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Vol. 39. Issue 10.
Pages 532-533 (December 2021)
Vol. 39. Issue 10.
Pages 532-533 (December 2021)
Letter to the Editor
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Reply to «Antigen-detecting rapid tests or real-time PCR, what test to use and why?»
Respuesta a «Test rápidos antigénicos o PCR en tiempo real para SARS-CoV-2, ¿qué test usar y por qué?»
Andrés Marcoa,b,
Corresponding author

Corresponding author.
, Concepció Soléc, Indiana J. Abdoc, Elisabet Turua
a Programa de Salud Penitenciaria, Instituto Catalán de la Salud, Catalonia, Spain
b CIBER Epidemiologia y Salud Pública (CIBERESP), Spain
c Equipo de Atención Primaria Penitenciaria, Figueres, Girona, Spain
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Dear Editor,

We appreciate Revollo and Llibre's comments1 on the letter recently published by our group on an outbreak of SARS-CoV-2 infection in Figueras prison (Girona)2. As a reminder, infection was detected by rapid antigen test (RAT) in three mildly symptomatic inmates between 23 and 25 December. As a result, in the afternoon of 25 December, the 81 remaining inmates of that prison block were screened using RAT and nine positive results were identified. On 28 December, the 72 inmates who tested negative by RAT underwent rt-PCR testing, 27 (37.5%) of which were positive. The sensitivity of the RAT in this scenario was very low at just 25%, which is why we reported it.

For reasons of brevity, we did not include any information about the population studied in our original letter, which, according to Revollo and Llibre's comments, could be relevant. Since 1 July 2020, new prisoners in Catalonia have been screened by rt-PCR. In total, 46.2% of those infected by the outbreak had been incarcerated after that date and had a prior negative rt-PCR test. The rest of the infected inmates had been in prison for many months and had not been diagnosed with SARS-CoV-2 infection nor monitored due to close contact with an infected individual. As such, the risk of there being a persistently positive or residual rt-PCR result in an infected inmate, as raised by Revollo and Llibre, we consider to be extremely small. Regarding the use of rt-cycle thresholds (Ct) that Revollo and Llibre also discuss, their use in initial phases of infection is low as the values vary over time3. In fact, we only use them very rarely, almost exclusively to assess infection risk in cases with persistently positive PCR results that require prolonged isolation, as "discharge" without knowing whether or not the subject is infectious is a risk in a confined environment.

We agree with Revollo and Llibre's assessment of RATs' high sensitivity for detecting symptomatic cases with a high viral load and transmission potential, typically in the first five days. However, current data are not as conclusive when it comes to their use in pre-symptomatic or asymptomatic patients. The Centers for Disease Control and Prevention (CDC) suggest that negative RAT results should sometimes be considered "presumptive", and in some circumstances (contact with an infected person or high prevalence of infection in the community), it is advisable to confirm the result with a SARS-CoV-2 nucleic acid amplification test (NAAT)4. Other organisations, such as Cochrane, have also confirmed that RATs are generally less sensitive in asymptomatic patients and more sensitive in settings with a high prevalence of infection5. Although it is true that RATs have shown high sensitivity in infected subjects with Ct <25, as pointed out by Revollo and Llibre, the cycle thresholds, as has already been mentioned, are dynamic and vary over time. Moreover, cases with Ct <25 may not include all potential risk cases.

In addition, the diagnostic strategy in a scenario with low or no viral circulation (scenario A) cannot be similar to a scenario with high viral circulation and localised outbreaks (scenario B) where infection of asymptomatic patients can be 70% or higher6. The specificity of rapid antigen tests is high (close to 100%) and they may be suitable for screening populations in scenario A, even assuming that they entail defined and potentially acceptable risks in certain circumstances5. However, what may be acceptable in scenario A, such as the Barcelona Love of Lesbian pilot concert that Revollo and Llibre participated in7, is not acceptable in the context of an outbreak, and even less so in a confined environment like a prison. Exception to this rule is when RAT screening negative results are subsequently confirmed by rt-PCR, as currently recommended by the guidelines and protocols of Spain's Ministry of Health8, the European Centre for Disease Prevention and Control (ECDC)9 and the CDC4.


The protocols governing the publication of patient data of our place of work have been followed.



Conflicts of interest


B. Revollo, J.M. Llibre.
Test rápidos antigénicos o PCR en tiempo real para SARS-CoV-2, ¿que test usar y por qué?.
Enferm Infecc Microbiol Clin., (2021),
A. Marco, C. Solé, I.J. Abdo, E. Turu.
Baja sensibilidad de los test rápidos antigénicos como método de cribado en un brote de infección por SARS-CoV-2 en prisión.
Enferm Infecc Microbiol Clin., (2021),
D. Romero-Alvarez, D. Garzon-Chavez, F. Espinosa, E. Ligña, E. Teran, F. Mora, et al.
Cycle threshold values in the context of multiple RT-PCR testing for SARS-CoV-2.
Risk Manag Healthc Policy., 14 (2021), pp. 1311-1317
Centers for Disease Control and Prevention (CDC). Interim Guidance for Antigen Testing for SARS-CoV-2. Update 13 May 13 2021. Available from: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html [Accessed 6 June 2021].
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Rapid, point-of-care antigen and molecular-based tests for diagnosis of SARS-CoV-2 infection.
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Centers for Disease Control and Prevention (CDC). COVID-19 Pandemic Planning Scenarios. Available from: https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.html [Accessed 3 June 2021].
Diario Médico. Solo 6 positivos en el concierto masivo ‘piloto’ de Love of Lesbian en Barcelona. Available from: https://www.diariomedico.com/medicina/medicina-preventiva/solo-6-positivos-en-el-concierto-masivo-piloto-de-love-lesbian-en-barcelona.html [Accessed 6 June 2021].
Estrategia de detección precoz, vigilancia y control de COVID-19. Centro de Coordinación de Alertas y Emergencias Sanitarias. Ministerio de Sanidad. Actualizado 26 Feb 2021. Available from: https://www.mscbs.gob.es/profesionales/saludPublica/ccayes/alertasActual/nCov/documentos/COVID19_Estrategia_vigilancia_y_control_e_indicadores.pdf [Accessed 6 June 2021.
European Centre for Disease Prevention and Control: Options for the use of rapid antigen tests for COVID-19 in the EU/EEA and the UK; 19 Nov 2020. Available from: https://www.ecdc.europa.eu/en/publications-data/options-use-rapid-antigen-tests-covid-19-eueea-and-uk [Accessed 6 June 2021].

Please cite this article as: Marco A, Solé C, Abdo IJ, Turu E. Respuesta a «Test rápidos antigénicos o PCR en tiempo real para SARS-CoV-2, ¿qué test usar y por qué?». Enferm Infecc Microbiol Clin. 2021;39:532–533.

Copyright © 2021. Sociedad Española de Enfermedades Infecciosas y Microbiología Clínica
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