A diagnosis of tuberculosis (TB) is rarely disputed if
Mycobacterium tuberculosis is isolated from a clinical specimen; however, specimen contamination may occur
(1--3). Identification of TB strain patterns through molecular typing or DNA fingerprinting is
a recent advancement in TB laboratory techniques
(3--7). CDC's National Tuberculosis Genotyping and Surveillance Network (NTGSN) performs DNA fingerprinting on
TB isolates to determine the frequency of clustering among
M. tuberculosis strains in project surveillance sites. In November 1998, NTGSN detected 11 isolates
from previously reported TB cases among persons in New Jersey whose DNA
fingerprints matched the avirulent laboratory M.
tuberculosis control strain H37Ra. H37Ra does not cause active TB in humans, but it has been reported as a source of
cross-contamination (8). In collaboration with the New Jersey Department of Health
and Senior Services, CDC investigated H37Ra as a possible cause of TB disease and/or
TB misdiagnoses caused by laboratory cross-contamination in the 11 case-patients.
This report describes findings from two of the 11 cases and summarizes the results of
this investigation, which indicate that TB was misdiagnosed and demonstrate the value
of DNA fingerprinting to identify occurrences of cross-contamination of
Case 1. In October 1998, a 44-year-old woman with multiple sclerosis and
no known exposure to a person with active TB had TB diagnosed on the basis of
a positive culture result. Cerebrospinal fluid revealed no signs of infection, but
the culture grew M. tuberculosis at 7 weeks. Her chest radiograph was normal, and
a tuberculin skin test (TST) was not documented. Anti-TB therapy was not
initiated because no development or progression of symptoms consistent with TB
occurred. The cerebrospinal fluid was retested in the same laboratory (7 weeks after
the original specimen was obtained) and revealed a stain with 1+ acid-fast bacilli
(AFB). The patient was started on anti-TB medications. The culture for the second
specimen was negative for TB. This patient had received 4 months of anti-TB treatment at
the time of the investigation.
Case 2. A 58-year-old woman with a history of reactive airway disease and
angioedema was taken to a local emergency department with shortness of breath
and cough. Her chest radiograph was normal, and a TST was not documented. A
sputum specimen obtained at that time was AFB smear-negative, but
M. tuberculosis culture was positive at 6 weeks. Although the patient had recovered after treatment for
acute asthma, she was started on anti-TB treatment. Treatment was discontinued after
2 weeks when health-care providers determined her illness was not TB.
A list of the 11 case-patients with an isolate with a fingerprint matching H37Ra
was compiled, and information on the origin of each case-specimen was
obtained. Investigators reviewed hospital, clinic, and health department records for each
case-patient to establish the clinical events leading to TB diagnosis. Investigators visited
the laboratories where the 11 specimens were processed to interview
laboratory personnel about specimen processing techniques and to review laboratory logs
for mycobacterial specimen testing.
The 11 case-patients had TB diagnosed and reported during 1996--1998. Mean
age of patients was 60 years (range: 36--81 years); eight were women, and three
were human immunodeficiency virus (HIV)-positive. Eight cases were classified
as pulmonary and three as extrapulmonary. Seven patients had abnormal
chest radiograph findings, and two had documented positive TSTs. All case-patients
received partial or full-course therapy for TB; treatment durations ranged from 2 weeks to
6 months. Seven patients had contact investigations performed; four of the 32
contacts identified were tested and treated for latent TB infection. Each case met at least
one criterion for suspected laboratory cross-contamination with
M. tuberculosis*. In addition, each of the eight pulmonary patients had clinical courses suggestive of
an illness other than TB (i.e., bacterial pneumonia [four], reactive airways disease
[two], interstitial lung disease [one], and congestive heart failure [one]).
The laboratory investigation revealed that the 11 specimens were
processed during February 1996--October 1998 at four laboratories in New Jersey (three
hospital laboratories and one commercial laboratory). Each of the laboratories either used
the strain H37Ra or participated in laboratory proficiency testing using H37Ra;
however, laboratory logs did not include the specific times when H37Ra was handled on
the same day as any of the 11 specimens. In addition, personnel at the laboratories
could not recall instances when the control strain may have been mishandled. The
average number of specimens collected for AFB culture per patient was four (range: two to
12). All culture-positive patient specimens were smear-negative. Mean number of days
to M. tuberculosis growth for patient specimens was 38 (range: 17--54 days).
These misdiagnosed cases of TB illustrate the need for
heightened awareness of laboratory cross-contamination with
M. tuberculosis. Clinicians and health department personnel did not suspect laboratory cross-contamination in
these 11 cases; therefore, this oversight would not have been detected without the use
of DNA fingerprinting through NTGSN. The putative source of cross-contamination
for the 11 cases, H37Ra, is a laboratory control strain that is used weekly in
some laboratories for routine drug susceptibility testing. H37Ra also is distributed
to mycobacteriology laboratories as part of a biyearly proficiency testing required
by the Clinical Laboratory Improvement Amendments
(9). The control strains for proficiency testing often are processed simultaneously with patient specimens,
but many laboratories do not document consistently specific times when
proficiency testing is conducted. As a result, it is difficult to prove that the control strain is
the source of cross-contamination in a specific case. In addition, several
opportunities exist for specimen carryover, spillage, or inadvertent contamination during
specimen processing, but these occurrences are difficult to discover retrospectively.
Given these obstacles in discovering cross-contamination, NTGSN has established
criteria for suspected laboratory cross-contamination of TB (CDC, unpublished data, 1998).
Reliance on clinical judgment and the presence of corroborating clinical signs
and symptoms play pivotal roles in interpreting laboratory data. Systemic symptoms
of fever, loss of appetite, weight loss, weakness, night sweats, and malaise are
common but not specific for TB. Other signs and symptoms vary according to the site
involved. In pulmonary TB, prolonged cough with or without sputum production, and
ensuing pulmonary inflammation and necrosis are manifest. Chest radiograph findings
of adenopathy, lung infiltrates, and pleural reaction are important correlates in
the diagnosis, but these findings may be due to illnesses other than TB, particularly in
the presence of HIV. These scenarios often create clinical dilemmas when initial
laboratory data support a TB diagnosis. A positive TST is evidence for TB, but the
positive predictive value depends on the cut-off value used to determine a positive test and
the prevalence of TB infection in the population
(10). In the appropriate clinical setting,
the presence of a positive AFB smear should raise suspicion for TB; however, a
positive smear with a concomitant inconsistent clinical history may represent the presence
of H37Ra, a nontuberculous organism, such as Mycobacterium avium
complex, or environmental contamination with a ubiquitous acid-fast species such
as Mycobacterium gordonae. H37Ra and nontuberculous organisms are
indistinguishable from pathogenic strains of M.
tuberculosis on a laboratory smear.
For some patients, signs, symptoms, and test results are lacking or conflicting,
as illustrated by the case-patients described in this report. If discrepancies exist
among clinical and laboratory data, and at least one criterion for laboratory
cross-contamination is met, an investigation should ensue to determine whether the
patient has a potential TB exposure, whether specimens from the laboratory strain
orother TB patients were processed simultaneously with the specimen in question,
and whether performance of DNA fingerprinting is appropriate. To identify
occurrences and sources of cross-contamination, it also is important for
mycobacteriology laboratories to determine the DNA fingerprint pattern of the
M. tuberculosis control strain used in their respective laboratories.
The patients described in this report received unnecessary treatment for TB
and more than half had a contact investigation initiated. Recognition by
professionals and laboratorians of the potential for laboratory
cross-contamination with M. tuberculosis should help avert erroneous TB diagnoses and
avoid unnecessary treatment and associated toxicity. In addition, this awareness assists
TB-control programs in avoiding unnecessary patient care costs and futile
contact investigations and helps maintain accurate TB case reporting.
- Maurer S, Kreiswirth B, Burns J, et al.
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- Braden C, Templeton G, Stead W, et al. Retrospective detection of laboratory
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- Small P, Hopewell P, Singh S, et al. The epidemiology of tuberculosis in San Francisco:
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- French A, Welbel S, Dietrich S, et al. Use of DNA fingerprinting to assess
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- Braden C, Templeton G, Cave M, et al. Interpretation of restriction length
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- CDC. Laboratory practices for diagnosis of tuberculosis---United States, 1994.
- American Thoracic Society. Diagnostic standards and classification of tuberculosis
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*Suspected laboratory cross-contamination with
M. tuberculosis may include at least one of the following: 1) patient's clinical course is inconsistent with TB; 2) single positive
M. tuberculosis culture with no AFB seen in any specimen; 3) culture-positive
specimen from a different patient processed or handled on the same day has an identical
DNA fingerprint, and no epidemiologic connections exist between patients; 4) laboratory
control strain has an identical fingerprint; and 5) time to growth detection is >30 days.