A prospective study of false-positive cultures of
Culture remains the reference standard for diagnosis of disease caused by
This study was conducted by staff of the Microbial Diseases Laboratory, California Department of Health Services, Berkeley, California, which is one of seven participants in the National Tuberculosis Genotyping and Surveillance Network of the Centers for Disease Control and Prevention. Participating California laboratories included those at the San Francisco General Hospital, Santa Clara Valley Medical Center (San Jose), and Solano County Public Health Laboratory (Vallejo). The study was conducted from January 1, 1998, to June 30, 1999.
RFLP pattern images were entered into a database and compared to identify isolates with matching genotypes. Any of the following cultures were considered potentially cross-contaminated and underwent further investigation: 1) the first
Patients with specimens meeting the above criteria and for whom potential source isolates were identified (i.e., their isolates had a genotype that matched that of a potential source isolate) underwent further investigation. The investigation included a review of all clinical data and radiologic studies, if applicable, and possible epidemiologic connections between patients with potential source and contaminated specimens. Clinical data included prior history of TB, results of tuberculin skin tests, treatment of latent TB infection, symptoms of present illness, results of diagnostic evaluations for TB, and alternative diagnoses. Personnel in laboratories where the potentially cross-contaminated specimens were processed also investigated potential sources of cross-contamination within their facilities.
A final determination of laboratory cross-contamination was made after a panel of experts reviewed each case. The panel comprised three genotyping network investigators from sites other than California. Panel members met once at the conclusion of the study and reviewed all information from laboratory and medical records, epidemiologic investigations, and genotyping images (upon request). Cases were presented to the panel members in person by one of the authors (RJ). Panel members independently recorded their conclusions on whether cross-contamination was likely, as well as their final diagnosis, and submitted these results to the senior investigator of the study (ED).
Review showed that similar methods for specimen processing were used in all three participating laboratories. However, one laboratory used a common flask for dispensing decontaminant reagent and phosphate buffer rather than an individual tube or pipette for each specimen.
During the study period, 21,835 specimens were submitted for mycobacterial culturing at the three laboratories. Of these, 988 (4.5%) from 296 different patients were positive for
| Patient | Lab | Specimen type | Initial or follow-up specimen | Sputum smear result | Clinical signs and symptoms | Comments | Panel decision | Final diagnosis |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | Sputum | Initial | Negative | 36-yr-old man with AIDS hospitalized with cough, dyspnea, right lower lobe infiltrate; he improved with trimethoprim and sulfamethoxazole alone | Genotype of isolate matched that of another patient whose specimen was processed same day | Cross-contamination | Bacterial pneumonia |
| 2 | 2 | Sputum | Initial | 2+ Positive | 28-yr-old man with HIV infection with fever, cough, right lower lobe infiltrate; improved with ceftazidime | Genotype of isolate matched that of another patient hospitalized on the same ward | Mislabeled specimen | Bacterial pneumonia |
| 3 | 2 | Sputum | Initial | Negative | 31-yr-old woman hospitalized with cough, left lower lobe infiltrate, and leukocytosis; chest radiograph showed improvement with clindamycin and ofloxacin | Genotype of isolate matched that of another patient whose specimen was processed same day | Cross-contamination | Bacterial pneumonia |
| 4 | 2 | Sputum | Initial | Negative | 44-yr-old man hospitalized with cough and right lower lobe infiltrate; improved with levofloxacin before anti-TB therapy initiated | Genotype of isolate matched that of another patient whose specimen in the BACTEC instrument immediately preceded the case patient | Cross-contamination | Lung abscess |
| 5 | 2 | Sputum | Follow-up | Negative | 68-yr-old woman newly immigrated from China with cough; chest radiograph showed bi-apical fibronodular changes | Genotype of second isolate >30 days later did not match that of initial isolate or any other isolate in database | Mislabeled specimen or mixed infection | Bacterial pneumonia versus TB with mixed infection |
| 6 | 2 | Sputum | Initial | 4+ Positive | 82-yr-old man with cough, fever; chest radiograph showed chronically increased right mid-lung interstitial markings | Genotype of isolate matched that of another patient hospitalized on the same ward | Mislabeled specimen or cross-contamination | Bacterial pneumonia |
| 7 | 2 | Sputum | Initial | Negative | 27-yr-old woman with upper respiratory symptoms; chest radiograph was normal | Genotype of isolate matched that of another patient whose specimen was processed same day | Cross-contamination | Upper respiratory tract infection |
| 8 | 2 | Sputum | Initial | Negative | 44-yr-old man with hemoptysis and known pulmonary metastases of squamous cell carcinoma of trachea; chest radiograph showed three large cavities with air-fluid levels; bronchoscopy culture grew | Genotype of isolate matched that of another patient whose specimen was processed same day | Cross-contamination | Lung abscess |
| 9 | 2 | Broncho-alveloar lavage | Initial | Negative | 55-yr-old woman hospitalized with dyspnea and left lower lobe infiltrate; she improved with broad-spectrum antibiotics alone | Genotype of isolate matched that of another patient whose specimen was processed same day | Cross-contamination | Bacterial pneumonia |
| 10 | 3 | Sputum | Follow-up | Negative | 34-yr-old homeless man with HIV infection, fever, and cervical lymphadenopathy; chest radiograph showed left lung nodule | Genotype of initial isolate matched that of another homeless TB patient; follow-up specimen (35 days later) had a unique genotype | TB | TB |
aLab, laboratory; TB, tuberculosis;
After the panel’s review, laboratory cross-contamination was identified as the cause for positive culture results for six patients (2% of all patients with cultures positive for
Laboratory cross-contamination was the cause for a positive culture result in 2% of all patients with
The rate of cross-contaminated cultures in our study is similar to the rates in previous studies of
Multiple factors can cause false-positive cultures, including contaminated clinical equipment (e.g., bronchoscope), clerical errors, and cross-contamination that occurs in the laboratory. The last category can be caused by batch processing, transfer of viable bacilli from the sample needle of a broth-culturing system, e.g., BACTEC (
Five of the six cross-contamination incidents were in a single laboratory. In four of these five cases, contamination probably occurred when reagents were dispensed with a common flask. Previous studies have reported that the step of adding the phosphate buffer was likely to have been the source of the cross-contamination (
As in other studies, we found that a single positive culture for
Clinical judgment is also important in raising suspicion about cross-contamination. TB classically is accompanied by symptoms of prolonged cough, fever, weight loss, and night sweats, but other diseases such as bacterial pneumonia can cause these symptoms. Therefore, no specific clinical criterion alone can be used to definitively state that TB is present. However, an inconsistent clinical course or absence of symptoms should certainly raise suspicion that cross-contamination may have occurred (
Our assessment of the rate of cross-contamination did not include private laboratories; thus, our results may not be generalizable to all types of clinical laboratories. In addition, our methods depended on identifying, obtaining, and genotyping an isolate from a positive source culture; thus, we may have underestimated the true rate of laboratory cross-contamination.
The authors gratefully acknowledge the efforts of Donald Cave and Lisa Fitzpatrick, who served as expert reviewers for cases.
This study was supported by funds from the Centers for Disease Control and Prevention (CDC Cooperative Agreement CCU900515-15-2).
Dr. Jasmer is an assistant professor of medicine in the Division of Pulmonary and Critical Care Medicine at the University of California, San Francisco. His primary research interest is in clinical studies of tuberculosis.