SARS-CoV-2 PCR testing of skin for COVID-19 diagnostics: a case report

Understanding the disease course and prevalence of COVID-19 is important not only for medical, but also for socioeconomic reasons. So far, COVID-19 has been understood as a multisystem disease, mainly affecting the lungs, kidneys, and heart.¹ In the past few months, different cutaneous manifestations, such as chilblain-like, vasculitis-like, or urticaria-like lesions, have been described in patients with COVID-19.² Colmenero and colleagues³ detected severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in endothelial cells of cutaneous chilblain lesions via immunohistochemistry methods in seven paediatric patients with negative nasopharyngeal swabs.³

Here, we report the case of an 81-year-old woman who presented at the Department of Dermatology at the University Hospital of Basel, Basel, Switzerland, with a temperature of up to 39°C and a generalised macular eruption with partial vasculitis-like patterns and palmoplantar accentuation (appendix pp 1–2). Infection with SARS-CoV-2 was suspected and laboratory assessments of blood samples showed increased C-reactive protein (248 mg/L), decreased lymphocyte count (7·7%), and negative Treponema pallidum serology. A SARS-CoV-2 PCR (nasopharyngeal swab, Cobas SARS-CoV-2 Test, Roche Diagnostics, Rotkreuz, Switzerland) was negative. 2 days later, a lesional whole skin 4 mm punch biopsy sample was taken from the left flank, which showed a subacute lichenoid interface dermatitis with vacuolisation of the basal epidermal keratinocytes and scant lymphohistiocytic perivascular infiltration in the upper dermis. No leukocytoclastic vasculitis or microthrombosis was present (appendix pp 1–2). Over the next 2 weeks, the patient's rash gradually improved. 6 weeks later, serology tests against anti-SARS-CoV-2 antibodies (Elecsys Anti-SARS-CoV-2, Roche Diagnostics, Rotkreuz, Switzerland) were negative. However, PCR testing of the skin using established methods¹ detected SARS-CoV-2 at low copy numbers (37 per 1 × 10⁶ human RPPH1 copies).

This case is important because it highlights the shortcomings of currently available testing methods for SARS-CoV-2 infection. Although the sensitivity and specificity of currently available PCR and serology tests are high, swab samples that are taken incorrectly are known drivers of the relatively large number of false negative tests for SARS-CoV-2.⁴ Our finding that the patient's serology remained negative is compatible with the hypothesis that some patients with COVID-19 might not establish humoral immunity; an observation that has also been made for other coronaviruses.⁵

In summary, this case emphasises the use of SARS-CoV-2 PCR testing of skin biopsy samples as an additional diagnostic tool, helping to shed light on the actual prevalence of COVID-19 in the general population. Additionally, further studies are needed to understand to what extent and at what point during their disease course patients with COVID-19 actually develop immunity—a question of uttermost importance, especially with regards to the currently ongoing efforts to develop a vaccine to SARS-CoV-2, and the concept of herd immunity generation.