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Journal of Clinical Microbiology, September 2002, p. 3232-3236, Vol. 40, No. 9
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.9.3232-3236.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Sensitivity of Three Urinary Antigen Tests Associated with Clinical Severity in a Large Outbreak of Legionnaires' Disease in The Netherlands
Ed P. F. Yzerman,1,2* Jeroen W. den Boer,3 Kamilla D. Lettinga,2 Joop Schellekens,4 Jacob Dankert,2 and Marcel Peeters5
Regional Laboratory of Public Health Haarlem,1
Municipal Health Service Kennemerland, Haarlem,2
Academic Medical Center, Amsterdam,3
National Institute for Public Health and the Environment, Bilthoven,4
Regional Laboratory of Public Health Tilburg, Tilburg, The Netherlands5
Received 22 January 2002/
Returned for modification 12 March 2002/
Accepted 6 June 2002

ABSTRACT
In 1999 an outbreak involving 188 patients with Legionnaires'
disease (LD) occurred among visitors to a flower show in the
Netherlands. Two enzyme immunoassays (Binax and Biotest) and
one immunochromatographic assay (Binax NOW) were tested, using
urine samples from LD patients from the 1999 outbreak. Sensitivity
was calculated using positive culture and/or seroconversion
as the "gold standard" in outbreak-related patients with radiographically
confirmed pneumonia who fulfilled the epidemiological critera.
The Binax EIA, Biotest EIA, and Binax NOW assay showed overall
sensitivities of 69, 71, and 72%, respectively. When the tests
were performed with concentrated urine samples, the overall
sensitivities increased to 79, 74, and 81%, respectively. Using
multiple logistic regression analysis with backward elimination,
a statistically significant association was found between clinical
severity and test sensitivity for all tests. For patients with
mild LD, the test sensitivities ranged from 40 to 53%, whereas
for patients with severe LD who needed immediate special medical
care, the sensitivities reached 88 to 100%. These findings have
major implications for the diagnostic process in patients with
mild pneumonia and suggest that patients with mild pneumonia
may go underdiagnosed if urine antigen tests alone are used.

INTRODUCTION
Legionnaires' disease (LD) is an acute pneumonia caused by
Legionella,
a rod-shaped gram-negative bacillus ubiquitous in (man-made)
aquatic reservoirs. Currently 43
Legionella species and 65 serogroups
have been described. In the United States, over 90% of Legionnaires'
disease cases are caused by
Legionella pneumophila, of which
70% of strains belong to serogroup type 1 (
16).
Legionella spp.
are responsible for 1 to 5% of cases of community-acquired pneumonia
(CAP) (
5). Clinically and radiographically, LD cannot be distinguished
from pneumonias caused by other microbial pathogens. Because
of the high mortality rate in patients with LD requiring hospitalization,
early diagnosis to enable adequate antimicrobial treatment is
potentially life-saving. Diagnosis of LD in patients with symptomatic
pneumonia is based on culture, serologic testing, or antigen
detection in urine. Isolation of
Legionella from respiratory
secretions is not a very sensitive diagnostic test (25 to 75%
sensitivity) (
15) and has the disadvantage of delay, because
a positive result is not available until at least 3 days of
incubation. Seroconversion is a diagnostic test with a high
sensitivity and a high (serogroup-dependant) specificity, but
it is of limited clinical value since it may take up to 9 weeks
for patients to develop detectable antibodies (
10,
13).
In contrast to the other tests mentioned above, urinary antigen tests combine reasonable sensitivity and high specificity with rapid results. The reported sensitivities of both enzyme immunoassay (EIA) and immunochromatographic test (ICT) show great variation: 50 to 90% (3, 7, 8, 19). These variations may be explained by differences in patient characteristics, the serogroup with which the patient is infected, the timing of collection the urine sample in the course of illness, and whether the urine is concentrated before testing.
To assess the value of the urinary antigen tests in a large outbreak situation (6), we used three widely used and commercially available tests with urine specimens from patients with outbreak-related LD: the Biotest EIA, the Binax EIA, and the Binax NOW test.

MATERIALS AND METHODS
Patients.
In February 1999 an outbreak involving 188 cases of LD occurred
in Bovenkarspel, The Netherlands. The outbreak investigation
indicated that a whirlpool displayed at the consumer product
division of an annual flower show was the most likely source
of infection. Genotyping revealed that isolates from 27 patients
were identical to one of the environmental
L. pneumophila serogroup
1 strains (
6).
All 180 hospitalized patients with a confirmed Legionella pneumonia were included in this study after written consent was obtained from patients or their relatives. A confirmed case of LD ("gold standard") was defined as a patient who fulfilled the epidemiological criteria (visitor to the 1999 Bovenkarspel flowershow or member of the exhibition staff) and who suffered from symptoms compatible with pneumonia, who showed radiological signs of infiltration, and who showed laboratory evidence of infection with L. pneumophila. Laboratory evidence included (i) isolation of L. pneumophila from a respiratory sample cultured on buffered charcoal yeast extract supplemented with
-ketoglutarate followed by genotyping and subsequent comparison to the environmental strains of the Bovenkarspel outbreak; or (ii) a fourfold rise in the titer of immunoglobulin M (IgM) antibodies to L. pneumophila in paired acute-phase and convalescent-phase sera, with final titers of
1:32 in accord with the 99% cutoff values found in a serosurvey of healthy volunteers representative of the Dutch population (4) using a microagglutination IgM, serotype 1, antibody assay (12); or (iii) seroconversion to positive IgM or IgG antibodies to L. pneumophila in paired acute-phase and convalescent-phase sera, with age-specific titers in accord with the 99% cutoff values found in a serosurvey among healthy volunteers representative of the Dutch population (4), using a commercial enzyme-linked immunosorbent assay to detect IgM and IgG serotype 1 to 7 antibodies (Serion ELISA; Institut Virion\Serion GmbH, Würzburg, Germany) (11).
For isolation and genotyping of L. pneumophila from sputum samples, the National Institute for Public Health and the Environment was the reference laboratory; for detection of antibodies against L. pneumophila in serum, the Regional Laboratory of Public Health Tilburg was the reference laboratory.
Classification of severity of disease.
To investigate the relation between test sensitivity and severity of disease, the patients were divided into three clinical categories for CAP. Clinical data were collected from the hospital chart by using a standardized case record form. Severity of pneumonia was scored according to the minor criteria for severity of CAP advised by the American Thoracic Society (17), using the following clinical criteria at hospital admission: (i) respiratory frequency above 30 breaths per minute, (ii) PaO2 below 60 mm Hg or O2 saturation below 92%, (iii) bilateral or multilobar infiltration on chest X-ray, and (iv) systolic blood pressure below 90 mm Hg or diastolic blood pressure below 60 mm Hg.
Patients with a radiographically proven unilateral unilobar pneumonia, but without signs or symptoms according to the above-mentioned criteria, were classified as CAP category 1 (mild pneumonia). CAP category 2 consisted of patients with a proven pneumonia who fulfilled only one of the criteria; according to our definition, these patients were suffering from a moderately severe pneumonia. CAP category 3 consisted of patients who presented with two or more criteria; these patients were considered to be suffering from severe pneumonia.
Collection of urine samples.
All medical microbiologists who had assisted in the diagnosis and treatment of the pneumonia patients in to this outbreak were asked to send available urine samples from the patients to the Regional Laboratory of Public Health in Haarlem, The Netherlands. After collection, the urine samples were stored in portions at -70°C. All but eight available urine samples had been collected during the hospital stay. In four cases, the urine samples had been obtained shortly before admission, and in four cases, they had been obtained after discharge from hospital.
Urinary antigen tests.
The presence of L. pneumophila antigens in urine samples was investigated by using the Binax (Portland, Maine) and Biotest (Biotest AG, Dreieich, Germany) Legionella urinary antigen tests, both EIAs, and with the Binax NOW test, a qualitative ICT. All tests were used as specified by the manufacturers. However, to ensure maximum specificity for the ICT (14), samples giving positive tests were reexamined after 60 min. Urine samples were tested nonconcentrated and, to enhance the intensity of the reaction, after concentration by selective ultrafiltration (Minicon B15; Millipore Corp. Bed Ford, Mass.). This selective ultrafiltration system consists of a permeable membrane that permits the passage of water and substances with molecular weights less than 15,000.
Statistics.
Statistical analysis was performed with the statistical program SPSS version 10.0 (Statistical Product and Service Solutions, Chicago, Ill.). Univariate analysis (chi-square test for dichotomous and ordinal variables; independent t test for discrete variables) was used to calculate the association with positive urinary antigen test results for the following variables: age, gender, clinical severity, number of days between onset of symptoms and collection of first urine sample, and number of times a urine sample was collected. Variables that were (borderline) significant were entered in a multiple logistic regression model. Using backward elimination, independent predictors for test positivity were established. Variables were retained in the model if the likelihood ratio test was significant (P < 0.1).

RESULTS
Patient selection and classification of disease severity.
In the 1999 outbreak, 188 LD patients were diagnosed, of whom
133 fulfilled the criteria for a confirmed case. Of the patients
with confirmed cases, 132 had been hospitalized and were enrolled
in this study. A large number of patients in this outbreak (51
patients) were diagnosed by a urinary antigen test alone, leaving
81 patients for evaluation. For 58 (72%) of these 81 patients,
urine samples were available. The microbiological diagnosis
in these cases was established by culture alone in 11 cases,
by culture and serologic testing in 14 cases, and by serologic
testing alone in 33 cases. For 55 of the 58 confirmed cases
with available urine samples, data on clinical severity could
be collected. Women were overrepresented in CAP category 1 (67%),
in contrast to CAP categories 2 (25%) and 3 (39%). The median
age for women was lower then for men in categories 1 and 2 (64
and 73 years and 57 and 63 years, respectively), in contrast
to category 3 (70 and 62 years, respectively). Mean age did
not differ significantly between CAP categories, but the age
difference between male and female subgroups in the lowest CAP
category was significant (independent
t test;
P = 0.04). All
patients who were classified in CAP category 3 needed medical
attention in a specialized unit.
Nonconcentrated urine samples.
Using nonconcentrated urine, the sensitivities calculated for the three tests were 71, 69, and 72% for the Biotest EIA, Binax EIA, and Binax NOW test, respectively. The differences between the test sensitivities were not significant. When the LD patients were divided in three groups according to their CAP category, it appeared that the average sensitivities for the three urinary antigen tests increased from 42% for patients in the lowest CAP category to 75% for those in CAP category 2 and 93% for those in CAP category 3. The individual sensitivities of the three tests when used with nonconcentrated urine samples are shown in Fig. 1. Table 1 shows variables which in univariate analysis were significantly associated with test positivity. Age was not associated with test positivity.
View this table:
[in this window]
[in a new window]
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TABLE 1. Odds ratios and mean differences with confidence interval for variables associated with urinary L. pneumophila antigen test positivity
|
Multiple logistic regression analysis showed that the CAP category
remained associated with test positivity in all tests used (Table
2). Separate analysis for male and female patients gave identical
results, indicating that the association was not modified by
gender.
View this table:
[in this window]
[in a new window]
|
TABLE 2. Multiple logistic regression models showing odds ratios with confidence interval for variables associated with urinary L. pneumophila antigen test positivity
|
Concentrated urine samples.
After concentration of the urine samples for all three tests,
a clear but not statistically significant increase in sensitivities
was found: to 74, 79, and 81% for the Biotest EIA, Binax EIA,
and Binax NOW test, respectively. This increase in sensitivity
was small in the Biotest EIA (3%) and more prominent in the
Binax EIA (10%) and Binax NOW assay (9%). When test sensitivity
results were compared for categories with increasing clinical
severity, concentration of urine samples yielded higher sensitivities
predominantly for patients in CAP categories 1 and 2 (Fig.
1).
CAP category, male gender, number of urine samples, and shorter period between onset of symptoms and collection of the first urine sample were associated with a positive test result, but age was not (individual test results are given in Table 1). Multiple logistic regression analysis showed that the CAP category was the only factor associated with test positivity in all tests used (see Table 2). Separate analysis for male and female patients gave identical results.

DISCUSSION
The first urinary antigen tests, based on an ELISA, were described
in 1979 (
2,
21). Since then, numerous publications have followed
that confirmed the value of urinary antigen detection for the
diagnosis of Legionnaires' disease, regardless of the technical
configuration of the test (
1,
3,
7,
8,
18-
20). Based on prospective
and retrospective studies using data from solitary cases, moderate
to high urinary antigen test sensitivities have been described.
All reported test sensitivities are based on studies using sporadic
LD cases. Most of these studies used a selection of patients
or were retrospective (Table
3); they are thereby subject to
selection bias. Some of them include patients with LD caused
by other serogroups than serogroup 1, which leads to underestimation
of the test sensitivity. Furthermore, the clinical conditions
of the patients described in these studies were not taken into
account, which may explain the range of sensitivity values found
by different authors.
To our knowledge, there are no publications on test sensitivities
in an outbreak situation. The 1999 outbreak in The Netherlands
provided a unique opportunity to evaluate urinary antigen tests
in an outbreak caused by an identified
L. pneumophila serogroup
1 strain. A nationwide alert for LD cases ensured optimal case
finding, thereby decreasing patient selection bias. Furthermore,
the conditions for a gold standard were favorable: two national
reference laboratories performed all microbiological tests,
and Dutch reference titers were calculated using the distribution
of antibodies against
L. pneumophila serogroup 1 in a large
sample from a national serum bank (
4). In addition, all available
clinical data for the LD patients in this outbreak were recorded
centrally.
Assuming that patient inclusion in an outbreak situation resembles a prospective study design, our data are best compared with the published results of two prospective studies on urinary antigen test sensitivity in LD. One study (19) included a single high titer in the gold standard, inherently lowering the test sensitivity to be measured on the basis of misclassification. In this study, however, a higher sensitivity (84%) was found for patients with LD caused by L. pneumophila serogroup 1. The other prospective study (18) included patients with LD caused by L. pneumophila serogroups 1, 3, and 7, making a lower sensitivity more likely on the basis of a low urine antigen detection capacity for other serogroups than serogroup 1. Indeed, the reported sensitivity in this study was lower (57%). Since the 1999 outbreak in The Netherlands was caused by an L. pneumophila strain of serogroup 1, high sensitivities for the three tests were to be expected. However, overall test sensitivities found in this study were lower than those reported by Ruf et al. (19) and ranged from 69 to 72% for nonconcentrated urine samples. This lower sensitivity may be explained by a difference in the study populations. Active case finding for LD in this outbreak may have resulted in hospitalization of a higher proportion of patients with relatively mild LD compared to a nonoutbreak situation. As our results show, the sensitivity of urinary antigen tests is relatively low for cases in CAP categories 1 and 2.
We do not know of a published study in which an association between the severity of disease and the test sensitivity for LD has been demonstrated, although one study hinted at such an association (22). The association between sensitivity and clinical severity demonstrated in the present study has clinical and diagnostic consequences. Because of the high sensitivity in patients with a severe pneumonia, the early recognition of patients with life-threatening LD can prevent delay in initiating adequate antibiotic therapy. However, the urinary antigen test is less reliable in milder cases of LD, indicating that this diagnostic test, despite its rapid interpretation, cannot replace culture and serologic testing. Therefore, in the setting of persistent clinical or epidemiological suspicion of LD and a negative urinary antigen test result in patients in whom no other microorganism is identified, culture and serologic testing are recommended and treatment must include antibiotic coverage for Legionella.
When the detection of antigens in urine during an outbreak is used for epidemiological purposes, one has to keep in mind that 50 to 60% of cases of mild pneumonia will stay undiscovered, depending on whether urine is concentrated (concentration improves the chance of detection). This implies that, due to undiagnosed cases, the size of an outbreak will be underestimated unless complementary diagnostic serologic tests using paired sera are performed in all suspected cases. Because seroconversion can take up to 9 weeks after onset of the disease, a prolonged interval between collection of the two sera is advisable for reliable interpretation of serologic results.
Like other researchers (7, 14), we were unable to demonstrate a significant difference in sensitivity between the two EIAs that were tested and the ICT. Since the latter is very easy to perform without special laboratory equipment and the results are available at short notice even after concentration of the urine samples, this test may be preferable in outbreak situations if serogroup 1 is involved. Previous studies (8, 9) have also demonstrated that a higher sensitivity of urinary antigen detection was found using concentrated urine, regardless of the test used. Concentration by ultrafiltration is easy to perform and can facilitate an early diagnosis, especially in milder cases.
In conclusion, in outbreak situations the urinary antigen tests are a useful tool for early diagnosis of LD, especially in patients with severe cases. The ICT scored at least equal to the EIAs and has the advantage of ease of performance combined with rapid test results. Concentration of the urine samples increases the sensitivity, particularly in patients with less severe illness, and is therefore recommended. In outbreak situations, the use of urinary antigen tests alone for evaluation of the incidence rate will lead to underestimation of the actual incidence. Therefore, culture and serologic testing remain necessary diagnostic tools.

ACKNOWLEDGMENTS
We thank all hospital clinicians and microbiologists in requesting
patients' permission and allowing us to collect clinical data
and urine samples. Special thanks to Yvonne Boelens and Jacob
P. Bruin, who performed the urinary antigen tests.

FOOTNOTES
* Corresponding author. Mailing address: Regional Laboratory of Public Health Haarlem, Boerhaavelaan 26, 2035 RC Haarlem, The Netherlands. Phone: 31-23-5307838. Fax: 31-23-5307805. E-mail:
e.yzerman{at}streeklabhaarlem.nl.


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Journal of Clinical Microbiology, September 2002, p. 3232-3236, Vol. 40, No. 9
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.9.3232-3236.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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