Urine screening with the MS 2

Vol. 17, No. 6 JOURNALOFCLINICALMICROBIOLOGY, June 1983,p.1061-1065

0095-1137/83/061061-05$02.00/0

Copyright©1983,AmericanSociety for Microbiology

Urine

Screening

with the MS-2

DARYL J. HOBAN,*JOAN C. KOSS,CLAUDIA A.GRATTON, ANDALLANR. RONALD DepartmentofClinicalMicrobiology, Health Sciences Centre andDepartment ofMedicalMicrobiology,

University ofManitoba, Winnipeg,Manitoba, Canada R3E OW3

Received 20 December1982/Accepted22February1983

A study was undertaken to evaluate the effectiveness of the MS-2 (Abbott

Laboratories, Dallas, Tex.) in screening urine specimens in a large clinical

laboratory. A total of 15,319 urine specimens (9,954 midstream specimens and

5,365 catheter specimens)wereevaluated with the MS-2 andby asurface streak

procedure. The studywasconductedintwophases, differingin thatphaseIIurine

specimens were evaluated in the MS-2 by using a program software update

(03.01). For midstream urine specimens, MS-2 detectionratesinphases I andII

were, respectively, 94.5and 94.3% ataplate count of>105 CFU/ml, 74.4 and

65.3%at

plate

countsof5 x 100to5 CFU/ml,55.0 and 52.4% atplatecounts

of 1 x 10 to5 x

104

CFU/ml,31.2 and 20.5%atplatecountsof

103

to104CFU/ml,

and 15.7 and 6.4%atplatecountsof

<103

CFU/ml. For catheter urinespecimens,

the MS-2 detectionratesinphasesIand IIwere, respectively,95.4and96.8%at

plate counts>105 CFU/ml, 74.4and 85.7%atplatecounts of5 x 104to1 x 105

CFU/ml, 50.0 and44.4%atplatecounts104to5 x 104 CFU/ml,25.6and 14.9%at

plate counts of 103 to 104 CFU/ml, and 14.7 and 5.2% at plate counts <103

CFU/ml.

Urine cultures are the most frequent

proce-duredone inroutine hospital laboratories. At the

Health Sciences Centre in Winnipeg, over

50,000 urinespecimensarecultured forbacteria

eachyear. Ascurrently carriedout,preliminary

resultsarenotavailabletothe clinician until24 h

after the culture is received. At one time, a

single organism at a concentration of

105

CFU/ml inaclean-voided urinewasconsidered

significant (9). Recently it has beenshown that

symptomatic bacterial urinary infection isoften

presentwithcounts aslowas

104

CFU/ml(16).

Many hospital laboratories are now reporting

thelowerbacterialcountsifasingle organism is

present, and decisions about their importance

aremade by clinicians.

Since only 20 to 30% of urine specimens

containover

104 CFU/ml

and35 to

45%

of urine

specimensarenegative

(<103

CFU/ml),

consid-erable time and effort is expended processing

negative urine specimens. A variety of rapid

screening tests have been introduced to

deter-mine whether infection is present without

fur-ther cultural investigation. Screening methods

include direct microscopy of stained smears (4,

12),microscopyoffresh urinespecimens (2, 10),

chemical testing ofurine specimens(1, 4, 5, 8),

and acolorimetricmethod (17). Recently,

sever-al automated systems have been marketed to

detect organismsin urine based upon changes in

light transmission. One of these, the MS-2

(Ab-bottLaboratories, Dallas, Tex.) has previously

been

reported

todetectbacteriuria

(6,

13,

14).

In

each of these reports, the number of urine

specimens screened was

small,

and

only

mid-stream urine

specimens (msu)

were examined.

We

compared

the MS-2 to a

quantitative

loop

surface streak

technique

to screenforinfection

in 15,319 urine

specimens.

MATERIALS AND METHODS

Specimens. In the overallstudy,15,319 urine speci-mens from both in- and outpatients at the Health Sciences Centre were studied. The Health Sciences Centre isa1,200-bedtertiarycare

university-associat-edhospital. Inpatientsaccountedfor 68% of the

proc-essed urinespecimens,whereasoutpatientsaccounted for32%. Of thistotal, 9,954weremsu,whereas5,365 werecatheterurine

specirmens

(cu). Specimens were received andprocessed in thelaboratorywithin1 hof collection or were refrigerated after collection and processed within10h.

Study design. Urine screening with the MS-2 was

carriedout in two phases. In phase I, 11,541 urine specimenswere processedintheMS-2, utilizing pro-gramsoftware02.02during the period15October 1981 to 5 February 1982. At the end ofphase I, Abbott Laboratories introduced a software revision to the urine screening program (03.01). During phase II, 3,778 urinespecimens werescreened between8 Feb-ruary1982and10March 1982.

Referenceprocedure.Asurface streakprocedureas describedby Barryetal.(3)wasused as thereference method.Usingacalibrated nichromeloop,0.001 mlof

a well-mixed urine specimen was inoculated onto a

split plate consisting of 5% sheep blood agar and

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TABLE 1. Number and percentage of msu in each plate count category

No. in category No. of No. of msu specimens Platecount category (% of total phase) organisms (% ofcategory)

Phase I PhaseII isolated PhaseI Phase II

I(>105) 1,175(15.8) 366 (14.4) 1 519(44.2) 179(48.9)

2 329(28.0) 110(30.1)

3 232 (19.7) 56 (15.3)

4 95 (8.1) 21 (5.7)

II(5 x 104-1 x 105) 359 (4.8) 98 (3.9) 1 83 (23.1) 16(16.3)

2 134(37.3) 32(32.7)

3 89(24.8) 40(40.8)

4 53(14.8) 10(10.2)

III(1 x

10W-5

x 104) 1,029(13.8) 347(13.7) 1 263 (25.6) 94(27.1) 2 414 (40.2) 129(37.2)

3 251 (24.4) 95(27.4)

4 101 (9.8) 29 (8.4)

IV(103-104) 1,654 (22.3) 565(22.3) 1 1,072(64.8) 380(67.3)

2 416(25.2) 159 (28.1)

3 128 (7.7) 22 (3.9)

4 38 (2.3) 4 (0.7)

V(<103) 3,200(43.1) 1,161(45.7)

MacConkey agar. The inoculatedplateswereincubat- Plainview, N.Y.). Calibrated loops were quality

con-ed at 37°C aerobicallyovernight and examined. The trolled by using the Evans blue method previously

typesand numbers oforganismspresentonthe split described (3).

plate were recorded. All plates showing growth and MS-2. Urine specimens were inoculated into the

thosewhich weremacroscopicallynegativewereincu- MS-2according to the manufacturers instructions. In bated for a further 24 h at room temperature and thepast18months, Abbott Laboratories has instituted subsequently reexamined. Isolateswereidentifiedac- several modifications in the MS-2 urine screening

cording to conventional methods (11) and in some procedure. These have includedanimprovedmaterial

casesby theuseof the API 20E(Analytab Products, forsealing the Ampvettes,introduction of agar into the

TABLE 2. Number and percentage ofcuin eachplatecountcategory

No. incategory(%of total No.of No. of cuspecimens(%of

Plate countcategory phase) organisms category)

(CFU/ml) Phase I Phase II isolated Phase I Phase II

I(>105) 741(17.9) 219(17.6) 1 396(53.4) 122(55.7)

2 199(26.9) 61(27.9)

3 124(16.7) 29 (13.2)

4 22 (3.0) 7 (3.2)

II(5 x 104-1 x 105) 106 (2.6) 28 (2.3) 1 50(47.2) 17(60.7)

2 31(29.2) 7(25.0)

3 16(15.1) 4(14.3)

4 9 (8.5) 0 (0)

III(1 x 104-5 x104) 234 (5.7) 63 (5.1) 1 74(31.6) 30(47.6)

2 100(42.7) 23(36.5)

3 40(17.1) 6 (9.5)

4 20 (8.5) 4 (6.3)

IV(103-104) 371 (9.0) 101 (8.1) 1 272(73.3) 72(71.3)

2 84(22.6) 25(24.8)

3 11 (3.0) 3 (3.0)

4 4 (1.1) 1 (1.0)

V(<103) 2,672(64.7) 830(66.9)

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URINE SCREENING WITH THE MS-2 1063

TABLE 3. Number and percentage of msu cultures detected by the MS-2 in each study phase

Phase I Phase II

Platecount

category No. No. No. No.

(CFU/ml) posi- positive by posi- positive by

tive MS-2(%) tive MS-2(%)

I(>10W) 1,175 1,110(94.5) 366 345(94.3)

II(5 x 104- 359 267(74.4) 98 64(65.3) 1 X

105)

III(1 x 104- 1,029 566(55.0) 347 182(52.4) 5 x 10k)

IV(103-104) 1,654 516(31.2) 565 116(20.5)

V(<103) 0 503 (15.7) 0 74 (6.4)

Ampvette eugonic broth to prevent the settling of organisms, and softwaremodifications.

Data entry and retrieval. Culture results including organism(s)identificationandconcentrationaswellas

allMS-2resultswererecordedon acustom-designed computer sheet. These data were then keypunched andentered into the University of Manitoba Amdahl

computer facilities. Dataretrieval was facilitated by

usingacustomprogram.

Urinetypes. Submittedurinespecimenswere classi-fiedintotwotypes: (i) clean-catch msuand(ii)cu as determinedbytherequisition accompanying the speci-men.

Plate count categories. All urine specimens were

divided intofivecategories dependinguponorganism concentration. CategoryIincludedallurinespecimens with colonycounts greaterthan 105CFU/ml. Category II included all urine specimens with colony counts betweenSx 104and

10'

CFU/ml.Category III

includ-ed all urinespecimenswithcolonycountsgreaterthan 104CFU/mlbutless than5 x 104CFU/ml. Category IV included all urine specimens with colony counts

between103 and 104 CFU/ml. Category Vwerethose urinespecimens showingnogrowth (<103 CFU/ml).

Predictivevalues.Predictive valueswere calculated

bythemethodof Grineretal. (7). RESULTS

PhaseI versus phase II urine specimens. The

urine specimens cultured during both phases of

this study displayedvery similar culture results

TABLE 4. Number andpercentageofcucultures detectedby the MS-2 in eachstudy phase

Phase I Phase II

Plate count

category No. No. No. No.

(CFU/ml) posi- positiveby posi- positiveby

tive MS-2(%) tive MS-2(%)

I(>105) 738 704(95.4) 219 212(96.8)

II(5x 104- 106 88(74.4) 28 24(85.7) 1 x 10)

III(1 x104- 234 117(50.0) 63 28(44.4) 5 x 104)

IV(103-104) 371 95 (25.6) 101 15(14.9)

V(<103) 0 392(14.7) 0 43 (5.2)

(Table1 and 2).This pattern is evident for each

platecount categoryformsu(Table1)andforcu

(Table2).Differences exist betweenmsuandcu

results. CategoryVurine specimens accounted

for 44.7% (43.1 to45.7%) of all msu (Table 1),

whereas category V cu accounted for

65.8%

(64.7to 66.9%) of the total(Table 2).

MS-2 detection. Table 3 and 4 illustrate and

comparethe MS-2 detectionrateoforganismsin

msu(Table3)and incu(Table 4),irrespective of

thenumberoforganismspresentin theurine. In

msu,theMS-2 detected 94.5 and

94.3%

of those

containing

>105

CFU/ml when comparing phase

I and phase II, respectively. As the absolute

total number ofcolony-forming unitsper

millili-terdecreasedso did the percentage

detected by

theMS-2.At aplatecountof <10 CFU/ml, the

MS-2detected15.7% of thosemsuas

positive

in

phaseI. Thisratedecreasedto6.4% in phase II

msuwith platecounts

<103

CFU/ml.

Similar results were observed for cu. At a

platecountof

>105 CFU/ml,

the MS-2detected

95.4 and96.8% ofthose urine

specimens

in the

two study phases. The positive detection rate

dropped with each decreasing platecount

cate-gory.

MS-2 single isolate detection. Table 5 and 6

comparethedetectionrateof

organisms

inpure

culture in bothmsuandcuin both

study phases

at the various plate count

categories.

In msu

with counts

>105

CFU/ml, the MS-2 detected

96.5 and 96.6% of all msu in

phases

I and

II,

respectively. The cuwiththis plate count were

detected 94.2 and

95.1%

of the time in both

phases. Astheplate countdecreasedsodid the

rate of MS-2 detections. However, even when

noorganisms wereisolated

(<103 CFU/ml),

the

MS-2was still

positive (false

positive)

in

15.7%

of all

culture-negative

msuin

phase

Iand in 14.7

of all

culture-negative

cu inphase I.This

false-positive

rate dropped

significantly

in phase II

urine specimens. In phase II,

6.4%

of all

nega-tive msu were MS-2 positive, whereas 5.2% of

all

negative

cuwere

positive.

Predictive value. Tables 7 and 8 show the

sensitivity,

specificity,

and

positive

andnegative

predictive values ofthe MS-2 in screening for

bacteriuria. Values areforpure culture isolates

only. The bacterialcount considered bya

labo-ratory to be significant varies, and therefore

predictive values were calculated attwo levels

ofsignificance for both study phases.

For msu(Table 7),there was aslight

improve-ment in sensitivity by using phase II software

and a substantial improvement in

specificity,

falsenegatives, and falsepositives, irrespective

of

significance

level. Similar results were seen forcu

(Table

8).

False-positive

rates for msu with phase II

software were 11% or lower, whereas for cu

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TABLE 5. Comparison of conventional culture and MS-2 for screeningmsuwhereonlyasingle organismis isolated

PhaseI PhaseII

Platecountcategory No. No.

(CFU/ml) No. positive positive by No. positive positive by

MS-2(%) MS-2 (%)

I(>105) 519 500(96.3) 179 173 (96.6)

II(5 x 104-1 x 10') 83 67(80.7) 16 12(75.0)

III (1 x104-5 x104) 263 135(51.3) 94 41 (43.6)

IV(103-104) 1,072 271 (25.3) 380 55(14.5)

V(<103) 3,200 (no growth) 503(15.7) 1,161 (no growth) 74 (6.4)

they were 7.3% or lower, depending upon the

significance level demanded.

DISCUSSION

Inthe first phase ofa collaborative study by

McCarthy et al. (13), the MS-2 detected about

89% of those urine specimens containing

>101

CFU/ml with afalse positive rate of 1.5% for

those urine specimens containing<10i CFU/ml.

In theirsecondphase, the MS-2 detected about

84%of those urine specimens containing

>105

CFU/ml, whereas false positive detection rates

fellto0.8% inthose urinespecimens containing

<103

CFU/ml. Pezzloetal. (14) detected about

75%of those urine specimens withcounts

>105

CFU/ml and observed false

positive

rates of

around 1.4%at counts of <10 CFU/ml. In our

study, we found the MS-2 could detect about

95% of those urine specimens containing

>105

CFU/ml; however, our false-positive rate at

<103

CFU/ml wassignificantly higherat15.7%

inphaseI,droppingto6.4%inphase II formsu.

These variances in sensitivity aredue to many

factors such asthe number of urine specimens

examined, techniquesinprocedure, the number

and types or organisms found in msu in each

institution, the ratio of inpatient to outpatient

urines, as well as disposable and software

changes instituted by the manufacturer.

Predictive values thatweobtainedwere

simi-lar to those reported by Pezzlo et al. (14) for

pure pathogens. Our data do notexclude

con-taminating organisms inpureculture and

there-fore represent minimum achievement levels.

Only diphtheroids were considered to be

con-taminants. In both cases negative predictive

valuesexceed 99%.

Our study examining cu illustrates that the

MS-2candetectorganisms in this urinetypeas

accurately asitcaninmsu. However, since the

level ofbacteriuria that a laboratory considers

significant is generally low, the ability of the

MS-2todetectorganismsatlowcounts is

ques-tionable (Table 4).

A laboratory worker would have the MS-2

perform two different tasks. One would be to

screen out all negative urine specimens. The

MS-2 performs this task very efficiently in

screening msu with false-negative rates of 3.4

and 5.1% in phase II at significance levels of

>105

and 5x

104

to1 x

105

CFU/ml,

respective-ly (Table 7). Similar results were obtained for

cu. However, at significance levels below 5 x

104

CFU/ml, the false-negative rate in these

urine specimens rose dramatically. Second, a

laboratory worker desires an instrument that

wouldnotyieldahigh false-positiverate.Inthis

area, the software modification carried out for

TABLE 6. Comparisonof conventional cultureandMS-2forscreeningcuwhereonlyasingleorganismis isolated

Phase I Phase II

Platecountcategory No. No.

(CFU/ml) No. positive positive by No. positive positive by

MS-2(%) MS-2(%)

I(>105) 396 373(94.2) 122 116(95.1)

11(5 x 104-1 x 105) 50 45(90.0) 17 15(88.2)

III(1 x 104-5 x 104) 74 41(55.4) 30 12(40.0)

IV(103-104) 272 62(22.8) 72 9(12.5)

V(<103) 2,672(nogrowth) 392(14.7) 830(nogrowth) 43 (5.2)

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URINE SCREENING WITH THE MS-2 1065

TABLE 7. Predictive values for pure culture isolates frommsuinphases I and IIat two levelsof

significance

Predictive value(%) Phase I Phase II Parameter (significance (significance

level) level) 5

xl104_

>105 51x

104

Sensitivity 96.3 94.2 96.6 94.9

Specificity 78.9 80.0 89.0 89.6

Falsenegative 3.7 5.8 3.4 5.1

Falsepositive 21.1 20.0 11.0 10.4 Positivepredictive 33.9 38.4 48.7 52.1 Negativepredictive 99.5 99.0 99.6 99.3

phase II urine specimens was quite successful.

False-positiveratesforpureculture isolate urine

specimens dropped from21to11% formsu.The

sensitivity of the instrument is excellent, being

able to detect >94% ofpositive urines inboth

study phases, dependingupon levels of

signifi-cance required. The specificity now exceeds

89% in thenewersoftware.

Ifa laboratory sets asignificance level of >5

x

104

CFU/ml in either msu or cu specimens,

the MS-2 will provide assurance that in each

urine specimen it determines to be negative,

>99% of thosespecimenswill haveplatecounts

<5 x

104

CFU/ml. Thenewestsoftware

modifi-cations have increased both sensitivity and

specificity and have lowered the false positive

and falsenegativerate. Rapidurine screeningis

TABLE 8. Predictive values for pure culture isolates from cu in phasesIand II at twolevels of

significance

Predictive value(%)

PhaseI Phase II Parameter (significance (significance

level) level) >105 5 x104- >10 5 x

104-1X105 1X105

Sensitivity 94.2 93.7 95.1 94.2 Specificity 82.4 83.6 91.7 93.1

Falsenegative 5.8 6.3 4.9 5.8

Falsepositive 17.6 16.4 7.3 6.9 Positivepredictive 40.9 45.8 59.5 67.2 Negative predictive 99.1 98.9 99.3 99.1

important in a busy hospital, and the MS-2

pointsthe way tofuture directions in this area.

ACKNOWLEDGMENTS

This work was supported by a grant from the Manitoba MedicalServices Foundation Incorporated.

Weacknowledge the excellent clerical assistance of F. Rey in preparation of this manuscript.

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