Making sense of a negative COVID-19 swab test
Letter to the Editor

Making sense of a negative COVID-19 swab test

Qin Xiang Ng1, Mei Fong Liew2,3, Wee Song Yeo4

1MOH Holdings Pte Ltd, Singapore, Singapore; 2Division of Respiratory & Critical Care Medicine, Department of Medicine, National University Hospital, Singapore, Singapore; 3Fast and Chronic Programmes, Alexandra Hospital, National University Health System, Singapore, Singapore; 4Department of Paediatric Medicine, Mount Elizabeth Hospital, Singapore, Singapore

Correspondence to: Qin Xiang Ng. MOH Holdings Pte Ltd, 1 Maritime Square, Singapore 099253. Email:

Submitted Jul 30, 2020. Accepted for publication Aug 24, 2020.

doi: 10.21037/qims-20-929

We read with great interest the study by Li et al. (1). As the Coronavirus Disease 2019 (COVID-19) continues to evolve, there have been increasing reports of false-negative oropharyngeal swabs for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (2,3). In response, we would like to raise two points for consideration.

First, factors that may have contributed to the false-negative results include improper or suboptimal sampling technique, transportation or storage process. Although an experimental study found that angiotensin-converting enzyme 2 (ACE2) expression was found only on the basal and not surface layer of the nasal, oral, and nasopharynx (4), SARS-CoV-2 RNA detected in nasophargyneal aspirates might be derived from the infected lower respiratory tract. In patients with COVID-19, ciliated cells in the upper airways are thought to be infected (5). As demonstrated by an earlier study of the 2003 SARS coronavirus (SARS-CoV), human ACE2 was detected in ciliated airway epithelial cells and SARS-CoV could infect the proximal airways (6). By reasonable extrapolation, this should also apply to SARS-CoV-2.

At present, we also know that there is a significant amount of SARS-CoV-2 shedding in the upper respiratory tract, even amongst asymptomatic individuals, and they can go on to infect others (7). According to data from the World Health Organisation and the Chinese Center for Disease Control and Prevention, SARS-CoV-2 can be detected in upper respiratory samples one to two days prior to symptom onset and persist for up to 12 days in moderate cases and up to two weeks in severe cases (8).

In a recent report by Wang et al. (9), SARS-CoV-2 was detected in specimens taken from multiple sites, with lower respiratory tract samples most often testing positive for the virus. Viral RNA was detected only in 32% (126 of 398) of oropharyngeal swabs, which was lower than that of nasopharyngeal swabs (63%, 5 of 8). The findings also suggest that testing of specimens from multiple sites or obtaining lower respiratory tract samples may improve the sensitivity and reduce false-negative test results. The alternative is retesting after 1 to 2 days, especially for patients who are epidemiologically linked or with a high index of clinical suspicion. In a study of 610 hospitalized patients, among the 384 patients with initial negative results, a retest after 1 or 2 days returned positive in 48 of them (12.5%) (2). Higher viral loads may be present after symptom onset.

More recently, clinicians have also supported the utility of chest computed tomography (CT) imaging in screening patients in whom COVID-19 is clinically suspected, especially those with negative initial RT-PCR results (10), or living in areas of high disease prevalence, e.g., Wuhan, China (11). CT imaging may facilitate early diagnosis of COVID-19 in these patients and it could also be used to monitor for complications. However, CT involves a considerable amount of radiation and cost for the patient, and like most things in medicine, these risks have to be weighed against the potential benefits.

The second point we would like to highlight pertains to the discharge criteria for COVID-19 patients. As the number of COVID-19 cases continue to rise worldwide, many health systems have become increasingly overwhelmed. However, as the kinetics and infectivity of viral shedding in COVID-19 remain under investigation, the United States Centers for Disease Control and Prevention (CDC) and some countries (including China and Singapore) still recommend two consecutive negative respiratory specimens, collected more than 24 hours apart before discharge or discontinuation of transmission-based precautions (12,13). Naturally, collection of upper respiratory tract samples would be less invasive and cumbersome than that of lower respiratory samples. Although the viral particles detected by real-time reverse-transcriptase polymerase chain reaction (RT-PCR) may be nonviable, the precaution of testing negative twice over a time interval provides some assurance that a productive infection is not ongoing. We recognise that this may put immense pressure on already strapped health systems and that a time-based or symptom-based strategy may be preferred but there is always the concern of discharging COVID-19 patients prematurely even though patients appear to have clinically recovered (14).

All in all, we would like to emphasize that pre-test probability matters and a negative COVID-19 swab test must be cautiously interpreted in the given clinical context.


Funding: None.


Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at The authors have no conflicts of interest to declare.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See:


  1. Li C, Luo F, Xie L, Gao Y, Zhang N, Wu B. Chest CT study of fifteen COVID-19 patients with positive RT-PCR retest results after discharge. Quant Imaging Med Surg 2020;10:1318-24. [Crossref] [PubMed]
  2. Li Y, Yao L, Li J, Chen L, Song Y, Cai Z, Yang C. Stability issues of RT‐PCR testing of SARS‐CoV‐2 for hospitalized patients clinically diagnosed with COVID‐19. J Med Virol 2020;92:903-8. [Crossref] [PubMed]
  3. Rong Y, Wang F, Liu J, Zhou Y, Li X, Liang X, Zhang D, Zeng H, Wang J, Shi Y. Clinical characteristics and risk factors of mild‐to‐moderate COVID‐19 patients with false‐negative SARS‐CoV‐2 nucleic acid. J Med Virol 2020. Epub ahead of print. [Crossref] [PubMed]
  4. Hamming I, Timens W, Bulthuis ML, Lely AT, Navis GV, van Goor H. Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis. J Pathol 2004;203:631-7. [Crossref] [PubMed]
  5. Mason RJ. Pathogenesis of COVID-19 from a cell biology perspective. Eur Respir J 2020;55:2000607. [Crossref] [PubMed]
  6. Sims AC, Baric RS, Yount B, Burkett SE, Collins PL, Pickles RJ. Severe acute respiratory syndrome coronavirus infection of human ciliated airway epithelia: role of ciliated cells in viral spread in the conducting airways of the lungs. J Virol 2005;79:15511-24. [Crossref] [PubMed]
  7. Wölfel R, Corman VM, Guggemos W, Seilmaier M, Zange S, Müller MA, Niemeyer D, Jones TC, Vollmar P, Rothe C, Hoelscher M. Virological assessment of hospitalized patients with COVID-2019. Nature 2020;581:465-9. [Crossref] [PubMed]
  8. World Health Organization 2020. Report of the WHO-China Joint Mission on coronavirus disease 2019 (COVID-19). [last accessed 28 April 2020]. Available online:
  9. Wang W, Xu Y, Gao R, Lu R, Han K, Wu G, Tan W. Detection of SARS-CoV-2 in different types of clinical specimens. JAMA 2020;323:1843-4. [PubMed]
  10. Chen D, Jiang X, Hong Y, Wen Z, Wei S, Peng G, Wei X. Can Chest CT Features Distinguish Patients With Negative From Those With Positive Initial RT-PCR Results for Coronavirus Disease (COVID-19)? AJR Am J Roentgenol 2020. [Crossref] [PubMed]
  11. Yang W, Sirajuddin A, Zhang X, Liu G, Teng Z, Zhao S, Lu M. The role of imaging in 2019 novel coronavirus pneumonia (COVID-19). Eur Radiol 2020;30:4874-82. [Crossref] [PubMed]
  12. United States Centers for Disease Prevention and Control 2020. Discontinuation of transmission-based precautions and disposition of patients with COVID-19 in healthcare settings (interim guidance). [last accessed 15 May 2020]. Available online:
  13. Wáng YXJ. CT suggests discharged Covid-19 patients who were retested RT-PCR positive again for SARS-CoV-2 more likely had false negative RT-PCR tests before discharging. Quant Imaging Med Surg 2020;10:1396-400. [Crossref] [PubMed]
  14. Wang G, Yu N, Xiao W, Zhao C, Wang Z. Consecutive false‐negative rRT‐PCR test results for SARS‐CoV‐2 in patients after clinical recovery from COVID‐19. J Med Virol 2020. Epub ahead of print. [Crossref] [PubMed]
Cite this article as: Ng QX, Liew MF, Yeo WS. Making sense of a negative COVID-19 swab test. Quant Imaging Med Surg 2020;10(11):2212-2214. doi: 10.21037/qims-20-929