DISCUSSION
Two main conclusions have been reached in our study. The sensitivity and specificity levels of rapid antibody tests in detecting COVID-19 cases confirmed by RT-PCR in the emergency department are 57.5% and 85.5%, respectively. In addition, thorax CT sensitivity and specificity were determined as 76.1% and 43.1%, respectively.
Regardless of whether people with COVID-19 infection are symptomatic or asymptomatic, early and accurate diagnosis is important for treating patients and reducing the rate of disease spread. Molecular and serological tests were previously compared during the SARS-CoV-1 epidemic and demonstrated that molecular tests have high sensitivity and specificity. The current gold standard for SARS-CoV-2 detection is the SARS-CoV 2-specific quantitative RT-PCR test from a nasal and/or pharyngeal swab, sputum, or bronchoalveolar lavage (1,9,22–24). However, if the amount of viral genome in the sample is insufficient or if the correct window period of viral replication is missed, it may give false-negative results (25). This situation may result in false-negative results due to technical problems in sampling, laboratory practice standards, complex technical procedures and lack of experienced staff. As a result of a systematic review of COVID-19 test accuracy, false-negative rates ranging from 2-29% were reported based on the results of patients whose first RT-PCR result was negative and repeat tests were positive (26).
Due to the low sensitivity of the PCR test in the diagnosis and treatment algorithm of the disease, it is aimed to support the diagnosis and to prevent possible false negativities with thorax CT examination. In a study performed on 1,014 patients who underwent thorax CT and RT-PCR tests, the sensitivity of CT was found to be 97% in positive RT-PCR patients (14). It is thought that CT scanning can help distinguish COVID-19 positive and negative patients in the emergency room (27–29). Based on this, guidelines were prepared by the WHO for the combined use of thorax CT and RT-PCR in the diagnosis of COVID-19 (27).
Due to the lack of diagnostic reagents, some patients can be clinically diagnosed with thorax CT imaging (21). Some typical radiological images can be detected by CT in patients with COVID-19 pneumonia. Prominent CT findings of COVID-19 infection are the appearance of prominent ground glass density in bilateral, peripheral, and basal regions (30,31). To date, many descriptive studies and case reports have focused on the CT findings of COVID-19 (1,32–35). However, clinical and laboratory findings of COVID-19 infection are indistinguishable from pneumonia caused by some common respiratory pathogens such as influenza virus, Streptococcus pneumoniae and Mycoplasma pneumoniae (36).
Chung et al. (34) reported that thorax CT may be negative for viral pneumonia of COVID-19 at the first admission of patients (3/21 patients). Xi et al. (37) reported 5/167 (3%) patients with negative RT PCR for COVID-19 at first admission, despite the thorax CT findings specific to viral pneumonia. In our study, PPV and NPV values of thorax CT in detecting COVID-19 patients were found at low levels such as 48.2% and 72.1%. This situation was thought to be related to the normal tomography images of patients who presented especially in the early period of the disease. Today, with the gradual recognition of the radiological findings of COVID-19 pneumonia, guidelines are prepared for prompt and accurate diagnosis (38).
Successful management of disease spread will require serological detection of past infection to determine immunity (39). Antibodies specific to SARS-CoV-2 are usually detected within just more than a week after the onset of symptoms, limiting the role of serology in identifying acute infection (40). As stated in the literature, it has been shown that IgM and IgG levels can be measured in patients with SARS-CoV-2 infection from the first week of the disease or generally from the second week (41-43). These findings were found to be parallel to the antibody development characteristics of MERS-CoV infection (44). This situation restricts the use of antibody tests for screening purposes during the COVID-19 pandemic (43,45,46).
These limitations have led to the development of different serological microplate ELISA tests (45,47). Some authors stated that the combination of molecular and serological techniques can reach a sensitivity of 97% in the diagnosis of SARS-CoV-2 infection (43,45). However, these time-consuming tests based on ELISA are generally not as suitable for clinical use as rapid tests and are difficult to incorporate into management algorithms in emergency departments (43,45,48,49). Testing IgM and IgG production in response to viral infection can be a simple method to increase the sensitivity and accuracy of the molecular test (45). Additionally, it can be used for screening purposes to evaluate antibody profiles in a large population. Large-scale screening programs using the antibody test are currently carried out by different governments to reveal the percentage of population immunity.
In our study, the sensitivity of the rapid antibody test performed at the first admission of the patients was evaluated. According to our preliminary findings, despite the low sensitivity (57.5%), our having high specificity (85.5%) levels in rapid antibody tests suggests that the use of rapid antibody test combining with RT-PCR and thorax CT may prevent false-negative results in our society, which population immunity is still low.
There are some limitations in our study. First of all, the selection of patients among the patients who applied to the emergency department made it difficult to evaluate asymptomatic SARS-CoV-2 carriers. It was thought that taking the single NS RT-PCR tests taken at the time of admission as an index caused the false-negative patients to be excluded due to the low sensitivity of the PCR test. In addition, rapid antibody tests evaluated in the emergency department was thought to affect the test results as they could not reach sufficient levels as a result of not allowing the window time required for antibody development.
In conclusion, rapid antibody test and thorax CT examinations were found to have low diagnostic value in patients who applied to the emergency department of our hospital and had a positive first RT-PCR SARS-CoV-2 test. Studies involving larger patient groups are needed for their use alone in diagnosis and screening.