M K and T N performed RIAT. Declaration of competing interest The authors report no potential conflicts of interest exist with any companies/organisations whose products or services may be discussed in this letter.. coronaviruses contamination. Two of the 4 patients showed initial unfavorable to subsequent positive RIAT results, indicating seroconversion. RIAT was positive for IgG and IgM in viruses other than coronavirus in 2 (25.0%) and 1 (12.5%) patient. Because of high SLI incidence of false positive RIAT results, cross antigenicity between human common cold coronaviruses and SARS-CoV-2 can be considered. Results of RIAT should be interpreted in light of epidemics of human common cold coronaviruses infection. Prevalence of past SARS-CoV-2 contamination may be overestimated due to high incidence of false-positive RIAT results. antigen, antigen, and nasopharyngeal influenza computer virus and were all unfavorable. Serum antibodies against HIV and em Trichosporon asahii /em , which is the most frequent antigen of hypersensitivity pneumonitis in Japan, were unfavorable He was admitted to our hospital on day 17 (hospital day [HD] 1) and was followed up without antibiotics. However, his fever continued (Fig. 2), and general fatigue increased after admission. Blood gas analysis under ambient air on HD 5 showed a PaO2 of 72.6?Torr. Chest CT performed on HD 5 showed worsening of ground-glass opacities and consolidation (Fig. 1c). We performed RIAT using a commercially available kit (RF-NC0001, RF-NC0002 with lateral flow style, KURABO Ltd., Osaka, Japan) for IgM and IgG against SARS-CoV-2, which was positive for IgG. We repeated RIAT on HD 6 and received the same result. We repeated both PCR testing for SARS-CoV-2 and multiplex PCR using nasopharyngeal swab specimens, which were unfavorable for SARS-CoV-2 but again positive for human coronavirus HKU1. We performed RIAT using preserved frozen serum obtained on admission, which showed unfavorable results for both IgM and IgG, indicating seroconversion. His body temperature gradually improved, and his PaO2 on HD 9 had risen to 89.7?Torr. Pulmonary shadows on CT also improved, and he was discharged on HD 14. After returning to home, his Pifithrin-alpha symptoms have never relapsed. Serum antibodies against influenza computer virus, em Mycoplasma pneumoniae /em , em Chlamydophila pneumoniae /em , em C. psittaci /em , respiratory syncytial computer virus, adenovirus, and parainfluenza computer virus did not increase in the convalescent phase, and we ultimately diagnosed Pifithrin-alpha the patient as having primary human coronavirus HKU1 pneumonia. Open in a separate windows Fig. 2 Clinical course of the patient. Body temperature decreased to 37?C on hospital day 8. C-reactive protein gradually decreased. Blood gas analysis worsened after admission and then improved. IgG antibody against SARS-CoV-2 was unfavorable on admission but switched positive. IgM antibody against SARS-CoV-2 was unfavorable throughout the clinical course. RIAT, Pifithrin-alpha rapid immunochromatographic antibody test. HD, hospital day. 3.?Discussion We experienced a patient suffering human coronavirus HKU1 pneumonia who showed false-positive results for IgG against SARS-CoV-2 using an RIAT. An excellent sensitivity of RIAT for SARS-CoV-2 has been reported. We performed RIAT using a commercially available kit for IgM and IgG against SARS-CoV-2 in serum samples of 24 patients with laboratory-confirmed COVID-19 (admitted from February to April 2020), 7 patients with human common cold coronavirus pneumonia (Table 1), and 8 patients with viral pneumonia due to other than coronavirus (admitted from January 2015 to January 2019) admitted to Pifithrin-alpha our institution, all of whom showed fever and bilateral ground-glass opacities and consolidation on computed tomography. For RIAT in patients with human common cold coronavirus contamination and non-coronavirus contamination, serum samples stored at ?80?C were used. Respiratory pathogens were detected on a Rotor-Gene Q instrument (Qiagen, Hilden, Germany) with a multiplex, real-time PCR (RT-PCR) using an FTD Resp 21 Kit (Fast Track Diagnostics, Silema, Malta). RIAT was performed according to manufacturer’s instructions. Table 1 Results of rapid immunochromatographic test for detecting SARS-CoV-2 antibody. thead th rowspan=”2″ colspan=”1″ Case /th th rowspan=”2″ colspan=”1″ Onset (month 12 months) /th th rowspan=”2″ colspan=”1″ Age, sex /th th rowspan=”2″ colspan=”1″ Underlying disease /th th rowspan=”2″ colspan=”1″ Coronavirus subtype /th th rowspan=”2″ colspan=”1″ Specimen in which virus was detected /th th colspan=”5″ rowspan=”1″ Laboratory results hr / /th th colspan=”4″ rowspan=”1″ Serum antibody hr / /th th rowspan=”1″ colspan=”1″ WBC,/mm3 /th th rowspan=”1″ colspan=”1″ Lym,/mm3 /th th.