Construction of an interpretive pattern directory for the API 10 S kit and analysis of its diagnostic accuracy

JOURNALOFCLINICAL MICROBIOLOGY, June1975,p.515-520 Copyright01975 AmericanSociety for Microbiology

Vol. 1, No. 6 Printed inU.S.A.

Construction of

an

Interpretive Pattern Directory for the API

10

S Kit and Analysis

of its Diagnostic Accurary

E. ARTHURROBERTSON* AND JAMES D. MAcLOWRY

ClinicalPathology Department, Clinical Center,National Institutes of Health, Bethesda, Maryland20014 Received forpublication 18March1975

A directory oftest patterns and their interpretations has been prepared for

identification

of Enterobacteriaceae by using the 11-test API 10 S kit. The

diagnostic accuracy ofthe directory and kit wereevaluated by using recordsof

test results for 37,476 isolates studied with the 21-test API 20 Enteric kit.

Analysis indicates that 96.9% of the isolates would have been correctly identified

atthegenuslevel and 95.9%atthe species level by using only the subsetoftests

included in the API 10

S.

A

variety

of

commercial kits

are

availabe

for

the

diagnosis

of

bacteria

of

medical

signifi-cance,

mainly

inthe

family

Enterobacteriaceae.

Most of these kits consist of

6 to 21 tests

which

give a

pattern

of results

allowing bacterial

identification

to

the

genus or

species level with

varying

degrees

of certainty. It is

generally

conceded that

a

large number

of

appropriately

chosen

tests

will allow

more accurate

identifica-tion than will a

smaller number

of tests.

Recently

we

analyzed

one

such

system

(1),

the API 20 Enteric

(Analytab

Products Inc.,

Carle

Place, N.Y.), using

avery

large

data

base

and

a

computer-assisted

mathematical model.

This

analysis confirmed the high degree

of

diagnostic

accuracy

that

could be

expected

utilizing such

a21-test set.

The

same

manufac-turer

produces

a

product which contains

11 of

the

21 tests, the

API

10

S.

Unfortunately,

this

kit

does

not

have

an

appropriate interpretive

manual

to

allow

maximum

utilization

of its

diagnostic

potential. Startin'g

with the data

base which exists

for

the

21 tests, we

used the

results

corresponding

to

the

11-test

subset and

evaluated

theoretically

the

diagnostic

accuracy

of

this

subset.

In

addition,

atest

directory

was

constructed which

can

be

used

to

diagnose

the

pattems

encountered. The

potential

diagnostic

accuracy of

each

patternwas

evaluated.

MATERIALS AND METHODS

The API ProfileRegister (AnalytabProductsInc.,

CarlePlace, N.Y.)

provides

ascheme for

identifying

Enterobacteriaceae on the basis of 21 biochemical tests performed with the API 20 Enteric kit. The biochemical reactions

beta-galactosidase, arginine

dihydrolase, lysine

decarboxylase,

ornithine

decar-boxylase, citrate (Simmons),

hydrogen

sulfide,

urease, tryptophane deaminase,

indole,

Voges-Pros-kauer, gelatin, glucose, mannitol,

inositol,

sorbitol,

rhamnose, sucrose, melibiose, amygdalin, arabinose, and oxidasearereadaspositiveornegative at 18 to 24 h. A smaller version of the kit (API 10S) includes the eleven tests beta-galactosidase, glucose, arabinose, lysine decarboxylase, ornithine decarboxylase, cit-rate, hydrogen sulfide, urease, tryptophane deami-nase, indole, and oxidase.

By using theplastic APICoder(AnalytabProducts Inc.), testresultsarereducedto auniqueprofile num-ber. The coding scheme groups the testinto triads. The threetestswithin eachtriadaregivenweightsof 1, 2,and 4,respectively. Addingup theweightsof the positivetests in a triad gives the code digit for that triad. The codedigitsforall the triads takeninorder form aprofilenumber, which uniquely represents the pattern of test results (Table 1). The API Coder mechanizes this process, eliminating manual group-ing,weighting, and adding. With theAPI20Enteric, the user simply looks up the profile number in the Profile Register to find the genus and usually the speciescorrespondingtotheobserved pattern oftest results. The manufacturer does not provide a register for interpretation of profile numbers obtained with the API 10S.

Thisstudyused theprofile list and identifications

found in the November 1974 edition of the Profile Register. The manufacturer made available to us a large fileof test resultson isolates which have been examined using the API 20 Enteric. Thus we were abletoassignafrequency ofoccurence toeachpattern

listed in the Profile Register forEnterobacteriaceae. When the Profile Register listed twopossible identifi-cations for a single pattem, the frequency of occu-rencewas divided between the two, and the pattem was processed under both identifications. If this pattern had beenobserved only once, bothpossible

identifications were assigned afrequency of 1. Since

the API 20 Enteric Profile Rgister includes some theoretical profiles which have not actually been observed, occasional frequencies are listedas 0. The 1,248 patterns included in theProfileRegister repre-sentedatotalof37,476 isolates. Theoriginalcultures themselveswerenotavailableto usfordirect

microbi-ologicalstudy.

515

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516 ROBERTSON AND MAcLOWRY

TABLE 1. Sample calculation of profilenumberfrom

testpattern

Testa [ Weigt

jSample

Weightsof Profile

Weight pattern positives no.b

1

2

4

2

4

1

2

4

1

2

+ 1

2 4

1

4 ONPG

GLU ARA

LDC ODC CIT

H2S URE TDA

IND

OX

aAbbreviations: ONPG, beta-galactosidase; GLU,

glucose; ARA, arabinose; LDC, lysine decarboxylase; ODC, omithine decarboxylase; CIT, citrate; H2S, hydrogen sulfide; URE, urease; TDA, tryptophane deaminase; IND,indole; and OX, oxidase.

bSum ofweightsofpositives.

These data were placed in computer files on an IBM 370 time-sharing computer system. For each 21-testpattern("longpattern") inthe ProfileRgister, the pattern of results corresponding to the 11 tests included in the API 10 S was determined. Foreach 11-test pattern ("short pattern") a four-digit profile number was calculated (Table 1). Using the com-puter, alistwasthen preparedshowingthefollowing information foreach four-digit profile number (i.e., for eachuniqueshortpattern):theseven-digitprofile number ofeach longpattern which reduced to this particular short pattern, the identification listed in

the Profile Register for each of these long patterns, and the number of isolatesactuallyobserved exhibit-ing eachofthesepatterns.

Each short pattern results from the reduction or

one or more long patterns. If the long patterns reducing to a given short pattern have only one identification, the short pattern is unambiguous. Ii longpattemsrepresentingdifferentgenusesorspecies

reducetothesameshortpattern,thatshortpatternis ambiguous. Also, if the Profile Register lists twc possible identifications forasingle longpattern, the short pattern towhich itreduces willof necessitybe ambiguous.

Theisolateswhose longpatternsreducedtounam

biguousshortpatterns,shortpatternsambiguous

only

for thespecies, and shortpatterns ambiguousatthe genuslevelweretabulated (Table2).

To assess the diagnostic adequacy of the 11-test

battery used in the API 10 S, we tabulated thE

identification errors which would be made if thes 37,476 isolateswerestudiedwith only the 11 testsol

the API 10 S strip. Using a technique of "besi

judgment," those isolates exhibiting an ambiguou short pattern wereassigned the identification whici

J. CLIN. MICROBIOL.

had been observed most frequently for that pattern (Table 3).

In additioneach short pattern was analyzed bya previously described computer program (1) which assigns an identification onthe basis ofamaximum likelihooddiagnostic model.

Sincethe Profile Registernowincludes adirectory of patterns produced by non-Enterobacteriaceae gram-negativerods,wereducedtheselongpatternsto

short patterns to determine which short patterns exhibited by Enterobacteriaceae mightalso be shown byother gram-negativerods.

Finally, asummarydirectoryofshort-pattern pro-file numberswasprepared (Table 4).Foreachpattern

the numberofEnterobacteriaceae ineach diagnostic category reducing to that short pattern is shown. Footnotes indicate which patterns might also repre-sent non-Enterobacteriaceae and which include iso-latesthat are ambiguous evenwhen all 21tests are performed.

RESULTS

Table 2 summarizes the results of reducing

each of the API-20

profiles

to their 11-test

profile. The original

data base

consisted

of

37,476 isolates

representing

1,248

profiles.

For

21,410 isolates, or 57.13%, the 21-test pattern

reduced

to an unambiguous 11-test pattern.

There was

ambiguity

in the 11-test

profile

diagnosis

at the genus level for 14,506 isolates,

or

38.71%.

Ambiguity

existed at the

species

level

for an

additional

1,560isolates,or4.16%.A

very small

subcategory

of the

unambiguous

patterns

emerged

in which the 21-test

profile

numbers

represented

one

diagnostic

entity, but

on

reduction

tothe11-test

profile,

thecomputer

made adifferent

diagnosis.

This

occurred

at

the

genuslevel with 91 isolates,or0.24%, andatthe

r TABLE 2. Resultsofreducing the21-testpatterns to

R 11-testpatterns

Type of shortpatternproduced _isolatesNo. of %Total

Unambiguous 21,410 57.13

Ambiguousatgenuslevel 14,506 38.71 Ambiguousatspecies level only 1,560 4.16

b

e e

f it

h

TABLE 3. Remainingmisidentifications after applying the best judgment technique to the short

patterns

Categoryofmisidentification No. of %Total isolates

Misidentified at genus level 1165 3.11 Misidentified only at species 382 1.01

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INTERPRETIVE PATTERN DIRECTORY FORAPI 10SKIT

TABLE 4. Directory of profile numbers, identifications, and frequencies

Profile Identification Frequency |

Profile

Identification Frequency

0071 0221 0261 0270 0271 0630 0660 0661a 0670 1301 2000b 2001b 2040 2041 2051 2060a 2061 2070 2071 2100 2101 2110 2200 2201b 2220 2221 2230 2250 2260a 2261 2270 2271 2300 2301 2310a 2311 2320 2370 2421 2440 Proteus vulgaris Proteus morganii Proteus morganii Proteus mirabilis Proteus mirabilis Proteus mirabilis Proteus mirabilis Proteus mirabilis Proteus morganii Proteus mirabilis Escherichia coli Shigella sp. Klebsiellarhinoscleromatis Shigella sp. Providencia alcalifaciens Providencia alcalifaciens Proteus vulgaris Proteus vulgaris Proteusmorganii Proteus mirabilis Proteus vulgaris Proteus vulgaris Proteus rettgeri Proteus mirabilis Proteus vulgaris Proteus vulgaris Salmonellatyphi Escherichia coli Salmonellatyphi Shigella sp. Escherichia coli Yersiniaenterocolitica Proteusmorganii Yersinia enterocolitica Proteus mirabilis Proteusmirabilis Proteusmirabilis Proteusmorganii Proteusmorganii Proteus mirabilis Proteus mirabilis Proteus mirabilis Enterobacterhafniae Salmonella cholerae-suis Serratiamarcescens Edwardsiella Escherichia coli Salmonellacholerae-suis Salmonella enteritidis Edwardsiella Escherichia coli Enterobacterhafniae Proteusmirabilis Proteusrettgeri Providenciaalcalifaciens 3 6 21 23 0 1 8 0 0 24 5 29 8 29 2 18 1 4 4 3 1 21 11 9 6 198 16 13 2 2 12 5 25 1 14 6 102 34 1011 7 1801 6 2 1 1 7 4 4 1 13 1 3 4 5 16 2441 2450 2451 2460 2461 2470 2471 2541 2620 2630 2641 2650 2660 2661a 2670 2671 2700 2710 2720 2760 2770 3000a 3001a 3020 3100 3101 3111 3200 3201a 3220 3221 3270 3300 3301 3311 Providencia stuartii Providenciaalcalifaciens Proteus rettgeri Providencia stuartii Providencia sturatii Providenciaalcalifaciens Proteusrettgeri Proteusvulgaris Proteusmirabilis Proteusrettgeri Proteus mirabilis Proteusrettgeri Proteusvulgaris Proteusrettgeri Providencia stuartii Proteus mirabilis Proteus mirabilis Providenciaalcalifaciens Proteusmirabilis Proteus mirabilis Proteus mirabilis Proteusmorganii Proteusmirabilis Proteus mirabilis Serratia marcescens Salmonella enteritidis Serratia marcescens Proteus mirabilis Proteus mirabilis Shigella sp. Enterobacteragglomerans Pectobacterium Yersiniapestis Klebsiellaozaenae Escherichia coli Shigella sp. Yersiniapseudotuberculosis Escherichia coli Serratiamarcescens Klebsiella ozaenae Escherichia coli Escherichia coli Escherichia coli Shigella sp. Yersinia enterocolitica Escherichia coli Yersinia enterocolitica Citrobacter sp. Yersinia enterocolitica Yersinia enterocolitica Proteusmirabilis Serratia marcescens Enterobacterhafniae Escherichia coli Escherichia coli 481 67 1 0 9 2 10 4 3 200 9 0 99 10 3 6 1 2 6 389 67 65 2213 27 16 0 2 4 5 2 1 1 1 0 87 8 0 7 3 0 108 3 2 2 2 105 23 3 1 0 1 12 3 382 1 aSome isolates

exhibiting

thispatternremain

ambiguous

evenwhentheir

long pattern

hasbeen determined with theAPI 20Enteric. IThis patterncanalso be exhibitedbyothergram-negativerods notbelongingto the

family Enterobacteriaceae.

I11i

I

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ROBERTSON AND MACLOWRY

TABLE 4-Continued

Profile 1 Identification I Frequency

I|

Profile 1 Identification Frequency

no. _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ no. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

3400b 3410 3420 3461 3500 3520 3600 3601 3610 3611 3620 3621 3670 3700 3701 3720 4261 4520 5301 5520 6000b 6001 6041 6061 6071 6101 6200 6201 6210 6220 6221 6261 6270 6300" 6301 6310 6311 6320 6410 6441 6500 6510 6520 6521 6600 6601 6610" 6660 6670 6700 Serratiamarcescens Citrobacterfreundii Citrobacterfreundii Yersiniapseudotuberculosis Proteusrettgeri Serratiamarcescens Serratiamarcescens Serratiamarcescens Enterobacter cloacae Citrobactersp. Citrobacterfreundii Citrobacter

freundii

Serratialiquefaciens Yersinia enterocolitica Yersiniaenterocolitica Proteusmirabilis Serratia marcescens Enterobacter aerogenes Serratiamarcescens Serratiamarcescens Proteus morganii Klebsiella pneumoniae Escherichiacoli Klebsiellapneumoniae Shigellasp. Shigellasp. Escherichiacoli Providenciaalcalifaciens Proteusrettgeri Proteusvulgaris Escherichiacoli Shigellasp. Salmonella enteritidis. Escherichia coli Salmonella enteritidis Yersinia enterocolitica Yersinia enterocolitica Proteusmorganii Proteus mirabilis Enterobacter

hafniae

Salmonella enteritidis Escherichiacoli Salmonella enteritidis Edwardsiella Escherichiacoli Enterobacterhafniae Salmonella enteritidis Providencia stuartii Klebsiellapneumoniae Salmonellaenteritidis Klebsiellapneumoniae Klebsiellapneumoniae Enterobacter cloacae Citrobacter sp. Salmonellaenteritidis Citrobacter freundii Proteus mirabilis Proteusmirabilis Salmonella enteritidis Serratialiquefaciens 14 0 3 0 6 32 9 7 1 11 7 2 4 0 0 2 341 1 1 75 12 3 6 4 14 43 25 1 1 6 78 12 8 48 7 39 8 2 17 38 14 166 13 1 1 5 5 3 5 13 38 16 27 1 7 5 2 19 201 21 6710 6711 6720 7000a 7001a 7010 7011 7020a 7030 7040 7041 7100 7101 7110 7111 7120 7121 7200 7201a 7210 7211 7220 7221 7230 7300 7301 7311 7320 7400a b Enterobacteraerogenes Enterobacter hafniae Salmonella enteritidis Salmonella enteritidis Serratialiquefaciens Enterobacter agglomerans Klebsiella ozaenae Escherichia coli Citrobacterfreundii Shigella sp. Yersiniapestis Escherichiacoli Shigella sp. Citrobacterfreundii Citrobacterfreundii Enterobacter agglomerans Klebsiellapneumoniae Yersinia pseudotuberculosis Citrobacterfreundii Enterobacter agglomerans Enterobacter agglomerans Klebsiellapneumoniae Escherichia coli Klebsiella ozaenae Escherichiacoli Klebsiella pneumoniae Escherichia coli Escherichia coli Klebsiellapneumoniae Klebsiella ozaenae Klebsiellapneumoniae Shigellasp. Enterobacter cloacae Citrobactersp. Serratialiquefaciens Citrobacterfreundii Yersinia enterocolitica Escherichiacoli Citrobacter sp. Yersinia enterocolitica Citrobacterfreundii Citrobacterfreundii Yersinia enterocolitica Yersinia enterocolitica Citrobacterfreundii Enterobacter

hafniae

Enterobacteraerogenes Escherichiacoli Serratialiquefaciens Arizona sp. Escherichia coli Escherichia coli Enterobacter

hafniae

Enterobactercloacae Enterobacteragglomerans Citrobacter sp. Citrobacterfreundii Klebsiella sp. Pectobacterium Serratia rubidae 11 11 337 2 2 51 9 6 5 3 3 1113 8 101 8 7 6 1 18 0 1 31 30 10 2942 6 1 64 63 5 25 106 17 6 5 2 1 2401 37 15 24 1 6 5 2 128 17 17 7 1 8634 50 3 48 38 12 11 11 2 1 H -L

J. CLIN. MICROBIOL.

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INTERPRETIVE PATTERN DIRECTORY FOR API 10 S KIT

TABLE 4-Continued

Profile

Identification

Frequency Profile

Identification

Frequency

no. no.

7401a Enterobacter agglomerans 28 7601 Citrobacter sp. 204

Citrobacter freundii 19 Enterobactercloacae 23

Klebsiellapneumoniae 17 7610 Citrobacterfreundii 82

Citrobacter sp. 5 7611 Citrobacterfreundii 18

7410 Citrobacterfreundii 359 7620 Enterobactercloacae 31

7411 Citrobacterfreundii 12 Serratia liquefaciens 4

7420a Klebsiellapneumoniae 256 Yersiniaenterocolitica 0

Enterobacter agglomerans 171 7621 Citrobacter sp. 3

7421' Klebsiellapneumoniae 22 7630 Citrobacterfreundii 3

Enterobacter agglomerans 1 7700 Enterobacteraerogenes 751

7430 Citrobacterfreundii 22 Serratialiquefaciens 308

7500 Klebsiellapneumoniae 486 Enterobacter

hafniae

64

Serratia rubidae 4 Arizona sp. 6

Klebsiellaozaenae 1 Enterobactercloacae 1

7501 Klebsiellapneumoniae 558 7701 Citrobacter sp. 4

Escherichia coli 0 Arizona sp. 1

7510 Citrobacterfreundii 1 7710 Arizona sp. 283

7520 Klebsiellapneumoniae 4373 Salmonellaenteritidis 1

7521 Klebsiellapneumoniae 906 7720 Serratia liquefaciens 55

7600a Enterobacter cloacae 2144 Enterobacteraerogenes 28

Citrobacterfreundii 181 Enterobacter cloacae 2

Enterobacter agglomerans 32 7721 Klebsiellapneumoniae 41

Serratialiquefaciens 13

species

level with

13

isolates,

or0.03%.

In

analyzing the discrepancies,

it was found

that when

two or more species

reduce

to the

same 11-test

profile number, the

frequency in

the data base of

one

of these

organisms is often

greater

than

any of

the others

that cause

am-biguity.

Therefore,

we

chose

to

further analyze

the data using

a

technique

of

best judgment

to

make

a

diagnosis

for

each

of

these

ambiguous

11-test

profile numbers. This

was

done by

assigning

to

the

11-test

profile

numbers

the

diagnosis which had the

greatest

frequency

in

the

original data base.

In many cases

the

use of

the best

judgment

will

give

acceptable

diagnos-tic certainty. For

other

categories

additional

tests are

needed.

Table

3

outlines

the results

of

applying the

best

judgment criterion

to

the

11-test

patterns.

At

the

genus

level, 1,165 isolates,

or

3.11%,

would

not

have

been

diagnosed correctly.

An

additional

382

isolates,

or

1.01%,

would

not

have

been

diagnosed

correctly

at

the

species

level. In other

words, using the

unambiguous

diagnoses and

the

best

judgment

technique,

the

correct

diagnosis

can be made 96.89% of

the

timeatthe genus

level

and

95.88%

ofthe

time

at

the

species

level,

when

working with

the

family

Enterobacteriaceae.

An

additional

analysis

was

performed

onthe

21-test

profiles available

to us

by looking

at

those

organisms

whichwere not

members

ofthe

family Enterobacteriaceae. These

profiles

were

also reduced

to 11-testprofile numbers. Only six

of

the

Enterobacteriaceae

profiles were also

produced

by the reduction of patterns for other

gram-negative rods. Of the Enterobacteriaceae,

229

isolates,

or 0.61%, fell into these six

"mixed"

patterns, which

have been footnoted

as

b in the directory (Table

4).Since we lacked

information

on

the

frequency of the

non-Enterobacteriaceae which

give these patterns, it

is not

possible

to assess the uncertainty

associ-ated with

the diagnosis

ofthese profiles.

DISCUSSION

We have

attempted

toevaluate

the

adequacy

of

the API

10

S

for

identifying

clinical

isolates

of

Enterobacteriaceae,

to compare its accuracy

with

that of

the

API 20

Enteric,

and

to

provide

a

practical guide for interpreting results obtained

with

the

11-testkit.

By using

one or two

hundred

isolates,

it is

relatively

simple

to compare

the results

ob-tained

by

using

two

different

kits

ona

test-by-testbasis. In the clinical

laboratory,

however,

it

is not the

comparability

of

individual

test

results, but

the

equivalence

ofthe final

identifi-cationswhich is

important.

Satisfactory

evalua-tion of thisoverall

diagnostic

accuracy

requires

study

ofmany more isolates - isolates

repre-sentative ofthe

kinds and

frequencies

of

orga-nisms

identified

in a

clinical

laboratory.

Since

the rate ofcorrect

identifications

is

influenced

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ROBERTSON AND MACLOWRY

by the

assortment of

isolates used

to

evaluate

a

kit,

it is

difficult

to compare

the

accuracy of

different kits

if

they have

not

been tested

against

the

same set of

isolates.

We were

fortunate to

have records

on over

37,000

iso-lates representing

most of

the

clinically

recog-nized

Enterobacteriaceae

species.

Furthermore,

we were

able

to compare

the

expected

accuracy

of the API 10

S and the

API 20

Enteric when

applying

both

to

the

same setof

isolates.

Several limitations

of

this

study

should

be

pointed

out, however.

Although this study used

a

data base

of over

37,000

isolates,

these isolates

may not

be

typical

ofthe

distribution

of

orga-nisms

found

in a

particular

clinical

laboratory.

Furthermore we

did

not

have

access to

the

original cultures

or

control

over

the

biochemical

testing.

For

ambiguous

profiles which have been

ob-served only

a few

times,

calling the

identifica-tion

which

has

been

seen most often

the best

judgment

is

somewhat

arbitrary. With small

numbers the statistical

sampling

errors

become

much

more important

and the choice

of

the

most

likely identification becomes less

certain.

We have

considered

only

those

patterns

in-cluded

in

the API Profile

Register. Experience

indicates that this includes the

majority

of

clinically isolated Enterobacteriaceae.

None-theless,

a

small but real

fraction of

isolates

are

not in the Profile

Register

and

thus

have not

been

considered

in

constructing this

directory.

Incorporating these

patterns into

the data

base

as

they

are

observed would

not

be

expected

to

have a major effect on the

directory,

but it

would

doubtlessly change the relative

frequen-cies,

particularly

forthe uncommon patterns.

The

possible

confounding

effect of

gram-neg-ative rods not

belonging

to the family

Enterobacteriaceae

is more difficult to

evalu-ate. We have

marked with

a footnote thoseshort

patterns

which could result

from the reduction

of

the

long

pattern of a gram-negative rod listed

in the Profile Register for

non-Enterobacteri-aceae. However, we lack information on the

frequency of occurrence of the various

non-En-terobacteriaceae

patterns, and we do not know

howcomplete is the listing of thesepatterns.

With 11 tests there are 211 or 2,048 possible

test patterns. The 37, 476 Enterobactertaceace

on

which

our

directory is based all fell into

one

of

only

177 patterns. Most

non-Enterobac-teriaceae fall into one of the other

1,871

pat-terns; however, the exact degree of overlap is

uncertainatthis time.

Not surprisingly, an element of diagnostic

ambiguity

is

introduced when only

the

abbrevi-ated

set of tests is

used. Performing all

21tests

does

not,

however,

remove

all

ambiguity;

1,175

isolates

have

long

patterns

which have

two

possible

identifications in the

Profile

Register.

If noadditional tests were performed and these

isolates were

simply

assigned

to the most likely

identification

on the

basis

of their long pattern,

480

would be misidentified

at the genus

level

and

585 at the species

level.

This represents

1.28%

and

1.56%,respectively, of the total

num-ber

of

isolates.

Thus,

many

of

the

ambiguities

and misidentifications

which occur when

us-ing the

directory

of short patterns

result

from

ambiguities which

are not

resolved

even

with

the

battery

of 21tests.

In

practice

users of the

API

20

Enteric

per-form additional tests on an isolate

whose

pat-tern is

ambiguous

in

the Profile Register

(notes

beside the

pattern in

the Register generally

suggest

which additional

tests

would

resolve the

problem).

The same course of action is open to

users of

the

directory

of

short

patterns. For

example,

a

deoxyribonuclease

test

would

sepa-rate most of the 308

Serratia

species having

profile number

7700

from

the816 Enterobacter

species with the same

profile

number. In fact,

selectively performed deoxyribonuclease

tests

could reduce the overall rate of genus

misassign-mentsfrom

3.1%

to

2.2%.

ACKNOWLEDGMENTS

Weacknowledge the cooperationofPierre Janin of Analy-tab Products Inc. for making available to us unpublished statistical information from API records for use in this study.

LITERATURE CITED

1. Robertson, E. A., and J. D. MacLowry. 1974. Mathemati-calanalysis of the API Enteric 20 Profile Register using acomputer diagnostic model.Appl. Microbiol. 28:691-695.

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