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Journal of Clinical Microbiology, December 2004, p. 5489-5492, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5489-5492.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Infectious Disease Research Division, Hyogo Prefectural Institute of Public Health and Environmental Sciences,1 Kobe Institute of Health, Kobe,3 Okafuji Pediatric Clinic, Himeji,2 National Institute of Infectious Diseases, Musashi-murayama, Tokyo, Japan4
Received 27 May 2004/ Returned for modification 9 July 2004/ Accepted 24 August 2004
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Recently, four different kinds of diagnostic methods for HAdV infection became available. The virus isolation technique (isolation) is usually considered the "gold standard" but is time-consuming. PCR is a rapid and sensitive diagnostic technique that can be completed in a few hours, and sequencing the amplified products yields useful data. Real-time PCR for HAdVs has recently become available. Real-time PCR produces results quickly, and quantitative data can be obtained (5, 8, 18). In addition to these laboratory diagnostic techniques, immunochromatography (IC) kits have become available for HAdV diagnosis at the patient's bedside (6, 7, 16, 20, 21).
IC kits for HAdVs have been evaluated in comparison with an isolation technique (6, 20), an enzyme-linked immunosorbent assay kit (16), and PCR (7, 21). However, they have not been evaluated in comparison to the combination of isolation, PCR, and real-time PCR. Quantitative evaluation alone was insufficient; therefore, we evaluated an IC kit in comparison to these three methods. The sensitivity and specificity of the IC kit change according to which method is considered to be the gold standard (15).
The purpose of this study was to evaluate the IC kit, qualitatively and quantitatively, in comparison to multiple, sensitive laboratory diagnostic methods. The reliability and limitations of the IC kit were evaluated. The detection limit of the IC kit was evaluated with HAdV type 1, 2, 3, 5, 6, and 7 isolates.
We found that the IC kit had a higher sensitivity than reported previously, although it was less sensitive than isolation, which was less sensitive than PCR and real-time PCR. However, the IC kit was found to have 95% sensitivity compared to HAdV isolation, the diagnostic technique usually considered the gold standard.
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Clinical diagnosis at bedside. The IC kit used in this study was the SAS Adeno Test (SA Scientific, Inc., San Antonio, Tex.), which was sold under the name Check Ad (AZWELL, Osaka, Japan). The tonsils and posterior pharynx of each patient were vigorously rubbed with a cotton swab moistened in sterile physiological saline. The swabs were extracted with 500 µl of mucolytic agent provided by the manufacturer and rubbed in a soft tube. An aliquot of the extract (200 µl) was filtered and dropped into the IC kit device. Both the test tube and filter were provided by the manufacturer. The IC kit indicated an HAdV-positive result when two colored lines appeared in the device. When only one colored line appeared in the control area of the IC kit, the result was HAdV negative (Fig. 1). The remaining extract solution (about 200 µl) was transferred into a test tube containing 2 ml of Dulbecco's modified Eagles's medium (Sigma) and used as a clinical sample for a later laboratory diagnosis. The test tubes were kept at 80°C until use.
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FIG. 1. Detection of adenovirus in a clinical sample by the IC kit. The test is positive if two colored lines appear in the sample (S) and control (C) areas. The test is negative when only one colored line appears in the control area.
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Viral DNA preparation. HAdV genomes and enterovirus RNAs were prepared directly from the clinical samples with a QIAamp blood kit (QIAGEN) or a High Pure viral nucleic acid kit (Roche Diagnostics). DNAs from HAdV 1, 2, 3, 5, and 7 isolates were extracted with the High Pure kit.
PCR and sequencing. A single-tube multiplex PCR for HAdVs was carried out as reported previously (3). HAdVs were distinguished by the size of the amplified products. HAdVs which amplified to produce 188- and 301-bp DNA fragments were typed as HAdV 3 and non-subgroup B HAdVs, respectively.
Another primer pair for HAdV 3 (and HAdV 7) was designed and used for PCR and sequencing of the hexon region of HAdVs. The sequences of the primers were 5'-AGAATCATGGACTGATACTGATG-3' (sense) and 5'-AGCCTGTCATTGCCAGGCCAGC-3' (antisense). Amplification reactions were conducted in 50 µl of reaction mixture containing a 0.5 µM concentration of each of the primers, a 200 µM concentration of each dideoxynucleotide, 1.25 U of Taq polymerase (TaKaRa Shuzo, Shiga, Japan), 10 mM Tris-HCl (pH 8.3), 50 mM KCl, and 1.5 mM MgCl2. The reaction was carried out with a cycle of 94°C for 30 s, 56°C for 30 s, and 72°C for 1 min and was continued for 35 cycles. In the first cycle, the denaturation step continued for 5 min at 94°C, and in the last cycle, the extension step continued for 5 min at 72°C. The expected product size was 1,596 bp (from position 591 to 2186 based on the hexon sequence of HAdV3; GenBank accession number X76549).
Two RT-PCR methods for enteroviruses developed by Ishiko et al. (11) and Oberste et al. (14) were used for enterovirus identification. PCR-amplified products were sequenced directly as reported previously (3, 4, 17). PCR and RT-PCR were performed using a Thermal Cycler Dice (TaKaRa Shuzo). The nucleotide sequence was determined with a model 310 genetic analyzer (Applied Biosystems Japan [ABI]).
Real-time PCR. Real-time PCR for a wide range of HAdVs was used in this study. Primers derived from the highly conserved HAdV hexon 3 and 4 genes by Echavarria et al. (1) were used. The sequences of the primers were 5'-GACATGACTTTCGAGGTCGATCCCATGGA-3' (Hex3) and 5'-CCGGCTGAGAAGGGTGTGCGCAGGTA-3' (Hex4). The expected product size was 140 bp (from position 21589 to 21728 based on the complete sequence of HAdV2; GenBank accession number J01917). One microliter of template DNA was added to a final volume of 25 µl containing 1x SYBR green PCR master mix (ABI) and a 160 nM concentration of the primers Hex3 and Hex4. The real-time PCR was carried out with an ABI PRISM 7900 HT sequence detection system for a cycle of 94°C for 1 min, 57°C for 1 min, and 72°C for 1.5 min, which was continued for 40 cycles. During thermal cycling, the emissions from each sample were recorded and SDS (sequence detection system) software processed the raw fluorescence data to produce threshold cycle (CT) values for each sample. The SDS software then computed a standard curve from the CT value of the diluted standards and extrapolated absolute quantities for the unknown samples based on their CT values. The DNA of the prototype HAdV 2 was amplified by the PCR system of Echavarria et al. (1) as described above. The product (140 bp) was cloned into a pCR2.1 vector and used as a standard.
Detection limit of the IC kit. HAdV 1, 2, 3, 5, and 6 isolated in this study and HAdV 7 isolated in 1998 were diluted in phosphate-buffered saline and tested with the IC kit. Diluted solutions showing slight but clear positivity by the IC kit were used to determine the detection limits of the IC kit. The detection limits were evaluated by determining the 50% tissue culture infective dose (TCID50) per milliliter by use of HEp-2 cells (10) and real-time PCR. Additionally, repeated IC kit detection limit tests were performed with a twofold serially diluted HAdV 3 solution to check the reproducibility. At each concentration, 10 IC kits were tested.
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TABLE 1. IC kit and typing results of detected viruses (n = 138)
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TABLE 2. Results of real-time PCR and typing of the IC kit-negative samples (n = 22)
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TABLE 3. Detection limit of IC kit
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At the bedside, 40 patient samples were recorded as strongly positive, since in each case a positive colored line appeared before the control line appeared. Seventy-three samples were positive within 10 min. Only three samples required 20 min.
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All IC kit-positive samples were isolation positive. Two enterovirus-infected samples were negative by the IC kit and other HAdV diagnoses; the specificity of the IC kit was 100%, as reported previously (6, 20).
In this study, we were able to identify many HAdV-infected patients within a short period of time because a large outbreak of HAdV occurred in Japan in 2003 (13). This outbreak was the largest of its kind in the previous 10 years. The main causative agent was HAdV 3, which accounted for 69% (84 of 122) of the patients diagnosed by NT. According to previous reports, the IC kit is faster than the enzyme-linked immunosorbent assay kit (which takes at least 40 min) and is more useful for detection of respiratory HAdVs (21). Therefore, the IC kit is the only practical bedside diagnostic tool for respiratory HAdV infections.
The IC kit was useful for detecting HAdV in samples coinfected with HAdV and enterovirus. There was no clinical difference between coinfected patients and other HAdV-infected patients. Therefore, HAdV seemed to be the main causative agent of infection in these patients. Coinfection of HAdV3 and HAdV5 could be identified by PCR-based sequencing (17) and use of the HAdV 3- and 7-specific primers designed in this study.
We found that when the SYBR green real-time PCR method was used with isolated HAdV strains, the detection limit of the IC kit was 104.6 to 105.8 virus genome copies/ml for HAdV 1, 2, 3, 5, 6, and 7. However, there were clinical samples from which we isolated HAdV 3 (four samples) and HAdV 1 (two samples) that were negative by the IC kit despite concentrations of 9.0 x 107 to 2.5 x 109 virus genome copies/ml for HAdV 3 and 3.8 x 107 to 4.5 x 108 virus genome copies/ml for HAdV 1. These clinical samples had virus concentrations within the sensitivity limits of the IC kit; the detection limits for HAdV 3 and HAdV 1 were 105.8 and 104.6, respectively. According to Hierholzer (9), the bulk of newly synthesized HAdV product is not continuously released but remains cell associated. Only the products that are released into the extraction solution contribute to a positive result by the IC kit. This may explain the 5% false-negative rate.
The IC kit used in this study utilizes a monoclonal antibody made with the HAdV 2 hexon protein. Uchio et al. (21) reported that the IC kit had a lower sensitivity to HAdV 3 and 7 than to HAdV 4, when using serotypes 3, 4, 7, 8, 11, and 37, although these researchers used conjunctival swabs as samples. They reported that the minimum amounts of HAdV 3 and HAdV 8 detected by the IC kit were 4 x 103 and 4 x 104 PFU, respectively. Shimizu et al. (16) reported that the detection limit of an IC kit was 104.45 (2.8 x 104) TCID50/ml for HAdV 3 and that the sensitivities of other IC kits were similar. In our results, the IC kit was sensitive to 101.9 to 104.0 TCID50/ml for HAdV 1, 2, 3, 5, and 7. Detection limits for the six serotypes were 2.9 ± 1.0 log TCID50/ml and 5.4 ± 0.4 log copies of HAdV genome/ml (mean ± SD). The coefficients of variation were 0.34 and 0.08, respectively. Based on the real-time PCR results, the detection limit of the IC kit was approximately 105.4 copies of HAdV genome/ml. In repeated IC kit tests, the high reproducibility of the IC kit was confirmed.
In this study period, no clinical samples containing HAdV 7 were obtained. HAdV 7 is known to cause fatal pneumonic infections in children, including nosocomial infections. We confirmed that the detection limits for HAdV 7 were 105.6 copies of the HAdV 7 genome/ml and 101.6 TCID50 of HAdV 7/ml. The sensitivity of the IC kit was not lower for HAdV 7 than for the other serotypes in this study. According to Uchio et al. (21), the detection rate of the IC kit is 31% (8 of 26 patients) for HAdV 3 and 60% (3 of 5 patients) for HAdV 7 compared to that of PCR. We were able to detect the presence of HAdV 3 in respiratory samples with the IC kit 93% of the time (in 80 of 86 samples) in comparison to PCR.
The five samples that were IC kit and isolation negative but multiplex PCR positive contained 3.0 x 104 to 3.8 x 105 copies of the HAdV genome/ml. As isolation requires viable virus, PCR was more sensitive than isolation. It is known that many HAdV particles are not infectious, perhaps because they have genomic defects or lack fiber or some other protein (19).
In conclusion, the IC kit is a useful method for diagnosing HAdV diseases at the bedside because it has 95% sensitivity relative to that of isolation. To prevent nosocomial infections, rapid diagnosis at the bedside is necessary. Furthermore, the IC kit provides pediatricians with information about the prognosis of the respiratory diseases at the patient's first visit. However, because the IC kit test had a 5% false-negative rate compared to isolation, careful interpretation is required in cases where IC kit-negative results are obtained and HAdV infection is suspected.
This study was partly supported by a grant-in-aid for scientific research (15590568) from the Japan Society for the Promotion of Science.
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