DISCUSSION
Discrepancies in preoperative reverse transcriptase polymerase chain reaction (RT-PCR) screening for SARS-CoV-2 can create major concerns with regards to proceeding with otolaryngologic surgeries that are medically urgent yet risky for SARS-CoV-2 transmission. Justified fear for the safety of the patient and the healthcare providers involved in the patient’s care can cause significant changes to the course of care. Lei et al. (2020) have reported that operating during the prodrome of COVID-19 can lead to life-threatening complications. False negative results are widely known to occur and are well-documented in the medical literature, however nothing has been published about false positive results.21 Therefore, when faced with patients with suspected false positive results, our department and institution as a whole were left with little guidance on how to proceed in these circumstances. It would have been dangerous to providers to simply assume the single positive result was spurious; likewise, delaying urgent surgery for three weeks for what might be a spurious positive result carried significant implications for the patient.
We turned to the expertise of several colleagues, including pathology, radiology, and infectious disease, as well as internal departmental experts who were part of the Surgical Review Committee. It seemed reasonable that if repeat testing, multiple times in some instances, was negative, we could declare patients as falsely positive and safely proceed with surgery. Chest imaging, particularly computed tomography, and subsequent clinical course also supported the determination that these were false positives. These false positives tests were seen in a clinically significant 7.1% of our patients. As part of the decision process, pathologists were able to re-review tests to report the titer levels. Rather than accepting the results reported simply as “detected” or “not detected” in the electronic medical record, we discussed suspicious cases to determine if titers were borderline positive or strongly positive. We sparked conversation with infectious disease physicians, pulmonologists, and particularly infectious disease specialists who were now specializing in the care of COVID-19 positive patients to seek expert opinions.
To our knowledge, there are no papers reporting false positive SARS-CoV-2 RT-PCR tests. Tahamtan and Ardebili (2020) discuss possible factors causing false negative results of SARS-CoV-2 RT-PCR, namely mismatches between the testing primers and viral genome or low viral loads in samples due to timing of disease or location of collection.22 False positive RT-PCR for the diagnosis of norovirus and dengue virus have been discussed, and indicate higher rates of false positives for certain viral targets.23-25 This supports repeat testing of patients using a different testing platform with a different viral target for more accurate testing when the result is questioned.
We hypothesize that mechanisms for generation of false positive results for SARS-CoV-2 may include the following:
1) pure technical artifacts where fluorescence signal is generated due to non-specific nuclease degradation of the probe probably associated with off-target probe binding. In short, a technical artifact. This is most likely associated with a weak positive signal.
2) detection of another non-SARS-CoV-2 virus/microorganism that has not yet been accounted for in the global databases used to design the primers/probes in these assays. Recall that all of these assays are new.
3) technical cross-contamination at any point along the sample chain of testing. If a manual pipetting step is involved, it could a technologist who accidentally made an error. In some instrument configurations, it could be a sample carryover contamination event. We tend to trust robots and instruments, but they are not infallible.
Otolaryngology patients are unique in that the patient often has disease involving the mucosa in the upper aerodigestive where the SARS-CoV-2 virus resides, and from which our nasopharyngeal swabs are obtained. Changing the location in which the sample is obtained could be considered. One of our patients, who was heavily irradiated in the area swabbed, had a tracheostomy, and thus, a bronchioalveolar lavage would have been easy to obtain. However, this was not performed. There has been no previous discussion in the literature regarding whether local mucosal cancer, infection, or inflammation can affect RT-PCR testing for this disease. Individual history of radiation to the head and neck can affect saliva production which may theoretically alter the ability of nasopharyngeal swab to collect specimen. While there was no difference seen in our groups regarding mucosal pathology, cancer diagnosis, or history of radiation, our sample size is quite small and further investigation is warranted. In any case, it would seem logical that mucosal abnormalities would lead to false negatives more often than false positives due to inadequate sampling.
Chest computed tomography (CT) has never been described as a tool for screening asymptomatic preoperative patients with no history of COVID-19 exposure. It has, however, been discussed for its utility as both a sole diagnostic test and as an adjunct along with RT-PCR. Findings of COVID-19 have been reported as ground-glass opacities with segmental consolidations; these overlap with many other pulmonary disease and therefore on meta-analysis, pooled specificity and positive-predictive value is low, 37% and 1.5-30.7% respectively.26,27Sensitivity of CT chest alone is 94-97%, with a negative predictive value of 95.4 to 99.8% in reports out of China with high prevalance.26,28 Ai et al. (2020) showed 75% of patients with symptoms concerning for COVID-19 and a negative RT-PCR but positive CT chest later converted to positive RT-PCR.28 Individual case reports have similarly shown the utility of chest CT as adjunct with RT-PCR to diagnose COVID-19 in patients with negative RT-PCR.27,29
There are reports of asymptomatic positive patients with classic CT chest findings for COVID-19, however these patients were known to have exposure to SARS-CoV-2.30,31 Between 70-100% of patients were found to have CT findings consistent with COVID-19, and between one-fifth and one-fourth of patients later developed symptoms of the disease. Our preoperative patients were asymptomatic with no known history of direct COVID-19 exposure; their only risk factor was living in a city during the ascending portion of the COVID-19 pandemic. Regardless, these reports provide evidence for CT chest changes in pre-symptomatic patients. It therefore seems reasonable that CT chest can serve as an adjunct test to help providers determine if subclinical infection could be present. This may assist with determining which of the two results is false. A negative CT chest can help reassure providers that the positive RT-PCR test is spurious and allow the surgery to proceed as scheduled.
Among our four patients with discrepant results: five additional RT-PCR tests were performed, two patients had surgery delayed 3 or 4 days each, two patients were sent to COVID-19 designated units, and one patient left against medical advice without surgery for an orbital abscess. As the weeks progressed, we became familiar with the possibility of false positive findings and had raised suspicions for when positive results might indeed reflect false positives. In the most recent patient with positive preoperative testing without symptoms (Patient #3), pathologists recommended immediate re-testing based on the borderline titers in her test results rather than delaying surgery for weeks for a potential COVID-19 infection. We were therefore able to move swiftly and continue with the surgery, rather than delay for weeks anticipating a clinical COVID-19 infection, which a more strongly positive test might have required.
Prolonged inpatient stays pose risks for all patients at all times; however, in the midst of the pandemic there are heightened risks. Particularly for Patient #2, who falls into a highly at-risk group due both to age and underlying pulmonary disease, the importance of a prolonged stay should not be minimized. Fortunately, we were familiar with the possibility of false positives by the time that this patient’s positive test occurred, and we were able to keep her out of the COVID unit until additional testing was performed and negative on three further repeat tests. Placement in COVID-19 designated units, as seen with the earlier cases of Patient #1 and #4, creates safety concerns for both patients and providers. While COVID-specific units are appropriate and necessary for the safe treatment of COVID-positive patients, the misplacement of false positive patients in these units exposes the patients to SARS-CoV-2 at an unnecessarily high rate. Providers from the surgical team, who in ordinary circumstances would not be in these high-risk units, are also placed in an environment with increased risk of disease contraction for themselves and risk of transmission to other patients. Moreover, our young and healthy patient was so concerned about his placement in this high-risk ward that he left against medical advice despite the risk of progressive vision loss. The psychological effects of these wards on our surgical patients should not be dismissed.
At this point in time, there is no “gold standard” test to which the results of the RT-PCR can be compared, which limits the statistical validity in the reporting a specificity. We therefore favor reporting a probable false positive rate of 7.1% in lieu of specificity, until such a time that a more accurate test is available. However, if we are to use chest imaging, close clinical follow up, and expert opinion as a gold standard in the absence of a true gold standard, the specificity would be 0.97. Classic statistical teaching is that a positive result in a highly specific test, such as in this case, should rule in the disease. However, we believe these test results must be considered as part of the broader clinical picture.
Little data exists about the use of RT-PCR tests to screen preoperative patients without symptoms of COVID-19. In the preoperative setting, the use of a screening test such as RT-PCR for SARS-CoV-2 is faulty in that screening tests have a higher accepted false positive rate, and the pre-test probability of a positive test in these patients is already low. Ideally there should be a confirmation test for positive results with higher accuracy, and perhaps, once validated, testing for immunoglobulins against SARS-CoV-2 can fill this role in the future. As the prevalence of the disease continues to increase, concern over false positives will increase as many providers become hesitant to consider a test falsely positive. Unlike false positive tests in the general population for whom this result would cause self-quarantine, unrealized false positive tests in preoperative patients can have significant clinical implications and interfere with urgent cancer surgery or other emergency surgery.