Evaluation of the Quantum II and Rapid E identification systems

(1)

Evaluation of the

Quantum

II

and

Rapid E Identification

Systems

PATRICK R.

MURRAY,',2*

AMYGAUTHIER,' AND ANNNILES'

Clinical Microbiology Laboratory, Barnes Hospital,' and Washington University SchoolofMedicine,2* St. Louis, Missouri 63110

Received23February 1984/Accepted 30 May 1984

A totalof 492 clinical isolates fromthefamily Enterobacteriaceae weretested in theAPI20E, RapidE, and

Quantum II identification systems. Discrepant identifications among these three systems were resolved by

repeattesting intheidentification systemsor useof conventional biochemical tests. Of these isolates,94.1%

werecorrectlyidentified with the API 20EandRapidEsystems,and97.0% werecorrectlyidentified withthe

QuantumIIsystem.Anadditional 48 non-Enterobacteriaceae isolatesweretestedwith the Quantum IIsystem,

and83.3% werecorrectly identified. The majority of incorrect identificationswith the Rapid E and Quantum

IIsystemswerecausedbyasingle aberrant biochemicalreaction. Reproducibilityof the biochemical reactions obtained with thesetwo systemswasevaluated bytesting 40 organismsintriplicate. Identicalbiocodes for all threetestswereobtained for 10 organisms with the Quantum IIsystemandfor 19 organismswiththe Rapid E system. Reproducibility of the Quantum II test resultswas improved with a subsequentmodification of the photometer of this system. Boththe RapidE andQuantumIIsystemswereinexpensiveandwere technically

easytoinoculate and interpret.

Commercially

prepared

systems for the identification of gram-negative bacillihavebeen usedin the United States for almost 15 years. These systems offer a number of benefits including standardized selection of biochemical tests, im-proved identification accuracy

compared

with the conven-tional systems used

previously

in most

laboratories,

and decreased time

required

for identification

(1-3, 6,

8).

More recentimprovements in commercial systems have included

obtaining

results within5 hand

using

automatedinstruments tofacilitate

interpretation

of thetest reactions

(2,

4, 5, 7-9).

Despite

the

advantages

of automated

instruments,

many

laboratories do not use them because of their

high

initial purchase costs. Recently, two new identification systems wereintroduced: the

Quantum

II system

by

Abbott Labora-tories, North

Chicago, Ill.,

and the

Rapid

E system

by

DMS Laboratories,

Flemington,

N.J. Both systems

require

incu-bation for only 4 to 5

h,

and the

Quantum

II tests are

automatically

interpreted by

a

photometer

provided

by

the manufacturer. In this report, the two new systems are

compared

with the API 20E system

(Analytab

Products,

Inc.,

Plainview, N.Y.),

one of the first

commercially

pre-pared identification systemsused in the United States.

MATERIALS ANDMETHODS

Testorganisms. Atotalof492 recentclinical isolatesfrom the

family

Enterobacteriaceae were tested in the API 20E, Rapid E, and Quantum II identification systems. The orga-nisms were randomly selected from gram-negative bacilli isolatedatthe

Barnes

Hospital ClinicalMicrobiology Labo-ratory. A single subculture onto a tryptic soy blood agar plate was madefrom the initial isolation plate, after which eachorganismwastestedin the threeidentificationsystems. If the same identification was obtained with all three sys-tems, thenitwasconsideredtobe accurate. If adiscrepancy was observed among the three systems, then the organism was retested in all three systems. If thediscrepancy was not resolved by retesting, then definitive identification of the

*Corresponding author.

509

isolate was determined by conventional biochemical tests. Abbott Laboratories claimsthat theQuantumII system can beused toidentifysomespecies ofgram-negativebacillithat are not members ofthe family Enterobacteriaceae. There-fore, a total of 48 non-Enterobacteriaceae isolates were tested in the Quantum II system. Finally, an additional 40 isolates of Enterobacteriaceaeweretested on three consecu-tive days in the Rapid E and Quantum II systems to determine the reproducibility ofthe individual biochemical testresults.

API 20E. The API 20E system consists ofa plastic strip with dehydrated reagents for the following 20 biochemical tests:

3-D-galactosidase;

lysineandornithine decarboxylase; arginine dihydrolase; urease; citrate utilization; hydrogen sulfide and indole production; tryptophane deaminase; Voges-Proskauer; gelatinase;andfermentationofarabinose, rhamnose, sucrose, glucose, melibiose, mannitol, inositol, sorbitol, and amygdalin. Inoculation, 18 to 24 h of incuba-tion, and interpretation of the API 20E tests, as well as quality control tests, were performed according to the in-structions of the manufacturer and have been described previously (1,6, 10).

Rapid E. The

Rapid

E system for the identification of Enterobacteriaceaeconsists ofa

plastic

strip

with

dehydrat-ed reagents for the

following

20 biochemical tests:

P-D-galactosidase; lysine and ornithine

decarboxylase;

urease; citrate

utilization;

phenylalanine

deaminase;

malonate utili-zation; esculin

hydrolysis;

indole

production;

Voges-Pros-kauer; and fermentation of

arbinose, xylose,

adonitol,

rham-nose,sucrose,

glucose,

cellobiose,

melibiose, trehalose,

and raffinose. The testinoculumwas

prepared

by

picking

one or more isolated colonies and suspendingthemin the

Rapid

E suspensionmedium or in 1.25 mlof 0.85%sterilesaline.The

turbidity

ofthe suspensionwas

adjusted

tomatcha McFar-land 0.5 barium sulfate standard. Each well

(cupule)

was then filledaccordingto theinstructions ofthemanufacturer with ca. 50 ,ulofthebacterial suspension, and the wellsfor thelysine decarboxylase, ornithine

decarboxylase,

and ure-ase testswereoverlaid with mineraloil. After incubation of theRapidEtestsfor4h ina35°Cair

incubator,

thereactions were visually interpreted by comparing the colors of the

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510 MURRAY, GAUTHIER, AND NILES

reactions with a color chart provided by the manufacturer. Reagents for the indole and Voges-Proskauer tests were added to therespective wells before the reactions were read. An oxidase test (reactivity with 1% tetramethyl-p-phenyl-enediamine dihydrochloride) was also performed with an isolated colony. The results of the test reactions were converted into a 7-digit profile number, and the microbial identification was determined by matching the profile num-ber with the listing in the DMS identificationcodebook. Ifa profile number was not available in the codebook, the manufacturer was contacted to see if an identification was available in theirmore extensivedatabase. An identification was considered to be acceptable if the likelihood of the first choice was greater than or equal to 80%. If the likelihood was less than 80%, then one or more differential tests were selected from a panel of 33 reactions recommended by the manufacturer. Quality control for each lot of Rapid E strips was assessed by using the control organisms suggested by the manufacturer.

AbbottQuantum II. The Quantum IIsystem consists of a 20-chamber disposable plastic cartridge, a multipunch car-tridge perforator, and a dual wavelength photometer that measures colorimetric changes in the individual cartridge chambers. The lyophilized biochemical reagents in the cu-vette include lysine and ornithine decarboxylase; urease; citrate utilization; argininedihydrolase; malonate utilization; esculinhydrolysis;indoleproduction;acetamide; polymyxin B susceptibility; and fermentation of lactose, arabinose, xylose, adonitol,rhamnose, sucrose, glucose, inositol, man-nitol, andsorbitol. Thereadings for theindividual biochemi-cal reactions are automatically interpreted by the photom-eter and compared with a probability matrix byan internal microcomputer, andthe mostlikely identification, additional test information (e.g., percent likelihood of identification, supplemental tests), and a biotype code are automatically printed.

TheQuantumII tests wereperformed precisely according tothe recommendationsofthemanufacturer.Theinoculum forthe test waspreparedby suspending fourorfive isolated colonies in 5 mlof steriledeionized waterand adjusting the turbiditytomatch aMcFarland0.5bariumsulfate standard. Thetopof the bacterial identification cartridgewas perforat-ed, and 200 ,ul ofthe standardized bacterial inoculum was dispensed into each reaction chamber. The top of the

cartridge

was sealed with an adhesive covering, and the

cartridge

was

agitated

to mix the inoculum andreagents in each chamber. Cartridges were incubated in a 35°C air incubator for4 h for oxidase-negative organismsor 5 h for

oxidase-positive organisms.

Aftertheprescribed incubation

period,

indole reagent

(p-dimethylaminobenzaldehyde)

was added to the chamber containing L-tryptophan, and the chamber was immediately examined for the presence of indole (indicated by the formation of a pink-red color). Results ofthe indole reaction and oxidase test (performed with an isolated colony) were entered into the Quantum II Analyzer, together with the cartridge. The system then interpreted the individual reactions, compared the test re-sults with the available database, andprintedthe identifica-tion. If thelikelihood of first-choice identificationwas great-er than

80%,

an "excellent," "acceptable," or

"good"

identification was indicated. If the likelihood was less than 80%,then the message "additionalcharacteristics shouldbe considered" was printed. If the first- and second-choice organisms were oxidase-negative and fermented glucose, then one or more additional differential testswere selected fromapanel of23 reactions recommended bythe

manufac-turer. Appropriate quality control tests were performedwith each lot of Quantum II tests.

RESULTS

A total of 492 organisms from the family Enterobac-teriaceaewere tested in the API 20E, Rapid E, and Quantum II bacterial identification systems. Of these isolates, 463 (94.1%) were correctly identified with the API 20E and Rapid E systems, and 477 (97.0%) were correctly identified with the Quantum II system. These results are summarized in Tables 1 to 3.

Of the 29 misidentifications with the API 20E system, 8 were at the species level and 11 were at the genus level (Table 1). No identification (including low selectivity be-tween 2 or more isolates) was reported for 10 of the 29 misidentified isolates. With the Rapid E system, no identifi-cation was reported for 11 of the 29 incorrect Rapid E results, 3 isolates were misidentified at the species level, and 15isolates were misidentified at the genus level (Table 2). In addition to the 29 incorrect identifications, the biocodes for six of the correctly identified isolates (four isolates of Escherichiacoli and oneisolate eachofSerratia marcescens and Klebsiella pneumoniae) were not in the identification codebook. The manufacturer had to obtain the identification oftheseisolates from France; which requireda 1-week delay from the time the isolates were initially tested. With the Quantum II system, 15 isolates were not identified correctly including 4 isolates with no identification, 2 isolates that were misidentified at the species level, and 9 isolates mis-identified at the genus level (Table 3). All of theisolates that were identified incorrectly were retested with the three systems. Of the 29 isolates misidentified with the API 20E and Rapid E systems, 11 were identified correctly after retesting with the API 20E system, and 10 were identified correctly after retesting with the Rapid E system. Of the 15 organisms that were initially misidentified with the Quantum II system, 9 were correctly identified after retesting. When the reasons for the discrepancies with the Rapid E and Quantum II systems were analyzed, a single aberrant bio-chemical reaction was found to be responsible for 15 mis-identifications with the Rapid E system (Table 4) and for 7 misidentificationswith theQuantum II system(Table 5). The Rapid Ereactionresponsiblefor the most incorrect identifi-cations was ornithine decarboxylation, whereas lysine and ornithinedecarboxylationandfermentationofadonitol were responsible for many of the Quantum IImisidentifications.

In addition tomembers of the familyEnterobacteriaceae, DMSLaboratories claims AeromonashydrophilaandVibrio spp. can beidentifiedwith theRapid E system. Four isolates ofAeromonas hydrophila were tested, and three were cor-rectly identified with the Rapid E system. No identification wasgiven for one isolate. We did not test any

Vibrio

spp. in this study. Abbott Laboratories states the Quantum II system can be used to identify Aeromonas hydrophila, Acinetobacter calcoaceticus, Pseudomonas maltophilia, Pseudomonasfluorescens-putida, Flavobacterium meningo-septicumlIlb, Plesiomonas shigelloides, and Pasteurella multocida. From this group of organisms, we tested 48 isolates of which 40 (83.3%) were correctly identified (Table 6). Of the eight organisms that were initially identified incorrectly, six were accurately identified with retesting. Two isolates ofPseudomonas aeruginosa were incorrectly identified as Pseudomonas fluorescens-putida isolates be-cause theisolates failed to utilize acetamide in the Quantum II system.

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QUANTUM II AND RAPID E 511

TABLE 1. Identification ofEnterobacteriaceae with the API 20E system

No. oforganisms identified as:

Organism Z _L.

Citrobacteramalonaticus 1

Citrobacterdiversus 11 4

Citrobacterfreundii 8

Edwardsiella tarda 1

Enterobacteraerogenes 28 3

Enterobacteragglomerans 3

Enterobacter cloacae 49 1 1

Enterobactersakazakii 3

Escherichia coli 1 126 1

Klebsiella oxytoca 25 2

Klebsiella pneumoniae 2 1 1 99

Morganella morganii 11

Proteusmirabilis 45

Proteusvulgaris 10

Proteusrettgeri 3

Providencia stuartii 5 1 3

Salmonella enteritidis 1 1 5

Shigellasonnei 4

Serratialiquefaciens 1

Serratiamarcescens 1 29

Serratiaodorifera 1

Reproducibility of the biochemical reactionsobtained with for 30 organisms (75%) with the QuantumII systemand 37

the Quantum II and Rapid E systems was evaluated by organisms (93%) with the Rapid E system. Identical bio-testing 40 organisms on 3 consecutive days (Table 7). The codes for all threetestswereobtainedfor only 10organisms same biocode was obtained with at least twoof three tests (25%) with the Quantum II system and 19 organisms(48%)

TABLE 2. Identification of Enterobacteriaceae with theRapid E system

No.oforganismsidentified as:

a

Organism Z

~~~~

s

Z

Ontroatrganisom er ns 1~1.1

Citrobacter amalonaticus 1 Citrobacterdiversus 15

Citrobacterfreundii 7 1

Edwardsiella tarda 1

Enterobacteragglomerans 1 1 1

Enterobacter cloacae 1 45 2 3

Enterobactersakazakii 3

Escherichia coli 1 125

Klebsiella oxytoca 26 1

Kiebsiella pneumoniae 4 96 2

Morganella morganii 101

Proteus mirabilis 45

Proteus vulgaris 10

Proteus rettgeri 3

Providencia stuartii 6 3

Salmonella enteritidis 6

Shigella sonnei 4

Serratia liquefaciens 1

Serratia marcescens 30

Serratia odorifera 1

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TABLE 3. Identification of Enterobacteriiaceae with the QuantumIl system

No.oforganismsidentifiedas:

z~~~~~~~~~~~~~~~~~~~~~~~~~~

Organism

_-CC5~~~~~~~~~~~~~~~" i '

Citrobacteramalonaticus 1

Citrobacterdiversus 15

Citrobacterfreundii 7 1

Edwardsiellatarda 1

Enterobacter agglomerans 2 1

Enterobactercloacae 1 48 1

Enterobacter sakazakii 3

Escherichia coli 126 1

Klebsiella ozaenae 27

Klebsiellapneumoniae 1 102

Morganella morganii 8 2 1

Proteusmirabilis 45

Proteus vulgaris 10

Proteus rettgeri 3

Providencia stuartii 9

Salmonella enteritidis 7

Shigellasonnei 1 3

Serratialiquefaciens 1

Serratiamarcescens 29 1

Serratiaodorifera 1

with the Rapid E system. The biochemical reactions that hydrolysis of urea and esculin and fermentation of xylose were mostfrequentlynonreproducible withthe Quantum II were most frequently nonreproducible in the Rapid E sys-system were inhibition by polymyxin B, hydrolysis of urea tem. After the reproducibility experiment was completed, and esculin, and arginine dihydrolase activity, whereas, the manufacturer modified the mechanical platform that

TABLE 4. Analysis of incorrect identifications withtheRapidE system"

Correctidentification Initialtestresult Discrepant biochemicaltest(s)

Citrobacter amalonaticus Citrobacterdiv'ersus MNT

Citrobacterfreundiib Escherichia coli LDC

Enterobacteraerogenes Klebsiellapneumoniae ODC

Enterobacteraerogenes No ID ONPG,SAC

Enterobacter agglomerans Enterobacter cloacae CIT,RAF, MEL

Enterobacter agglomerans Klebsiellapneumoniae CIT, ADO, RAF

Enterobacter cloacae Citrobacter freundii CEL,VP

Enterobacter cloacae Klebsiellapneumoniae LDC,ODC,SAC

Enterobacter cloacae Klebsiellapneuimoniae LDC,ODC

Enterobactercloacaeb No ID RAF

Enterobacter cloacae NoID ONPG

Enterobacter cloacae No ID MEL.RAF,VP

Escherichia colib Enterobactercloacae CIT,MNT, ESC, CEL, IND, VP

Escherichia colib Hafnia alvei VP, IND

Escherichia coli NoID MNT,ESC,ADO

Klebsiellaoxytoca No ID ODC

Klebsiella

pneumoniae'

Enter-obacte

aerogenes

ODC

Klebsiellapneumoniaeb Enterobacteraerogenes ODC

Klebsiellapneumoniae Enterobacter aerogenes ODC,VP

Klebsiellapneumoniae Serratiarubidaea RHA,CEL

Klebsiellapneumoniaeb NoID TRE

Morganellamorganiib Providencia stuartii ODC

Providenciastuartii No ID ADO

Porvidencia stuartii No ID ADO

Providenciastuartii No ID LDC, ODC

Salmonella enteritidisb No ID CIT

Serratialiquefaciens Serratia marcescens RAF

aONPG, ,B-Galactosidase; LDC,

lysine

decarboxylase; ODC,ornithinedecarboxylase;CIT.citrate; MNT, malonate; ESC, esculin;ADO,adonitol;RHA,

rhamnose; CEL, cellobiose; MEL,melibiose; SAC, sucrose; TRE, trehalose; RAF,raffinose; IND,indole; VP,Voges-Proskauer;NoID, notidentified by

system.

bOrganisms correctly identified afterretesting.

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TABLE 5. Analysisof incorrect identifications with theQuantum II system"

Correct identification Initialtestresult Discrepantbiochemical test(s)

Citrobacterfreundii5 No ID LDC, ADO

Enterobacter aerogenes Klebsiellapneumoniae ODC

Enterobactercloacaeb Klebsiellaozaenae ADH, ODC, ADO,INO

Enterobacter cloacae Klebsiellapneumoniae LDC,ODC

Enterobacter cloacae Enterobacteraerogenes LDC

Enterobacteragglomerans Enterobactercloacae CIT, URE, ADO, PXB

Escherichia coli No ID LDC, ESC

Escherichiacoli Serratialiquefaciens CIT,ESC, RHA, XYL, IND, PXB

Klebsiellapneumoniaeb Enterobacter aerogenes ODC

Serratiamarcescensb No ID ADO

Serratiaodorifera Enterobacteraerogenes Organismnotindatabase

Morganellamorganiib Proteusmirabilis IND

Morganella morganiib Proteusmirabilis IND

Morganella

morganiib

NoID LDC, URE,ADO,ARA, IND

Shigellasonneib Salmonellaspp. CIT

aLDC, Lysine decarboxylase;ODC,ornithinedecarboxylase; ADH,arginine dihydrolase; URE, urease;ADO,adonitol; INO,inositol; CIT, citrate;PXB, polymyxin B;ESC, esculin; RHA,rhamnose; XYL,xylose; IND,indole; ARA,arabinose;NoID,notidentifiedbysystem.

bOrganismscorrectlyidentified afterretesting.

supports the Quantum II cuvette during the readings. Test

reproducibility with this modified instrument wasevaluated withanadditional 25 isolates. The same testreactions were obtained withatleasttwoofthethreetestsfor 23organisms (92%), and identical biocodes were obtained for all three

tests with 13organisms (52%).

DISCUSSION

Identification of isolates from the family Enterobac-teriaceae was veryaccurate with both theQuantum II and Rapid E systems (97.0 and 94.1%, respectively) and was comparable with or better than thatreported with the API 20Esystemand other manualorautomated systems(1, 3-9, 11, 12). In addition, many of the incorrect identifications observedinthisstudywerecorrectedwithretestingorwere due to a single aberrant reaction. Thetest accuracy of the three systems may be less than that reported in this study because if the identification ofanorganismwas thesamein all three systems, then it was assumed to be correct. However, ifanorganismwasmisidentified by this limitation in the study design, then theaccuracy of all three systems would be proportionately decreased.

The Rapid E system canonly be used toidentify Entero-bacteriaceae, Aeromonas spp., and Vibrio spp., whereas another identification system manufacturedby DMS Labo-ratoriesmustbe usedtoidentify other gram-negative bacilli.

In contrast with this limitation, 12 spp. of

non-Enterobac-teriaceae canbe identified with the Quantum II system. Of

the 48 non-Enterobacteriaceae isolates tested in the

Quan-tum II system, 83.3% were identified accurately including

TABLE 6. Identification of non-Enterobacteriaceae with the

QuantumII system

No. of identifications that

were:

Organism(no. tested)

Correct Incorrect

dNtbase

Aeromonashvdrophila (4) 4

Acinetobacter calcoaceticus (20) 17 3

Pseudomonasaeruginosa (13) 11 2

Pseudomonas maltophi/ia (5) 5

Pseudomonasfluorescenslputida(2) 1 1

Flavobacteriuim meningosepticum IlIb (2) 1 1

Pasteurella multocida (2) 1 1

85% ofthe isolatesofPseudomonas aeruginosa and Acine-tobactercalcoaceticus.

Only Aeromonas and Vibrio spp. and oxidase-negative, gram-negative bacillishould be tested in the Rapid E system. However, Acinetobacter spp. can be tested using these criteriawhich, inourexperience, resulted inthe misidentifi-cation ofsevenisolatesasShigellaspp. and oneisolateas a Salmonella sp. Therefore, all Shigella and Salmonella spp. identifications mustbe confirmedbyserologicaltesting as is recommended by DMS Laboratories. The database for the Rapid E system, however, should be modifiedto preclude these incorrect identifications.

Theresultsof boththeRapidEand QuantumII identifica-tion systems were not reproducible at a level sufficient for epidemiological biotyping, similar to results previously re-ported withtheAPI 20E (3, 10). We weresurprisedthat the manually performedRapidEtests wereinitiallymore repro-ducible than the semiautomated QuantumII tests. Despite thisobservation, identificationswith the Quantum II system were more frequently correct than those with the Rapid E system.Thus, thedifferences betweenthe two systems were probably due to the more extensive database with the Quantum II system rather than to improved accuracy in interpreting the test reactions. This is consistent with the observation that "noidentification" was obtained for more

isolates

tested with the Rapid E system than with the

Quantum II system(Tables 2 and 3). Itis possible that the

TABLE 7. Reproducibility ofRapid E andQuantum II

biochemicaltests

No. oforganisms No.oforganisms

with two identical with threeidentical Organism (no. tested) biocodes" by: biocodesby:

Rapid E Quantum 11 RapidE QuantumII

Escherichia coli(12) 11 8 7 3

Klebsiella pneumoniae(10) 9 8 1 3

Proteusmirabilis (5) 5 4 3 1

Serratiamarcescens (4) 4 3 0 0

Enterobacter aerogenes (3) 3 3 3 1

Enterobactercloacae(3) 2 1 2 0

Citrobacterdiversus (1) 1 1 1 1

Shigellasonnei(1) 1 1 1 0

Morganella moorganii(1) 1 1 1 1

"Eachorganismwastestedin bothidentification systemson 3consecutive days.

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testaccuracyof the Rapid E system will be furtherimproved withexpansion of the database.

Inoculation, incubation, and interpretation of the two systems were relatively problem-free. Both systems require working with an inoculum adjusted to the turbidity of a 0.5 McFarland Standard.However,this inoculum waseasier topreparewith the RapidEsystembecause theisolatewas suspendedin 1.25 mlratherthan 5 ml as withtheQuantumII system. DMALaboratories recommendspreparing the Rap-id E inoculum by picking colonies with their pipette. We found thisproceduretobecumbersome and useda bacterio-logical loopor cotton swabtoprepare theinoculumfor most of our tests. This change did not affect the accuracy of the testresults. Theincubation time for the RapidE systemis4 h, andfor the Quantum II it is 4 hfor Enterobacteriaceae and 5h for other bacilli. We found that the Rapid E system hadtobe read between4and4.5h, withidentificationerrors encountered if the system was incubated for a different period of time. We recommend incubating the Quantum II for 5h before reading. Although the results for Enterobac-teriaceae were accurate at 4h,weencountered problemsif thetestshadtobereincubated foranadditional 1h because theindole reaction had tobe read before thecartridge was inserted into the photometer. If the cartridgewas reincubat-ed, then the indole reaction would cause false-positive

reactions

in the adjacent wells of the

cartridge.

The actual

interpretation

of results obtained with both systems was simple. DMS Laboratories included a color reaction chart with the RapidE systemwhich helpedtoresolve interpreta-tion of borderline reacinterpreta-tions. This chartwasinpart responsi-ble for the excellent test

reproducibility

with this system. The Quantum II reactions were automatically read by the photometer of this system, and no problems were experi-enced with this instrument.

Afinal consideration with these identification systems is testcost.The lowest listprices for theRapidEand Quantum II systems are$1.93 and$1.64, respectively. The QuantumII price includes lease of the instrument fromAbbott Labora-tories. Another variablethat willinfluencethe costofusing an

identification

system is the personnel time

required

to

inoculate

andtoread thesystem.Inour

experience

both the

Quantum

IIand

Rapid

E systems

required

approximately

the same amountofprocessing time.

Although

the

interpretation

oftheQuantumII reactionswas

performed by

the photom-eter,the

reading

time was slow, and eachcuvettehadtobe manually inserted into the instrument. Thus, the

major

advantage of the Quantum II photometerwasthe standard-ized interpretation of thetest reactions rather thanspeedor reduction of technical processing time.

Insummary,we found both the

Rapid

Eand

Quantum

II systems tobe accurate for the identification of Enterobac-teriaceae. In

addition,

the

Quantum

II system

accurately

identifiedmany other

gram-negative

bacilli,

including

Pseu-domonas aeruginosa and Acinetobactercalcoaceticus. The identificationaccuracyoftheRapidE systemZm befurther improved by expanding the database to include more

bio-chemically

variant strains of Enterobacteriaceae and to excludeoxidase-negative strainsoforganismsnot

specifical-ly identified bythe system (e.g.,

Acinetobacter-spp.).

Both systems were easy touse and were inexpensive.

LITERATURE CITED

1. Aldridge, K.E.,andR. L.Hodges. 1981. Correlation studiesof Entero-Set 20, API 20E, andconventional mediasystems for Enterobacteriaceae identification. J. Clin. Microbiol. 13:120-125.

2. Bale, M.J., and J. M. Matsen. 1981. Time-motion and cost comparison study of Micro-ID, API 20E, and conventional biochemical testing in identification of Enterobacteriaceae. J. Clin. Microbiol. 14:665-670.

3. Barry, A.L., R. E. Badal,andL. J. Effinger. 1979. Identifica-tion of Enterobacteriaceae in frozen microdilution trays pre-paredby Micro-Media Systems. J. Clin. Microbiol. 10:492-496. 4. Barry, A. L., T.L. Gavan, R. E. Badal, and M.J. Telenson. 1982.Sensitivity, specificity,andreproducibility of the AutoMi-crobic system (with the Enterobacteriaceae-plus Biochemical Card)foridentifyingclinicalisolates of gram-negative bacilli. J. Clin. Microbiol. 15:582-588.

5. Costigan, W. J., and G. E. Hollic. 1984. Use of the Autobac IDX system for rapididentification of Enterobacteriaceae and nonfermentative gram-negative bacilli. J. Clin. Microbiol. 19:301-302.

6. Edberg, S. C., B. Atkinson, C. Chambers, M. H. Moore, L. Palumbo, C.F.Zorzon,andJ.M.Singer.1979.Clinical evalua-tion of the MICRO-ID, API 20E, and conventional media systemsfor identification of Enterobacteriaceae. J. Clin. Micro-biol. 10:161-167.

7. Isenberg, H. D.,T. L.Gavan,P. B.Smith,A.Sonnenwirth,W. Taylor, W. J. Martin, D. Rhoden, and A. Balows. 1980. Collabo-rative investigation of the AutoMicrobic System Enterobac-teriaceaebiochemical card. J. Clin. Microbiol. 11:694-702. 8. Kelly, M. T., and J. M. Latimer. 1980. Comparison of the

AutoMicrobicSystemwithAPI, Enterotube,Micro-ID, Micro-MediaSystems, and conventional methodfor identification of Enterobacteriaceae. J. Clin. Microbiol. 12:659-662.

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