Understanding the Different Diagnostic Methods of COVID-19
A guide for COVID-19 detection and diagnosis
It’s been an intense past few weeks for the entire world. Cases are exploding at an exponential rate and death tolls are ever-so steadily climbing, despite institution of major efforts and round-the-clock work on management, vaccine research, and drug clinical trials. At the time of writing (16 April), the number of cases worldwide has just surpassed the 2 million mark, and totalled close to 140 thousand deaths.
One of the reason for this upsurge in confirmed cases is the growing spread of diagnostic modalities for establishing a positive case on somebody who is showing signs of the SARS-CoV-2 infection.
Most countries are utilising some kind of risk score to determine who should get tested. USA’s Centers for Disease Control & Prevention (CDC) advise immediate implementation of recommended infection prevention and control practices if a patient is suspected of having COVID-19, but also recommends to “test for all other sources of respiratory infection first”, along with the general workup for COVID-19 — most of which is currently performed via remote consultations over the phone or web with primary care physicians.
The main reason for this is the frustratingly unspecific nature of COVID-19 symptoms. Most patients present with fever, cough, and general feelings of malaise, which could be attributed to many diseases, most of which are quite common and benign, like the common cold. Shortness of breath is a hallmark finding in patients with COVID-19, but they usually present later into the disease, when condition of the lungs have significantly worsened.
Epidemiological facts should also be factored in to aid in determining which patients need to be prioritised in testing. A paper by Marco Cascella of Italy’s National Cancer Institute in Napoli, which was published almost a month ago, reports that this “includes anyone who has had close contact with a patient with laboratory-confirmed COVID-19 within 14 days of symptom onset or a history of travel from affected geographic areas within 14 days of symptom onset.”
It’s understandable why triage is imperative in the macro-management of the COVID-19 pandemic. For one, testing can be expensive, and even after financial assistance is taken into account, the total sunk cost makes it unreasonably inefficient to test everyone who presents with mere signs of COVID-19. Diagnostic centres and facilities are also working night and day to process the bulk of patient samples, and over-saturating lab workers and equipment with excessive demand may counterintuitively increase burnout and decrease overall accuracy in diagnosis.
Elective testing is available in some parts of the world, but CDC reports that it may still be difficult to get tested despite increasing supplies for conducting particular tests — such as the rapid serologic testing, which we will get back to in just a bit.
David Cennimo, Assistant Professor of Medicine and Pediatrics at Rutgers New Jersey Medical School, writes on Medscape that the workup of COVID-19 cases, after thoughtful exclusion for other possible causes of the presenting symptoms, involves routine laboratory tests to assess the function of multiple organs, radiological evaluation using chest x-ray or CT scan, and finally the Gold Standard COVID-19 testing modalities if results point towards that direction for ultimate confirmation of the disease.
Laboratory Studies
Routine lab tests ordered for suspected cases of COVID-19 include a complete blood count (CBC), a metabolic panel to assess liver and kidney function, inflammatory markers, arterial blood gasses, and tentative blood cultures to exclude other possible bacterial co-infections.
Current data on COVID-19 associated lab results include low white blood cell count on CBC, elevated liver and kidney enzymes (which portrays extension of damage towards multiple organs), and an increase in inflammatory markers. Arterial blood gas analysis in COVID-19 testing is performed to evaluate the abundance of oxygen within the arteries, and can be used to determine severity of the disease. A low partial pressure of oxygen (PaO2≤100mmHg) is indicative of severe ARDS.
Additionally, new research suggests that elevation of cardiac enzymes such as CK-MB and troponins are associated with worse overall prognosis, so it may be sensible to test for these biomarkers, especially in patients with a known history for heart abnormalities.
Radiological Imaging
The two imaging modalities that are most commonly used in the field are chest radiography, better known as chest x-ray, and CT scan. The former is more widely available and is cheaper to perform than the more advanced CT scan, but consequently produces less accurate results.
David Cennimo highlights a Hongkong study which showed that the most common abnormalities found in chest x-ray of COVID-19 patients include consolidation (47%), and ground glass opacities (33%). This particular test has a low sensitivity (the ability to detect the true positives in a pool of positive results) owing to the fact that lung consolidations and ground glass opacities are merely signs of pneumonia — regardless of the cause.
CT scans have been shown to be more effective on this front. A Chinese paper which was published last February compared the results of CT scans to that of the gold standard Polymerase Chain Reaction — a test which ultimately confirms the diagnosis of COVID-19 (although they do have drawbacks as well, bear with me).
The sensitivity of chest CT scans was found to be 97% when compared to positive RT-PCR results, meaning that 97% of people who were identified to be positive by pure CT scan results turned out to be confirmed positive by the gold standard. A mere 3% turned out to be false negatives, meaning they actually did have the disease despite being identified by the CT scan as negative. In the world of diagnostic medicine, a sensitivity of 97% is impressive.
The specificity of the scans, however, were dismal. Only 25% of patients who were identified to be free of COVID-19 through CT scans ended up being ruled out of the disease by PCR. The other 75% were falsely identified as positive by the CT scans, contriving a high number of false positives. What happens is we end up with more positive results than in reality, which could quickly use up valuable resources that could instead be utilised for the true positives. Nevertheless, being conservative is better than being careless at times like these.
Gold Standard: PCR and NAAT
This method essentially amplifies specific viral particles, which doctors can later compare to known samples of the SARS-CoV-2. In order to perform these tests, samples are first obtained from nasopharyngeal oropharyngeal swabs, sputum, or broncho-alveolar lavage, then sent to appropriate testing facilities under special packaging and shipment conditions. WHO has published an entire PDF guide on this, highlighting adherence to biosafety regulations and ensuring good communications between involved laboratories.
NAAT, which stands for Nucleic Acid Amplification Test is the most accurate diagnostic method for establishing a SARS-CoV-2 infection. They are performed on similar premises to RT-PCR, and the two are often terminologically interchangeable. That being said, the WHO also emphasises that a negative result does not rule out the possibility of a COVID-19 infection. One article goes as far as disregarding the negative result if a strong clinical evidence for COVID-19 is present.
There are a number of factors that could lead to a false negative results, such as poor quality and handling of specimens. Obtaining the specimens too early into the disease course can also result in false negatives, as it takes 1–2 days for viral inoculation before the virus is detectable in the respective sampling locations.
Besides the possibility of false negatives, time could also pose as a drawback to NAAT testing. Even though a reliable diagnosis can be produced in as little as 6–8 hours, the sheer number of specimens on waiting list and time lost during fastidious transportation all amount to longer waiting time. One source reports that the tests could take anywhere from 2 days to 13 days to come back. The test is also quite expensive to run, so that’s why we currently have a more popular, cheaper and quicker alternative to NAAT, which is the rapid test.
Rapid Serologic Test
The rapid serologic test is what people commonly refer to as the ‘rapid test’, and it measures specific antibodies that are produced in response to SARS-CoV-2 in the serum. When a serum with antibodies are introduced into a viral medium, say a testing kit with a tiny amount of virus ‘mash’, the two would react and coagulate, denoting the presence of anti-virus antibodies.
As you can probably imagine, there are some caveats to this test, mainly the fact that antibodies must already be present in order for a positive result to appear. In other words, the patient would technically already be cured off the disease, or in the convalescent phase — after the body has successfully produced the proper antibodies to fight the virus.
They are therefore less accurate at detecting present cases, but are cheaper, easier to distribute, and yields faster results than the more advanced gold standard techniques. The test is also particularly useful at detecting asymptomatic cases of COVID-19, and can therefore be used for contact tracing for those patients.
Based on the principles of the rapid serologic testing, a new clinical trial is on the works for convalescent plasma. The idea is to transfer immunoglobulins from the serum of cured patients to those that require immediate therapy through plasmapheresis. It works like a silver bullet, similar to treatment of a venomous snake bite using its specific antivenin as a form of artificial passive immunity.
Phew. There’s a lot of information to digest here, but that’s the whole truth on testing. As practitioners (and patients), we all rely heavily on test results, knowing how a false negative result can result in otherwise preventable fatality, but there really isn’t a diagnostic method that is entirely foolproof. Understanding the limitations of each diagnostic modality is just as important as understanding how to interpret the results.
Currently, many countries are institutionalising the use of chest CT scans to exclude patients who present with suspicious clinical signs. They’re reliable (in terms of relegating patients off the PUI list) considering their high sensitivity, and they’re also capable of producing quick results (15–30 minutes). At the same token, don’t be completely crestfallen if yours come back positive — it might just be a false positive.
