<21 (~4)

0.0 0.0 0.0

0.0 0.0 2.9

0.0 0.0 12.7

~Very major, false-susceptible; Major, false-resistant; and Minor, intermediate result by one method and susceptible or resistant by the other test result. bMIC correlates in Ixg/ml. CAmpicillin criteria from Doern et al. (1990) and the NCCLS (1990a and b).

licates) are s u m m a r i z e d in Table 3. The statistical m e t h o d described in several earlier susceptibility test standardization studies (Gavan et al., 1981) was also applied to the lomefloxacin disk test. The lomefloxacin 10-txg disk m e a n a n d m e d i a n data were similar, and the r e c o m m e n d e d Q C z o n e range based on the G a v a n statistic was 35-39 mm. Using this guideline, 85.3% of the u n i q u e lot z o n e diameter a n d 94% of the c o m m o n lot z o n e diameters (87% overall) were within control. Two of the laboratories reported s o m e w h a t larger z o n e diameters with their u n i q u e lots of HTM agar base. Several of these values fell just outside the r e c o m m e n d e d Q C range s u g g e s t i n g that the G a v a n statistic was possibly too n a r r o w for practical purposes. Broadening the Q C range to include zone diameters of 33 a n d 41 m m (range, 3341 ram) w o u l d result in 100% of all u n i q u e a n d common-lot z o n e diameters being in control, a n d thus avoid the p r o b l e m of too m a n y "false alarms" that m a y be p r o d u c e d w h e n overly stringent Q C criteria are e m p l o y e d . The zone diameters from the five-laboratory study of 10-txg m e r o p e n e m disks (three lots, 300 total replicates) are s u m m a r i z e d in Table 4. The m e r o p e n e m 10-~g disk m e a n a n d m e d i a n data were similar, a n d

TABLE 2

H T M Broth Microdilution Test Results from a Five-Laboratory Trial (125 Total Results) Using Quality Control Strain Haemophilus influenzae ATCC 49247

Antimicrobial Agent

Meropenem Lomefloxacin

MIC (~g/ml) Reports a 0.03 0 [33

0.06 [21 92

0.12 59 0]b

0.25 41]b 0

0.5 4 0

qndudes results reported from six lots of HTM broth. The common lot results were also included. bBrackets indicate recommended mode _+1 log2 dilution quality control ranges.

the Q C zone range based o n the G a v a n statistic (Gavan et al., 1981) was 21-27 m m . Using this guideline, 84.7% of the u n i q u e lot z o n e diameters a n d 91% of the c o m m o n lot z o n e diameters (85.9% overall) were within control. B r o a d e n i n g the Q C range to include zone diameters of 20 a n d 28 m m (range, 20-28 ram) w o u l d result in 89.6% of the u n i q u e lot zone diameters a n d 96% of the c o m m o n lot zone diameters (91% overall) being in control. O n e of the laboratories (laboratory D) r e p o r t e d significantly smaller zone diameters with their u n i q u e lot of HTM agar base. Of the 31 z o n e diameters that fell outside of the Q C limits, (100%) were from a single lot of

TABLE 3

HTM Agar Disk Diffusion Test Results for Lomefloxacin 10-txg Disks from Five Laboratories Using Quality Control Strain Haemophilus influenzae ATCC 49247

10-txg Lomefloxacin Disk Zone (mm) 334 34 35 36 37 38 39 40 41a Median Mode Range

Zone-Diameter Occurrence by Sites A

B

C

D

E Total

Common Lot

1 1 -3 8 -15 29 - 24 10 - 14 11 3 2 1 19 18 1 - - 17 3

2 11 47 59 50 47 53 28 3

-1 12 19 18 12 5 3 --

39 37 36 35 39 39 36 36 35 38 2 4 7 5 4

37 36 8

37 36 6

-- 3 - - 25 - - 22 16 9 34 1 10 - -

aRecommended QC zone range for lomefloxacin. Statistical method described by Gavan et al. (1981) would dictate a 35 to 39-ram range.

HTM Tests for Lomefoxacin a n d M e r o p e n e m

TABLE

4

149

H T M Agar Disk Diffusion Test Results for M e r o p e n e m 10-,,g Disks from Five Laboratories Using Quality Control Strain Haemophilus influenzae ATCC 49247 Zone-Diameter Occurrence by Sites

10-p,g Meropenem Disk Zone (ram) A

Common Lot

B C D

E

Total

1 8 11 11 13 4 6 3 -2 -1

------1 6 23 18 9 3

1 8 11 11 14 9 26 35 52 61 55 16 1

---

24 25 13

25 25 7

16 17 18 19 2(Y 21 22 23 24 25 26 27 28a

9 17 22 12 ---

Median Mode Range

25 26 23 20 24.5 26 26 22 19 24 6 4 6 11 5

-5 12 30 12 1

1 5 19 17 7 7 4 --

--

3 3 7 8 8 27 14 5 ---

aRecommended QC zone range for meropenem. Statistical method described by Gavan et al. (1981) would dictate a 21 to 27-ram range. HTM agar base tested in laboratory D. W h e n the results of laboratory D w e r e excluded, 100% of the zone diameters g e n e r a t e d at the remaining four laboratories w e r e within control.

DISCUSSION Both lomefloxacin a n d m e r o p e n e m w e r e highly active against the isolates of H. influenzae included in this study. Similar to o t h e r fluoroquinolones (ciprofloxacin, ofloxacin) and c a r b a p e n e m s (imipenem), all of the test strains, including J3-1actamase-positive and -negative ampicillin-resistant strains, w e r e susceptible to both drugs (Doern et al., 1990; NCCLS 1990a and b). A l t h o u g h ampicillin-resistant strains t e n d e d to have h i g h e r m e r o p e n e m MICs, e v e n the most highly ampicillin-resistant strains r e m a i n e d susceptible to this agent. Because of the lack of lomefloxacin or m e r o p e n e m - r e s i s t a n t strains, it is not possible from o u r data to p r o p o s e precise z o n e size

or MIC interpretive criteria for these p o t e n t antimicrobial agents. H o w e v e r , o u r data suggest that a single susceptible interpretive z o n e size for H. influenzae of ~22 m m for lomefloxacin and ~13 m m for m e r o p e n e m , and susceptible MIC break points of -<2 p,g/ml and -<4 ~g/ml, respectively, m a y be clinically useful (Table 1). These tentative interpretive criteria for lomefloxacin and m e r o p e n e m m u s t be validated by testing frankly resistant strains if they are discovered in the future. If strains of H. influenzae resistant to fluoroquinolones or c a r b a p e n e m s become available, t h e n efforts should be m a d e to develop z o n e - d i a m e t e r interpretive criteria and MIC correlates for the resistant and intermediate or moderately susceptible categories for these antimicrobial agents using the m e t h o d s a n d media described previously (Doern et al., 1990 a n d 1991; NCCLS 1989, 1990a and b). The in vitro tests using H T M agar and broth base with lomefloxacin a n d m e r o p e n e m v e r s u s H. influenzae strains p e r f o r m e d acceptably, a n d the QC limits were readily established for both broth microdilution and disk diffusion tests. H o w e v e r , care m u s t be exercised to select H T M lots that have acceptable QC performance. In this investigation four (40%) of 10 lots of H T M agar failed initial QC screening with ampicillin, clearly d e m o n s t r a t i n g a great contemp o r a r y n e e d for p e r f o r m a n c e screening. In addition, one H T M agar lot (laboratory site D, m e r o p e n e m , Table 4) s h o w e d results that could be attributed to c o m p r o m i s e d m e d i u m quality, a l t h o u g h p e r f o r m i n g well for other drugs such as ampicillin, chloramphenicol, and "third-generation" cephalosporins. These data p r e s e n t e d here, e x p a n d the n u m b e r of reliable c o m p o u n d s with QC p e r f o r m a n c e guidelines for H T M and the NCCLS M2-A4 a n d M7-A2 m e t h o d s (NCCLS 1990a and b). A l t h o u g h a single lot of H T M agar base in one laboratory p e r f o r m e d poorly with m e r o p e n e m , in general, these p r o p o s e d MIC and disk z o n e - d i a m e t e r QC guidelines should p e r f o r m well o n H T M broth a n d agar base lots meeting NCCLS (1990a and b) p e r f o r m a n c e criteria.

The authors acknowledge the excellent technical support of the many medical technologists and laboratory technicians in the participating laboratories and thank Ruth Kjaer and Linda Schneekloth for excellent secretarial assistance. The Prepared Media Laboratory (Tualatin, OR), especially Barbara Horton, is acknowledged for support of HTM preparation and some performance tests.

150

M.A. Pfaller et al.

REFERENCES Barry AL, Fuchs PC, Jones RN, The Collaborative Antimicrobial Susceptibility Testing Group (1989) Statistical criteria for selecting quality control limits for broth microdilution susceptibility tests with 39 different antimicrobial agents. Diagn Microbiol Infect Dis 12:413-420. Campos J, Garcia-Tornel S, Sanfeliu I (1989) Susceptibility studies of multiply resistant Haemophilus influenzae isolated from pediatric patients and contacts. Antimicrob Agents and Chemother 25:706-709. Chin N-X, Novelli A, Neu HC (1988) In vitro activity of lomefloxacin (SC-47111; NY-198): a difluoroquinolone 3-carboxylic acid, compared with those of other quinolones. Antimicrob Agents Chemother 32:656-662. Doern GV, Jorgensen JH, Thornsberry C, et al. (1986) National collaborative study of the prevalence of antimicrobial resistance among clinical isolates of Haemophilus influenzae. Diagn Microbiol Infect Dis 4:95-107. Doern GV, Jorgensen JH, Thornsberry C, Preston DA The Haemophilus influenzae Surveillance Group (1988) Prevalence of antimicrobial resistance among clinical isolates of Haemophilus influenzae: a collaborative study. Antimicrob Agents Chemother 32:180-185. Doern GV, Jorgensen JH, Thornsberry C, Snapper H (1990) Disk diffusion susceptibility testing of Haemophilus influenzae using Haemophilus test medium: description of a method, zone diameter interpretive criteria and MIC correlates. Eur J Clin Microbiol Infect Dis 9:329-336. Doern GV, Gerlach EH, Jorgensen JH, Murray PR, Thornsberry C, Washington II JA (1991) Quality control limits for disk diffusion and broth microdilution susceptibility tests with Haemophilus test medium. Diagn Microbiol Infect Dis 14:485-493. Gavan TL, Jones RN, Barry AL, et al. (1981) Quality control limits for ampicillin, carbenicillin, mezlocillin, and piperacillin disk diffusion susceptibility tests: a collaborative study. J Clin Microbiol 14:67-72. Jones RN, Gardiner RV (1989) Stability of SM-7330: a new carbapenem in mediums recommended for the susceptibility testing of anaerobic bacteria and gonococci. D/agn Microbiol Infect Dis 12:271-273. Jones RN, Aldridge KE, Barry AL, et al. (1988) Multicenter in vitro evaluation of lomefloxacin (NY- 198, SC-47111), including tests against nearly 7,000 bacterial isolates and preliminary recommendations for susceptibility testing. Diagn Microbiol Infect Dis 10:221-240. Jones RN, Barry AL, Thornsberry C (1989a) In vitro studies of meropenem. J Antimicrob Chemother 24(Suppl A):929. Jones RN, Gavan TL, Thornsberry C, et al. (1989b) Standardization of disk diffusion and agar dilution susceptibility tests for Neisseria gonorrhoeae: interpretive criteria and quality control guidelines for ceftriaxone, penicil-

lin, spectinomycin and tetracycline. J Clin Microbiol 27:2758-2766. Jorgensen JH, Redding JS, Maher LA, Howell AW (1987) Improved medium for antimicrobial susceptibility testing of Haemophilus influenzae. ] Clin Microbiol 25:21052113. Jorgensen JH, Doern GV, Thornsberry C, et al. (1988a) Susceptibility of multiply resistant Haemophilus influenzae to newer antimicrobial agents. Diagn Microbiol Infect Dis 9:27-32. Jorgensen JH, Maher LA, Redding JS (1988b) Disk diffusion interpretive criteria for extended-spectrum cephalosporins with Haemophilus influenzae. J Clin Microbiol 26:1887-1889. Jorgensen JH, Howell AW, Maher LA (1990) Antimicrobial susceptibility testing of less commonly isolates Haemophilus species using Haemophilus test medium. ] CLin Microbiol 28:985-988. Mendelman PM, Wiley EA, Stull TL, Clausen C, Chiffen DO, Onay O (1990a) Problems with current recommendations for susceptibility testing of Haemophilus influenzae. Antimicrob Agents Chemother 34:1480-1484. Mendelman PM, Chaffin DO, Krilov LR, et al. (1990b) Cefuroxime treatment failure of nontypeable Haemophilus influenzae meningitis associated with alteration of penicillin-binding proteins. J Infect Dis 162:1118-1123. National Committee for Clinical Laboratory Standards (NCCLS) (1989) Tentative guideline M23-T: development of in vitro susceptibility testing criteria and quality control parameters. Villanova, PA: NCCLS. National Committee for Clinical Laboratory Standards (NCCLS) (1990a) Approved standard, M2-A4: performance standards for antimicrobial disk susceptibility tests. Villanova, PA: NCCLS. National Committee for Clinical Laboratory Standards (NCCLS) (1990b) Approved standard M7-A2: methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically. Villanova, PA: NCCLS. Powell M, Koutsia-Carouzou C, Voutsinas D, Seymour A, Williams JD (1987) Resistance of clinical isolates of Haernophilus influenzae in United Kingdom, 1986. Br Med J [Clin Res] 295:176-179. Powell M. Seetulsingh P, Williams JD (1989) In vitro susceptibility of Haemophilus influenzae to meropenem compared with imipenem, five other beta-lactams, chloramphenicol and ciprofloxacin. J Antimicrob Chemother 24:(Suppl A):175-181. Slaney L, Chubb H, Mohammed Z, Ronald A (1989) In vitro activity of meropenem against Neisseria gonorrhoeae, Haemophilus influenzae and H. ducreyi from Canada and Kenya. JAntimicrob Chemother 24(Suppl A):183186.