Reverse-transcriptase polymerase chain reaction (RT-PCR) examination using salivary specimens or saliva tests is starting to be considered to establish COVID-19 as an alternative to nasopharyngeal swab specimens. Nasopharyngeal swabs have several disadvantages, namely discomfort for the patient, stimulating coughing and sneezing, and the potential to produce aerosols, thus causing the saliva test to be increasingly studied as the preferred sampling option for patients and healthcare professionals.

However, it is necessary to evaluate the scientific evidence comparing the effectiveness of salivary specimens and nasopharyngeal swabs to confirm the role of saliva collection methods in diagnosing COVID-19. In this article, we will discuss some of the growing scientific evidence regarding its effectiveness, technical limitations, and the potential application of saliva tests to establish COVID-19.



Scientific Evidence of Saliva Test for Diagnosis of COVID-19
The scientific evidence that studies about saliva tests to diagnose COVID-19 is related to the salivary glands' secretory function. Saliva has been widely used as a reservoir for various biological indicators, such as nucleic acids, proteins, and biochemical compounds derived from microorganisms in the oral cavity.
The use of saliva as a diagnostic specimen for SARS-CoV-2 has two advantages. First, the saliva collection method is less invasive than nasopharyngeal swabs, thereby reducing patient discomfort. Second, saliva can be used to detect SARS-CoV-2 in patients with or without symptoms. With these two advantages, diagnosis using salivary specimens has become a widely studied topic in the era of the COVID-19 pandemic.
Several studies have studied the use of saliva for the detection of SARS-CoV-2. To et al. studied a method of collecting saliva by asking the patient to cough the saliva from the throat into a sterile container added with viral transport medium (VTM).
Collecting saliva by this method increases the secretion from the tracheobronchial and lower nasopharyngeal branches. The study results revealed that salivary specimens had a sensitivity of up to 87% in detecting SARS-CoV-2 compared to nasopharyngeal swab or sputum collection by RT-PCR.
Meanwhile, salivary specimens also showed negative results in all samples (n = 33), which showed negative results for SARS-CoV-2 based on RT-PCR on nasopharyngeal swab specimens.
In another study, Azzi et al. studied the role of saliva in helping diagnose COVID-19. They collect the saliva using a pipette with a drooling technique in severe to critical cases of COVID-19. The results of this study revealed that all salivary samples were positive for SARS-CoV-2 and following RT-PCR results from the throat or bronchoalveolar swab specimens.
However, RT-PCR of 2 salivary specimens showed positive results. In fact, on the same day, RT-PCR from the throat or bronchoalveolar swab specimens showed negative results. This suggests that someone still can transmit SARS-CoV-2, even though the throat swab is negative. Additionally, it may also be related to the salivary glands' potential as a reservoir for SARS-CoV-2 in patients with confirmed cases of COVID-19.
Meanwhile, Han et al. analyzed viral load kinetics in neonates diagnosed with COVID-19. They found that viral copy counts in salivary specimens and throat swabs in neonates with COVID-19 appeared comparable, with a range of 105 copies / mL. SARS-CoV-2 ribonucleic acid (RNA) was detected in throat and saliva swab specimens and all airway and stool specimens.
The study also revealed that SARS-CoV-2 infection in neonates could be systemic and cause RNAemia. However, it is unknown whether a high viral load influenced the identification of SARS-CoV-2 RNA in this patient's salivary specimen or whether the salivary glands could be a reservoir for SARS-CoV-2 in infants and children.


Effectiveness and Limitations of Saliva Test in Diagnosing COVID-19
Multiple research data with an adequate number of samples is needed to study the saliva test's effectiveness and limitations in diagnosing COVID-19. Czumbel et al. developed a meta-analysis to determine the saliva test's sensitivity in detecting COVID-19.
In the study, researchers found that the sensitivity of throat swab specimens and saliva tests in the detection of SARS-CoV-2 was 98% (89-100% CI) and 91% (80-99%). The overlapping confidence intervals may indicate that the proportion of test results showing a positive value between the saliva specimen and throat swab is not significantly different. However, this needs to be studied further in testing the accuracy of a larger study.
In the meta-analysis conducted by Czumbel et al., only two studies reported the saliva test's specificity in detecting SARS-CoV-2. One study involving 50 healthy individuals revealed that one saliva sample was positive, while all throat swabs were negative.
Meanwhile, in another study, 2 out of 98 salivary specimens were detected positive for SARS-CoV-2, even though the throat swab specimens' RT-PCR results were all negative. The difference in specificity between the two types of specimens in detecting SARS-CoV-2 can be caused by a false-positive result on the saliva specimen or a false negative on the throat swab specimen.
Reporting internal quality control data can be a way of determining the likelihood of false-positive or negative results. However, there was no detailed explanation in the sample in this study, so it is unknown whether the internal quality control mechanisms adequately explained the discrepancy of positive results between the two specimens.
Another limitation of the study regarding applying the saliva test in diagnosing COVID-19 is the lack of optimization and validation of the saliva collection protocol, pre-PCR saliva transport and storage methods, and standardized viral RNA examination methods.
Existing data are also insufficient to support saliva collection techniques in infant and pediatric populations. Further research on this matter will help provide clinical recommendations regarding the use of salivary specimens to detect SARS-CoV-2.


Potential of Saliva Test for Diagnosis of COVID-19
The potential of a saliva test for the diagnosis of COVID-19 lies in its simplicity, scalability, and cost savings. First, if the method of collecting specimens through drooling proves to be effective in detecting SARS-CoV-2, this method is simple enough to be taught to patients and requires almost no help from health workers.
Second, from the point of view of potential scalability, by this simple method of specimen collection, patients may be able to do it without having to come to the hospital or even simply use a special courier to ensure that the sample is sent to the laboratory.
Third, the method of collecting saliva utilizing self-drooling can save the cost of human resources for health workers, as required in the throat swab method. Cost savings can also come from minimizing unnecessary personal protective equipment for the saliva collection method through self-drooling.
Meanwhile, before the saliva test is developed in the specimen collection policy scheme for SARS-CoV-2 detection, some basic assumptions about this procedure's risks and benefits need to be confirmed first. First, an adequate data set needs to be collected to assess the saliva test's effectiveness and limitations in various demographics and health care situations.
Second, the information on viral load dynamics in positive salivary specimens needs to be compared in parallel with positive specimens from throat swabs and bronchoalveolar rinses. Thus, a positive result can be interpreted appropriately according to the patient's clinical condition.
Third, the saliva test placement against a standardized standard (nasopharyngeal swab) needs to be ascertained whether used as an alternative method in areas with limited health personnel and PPE availability or can serve as complementary data cases where PCR results were previously indeterminate.
If these basic assumptions can be clarified before saliva testing becomes part of the policy for diagnosing COVID-19, health professionals recommend this technique to patients properly.


Conclusion
Various study results support the potential of saliva testing for COVID-19. With a sensitivity level of 87–91%, RT-PCR with salivary specimens can be a simpler, safer, and cost-effective method of specimen collection of choice. However, compared with nasopharyngeal swabs, data on the saliva test's specificity, the standard saliva collection method, the method of transport and storage of saliva before PCR, and the standard technique for detection of SARS-CoV-2 RNA from salivary specimens are still limited.
The saliva test to detect SARS-CoV-2 has great potential to be applied. Compared to nasopharyngeal swabs, the saliva test has the advantage of being a simple method to be done independently by the patient, the potential to be done without having to come to the hospital or laboratory as having health costs. However, the saliva test application to diagnose COVID-19 requires more complete data support on the effectiveness and limitations of this procedure in various conditions of care and viral load dynamics between specimens.
References
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