Frequently Asked Questions About COVID-19 Testing

Ordering, Reports, and Turnaround Time

What SARS-CoV-2 (COVID-19) tests do you offer?

We currently offer RT-PCR for detection of SARS-CoV-2 SARS-CoV-2 (COVID-19) Qualitative PCR [NCVQLT] and IgG antibody testing as an indication of prior infection with SARS-CoV-2 COVID-19 Antibody (IgG) [NCVIGG].

I am not a UW provider. Will you accept specimens from my hospital?

Yes.

What is the turnaround time for RT-PCR testing?

Our median turnaround times have been approximately 7-8 hours once the sample is received.

I am at an outside location. Does it matter what carrier (e.g. FedEx, UPS) I send my samples by?

It does not matter as long as appropriate specimen handling conditions are met.

Who is eligible for testing?

  • Test orders must be medically necessary and accompanied by physician orders.
  • UW Medicine phlebotomists only draw blood from patients with a UW Medicine-related requisition and provider.

Sampling for RT-PCR testing

What specimen types do you accept?

Please see the “Ordering & Collection” section of SARS-CoV-2 (COVID-19) Qualitative PCR [NCVQLT] or COVID-19 Antibody (IgG) [NCVIGG] for up to date information.

What swab types do you accept?

All of our tests are currently only validated for flocked swabs. We are currently working to validate other (e.g. foam) swabs.

Is an NP swab alone sufficient, or do I need to send both NP and OP?

An NP swab alone is sufficient.

I have ordered multiple respiratory tests. How many swabs do I send?

In addition to SARS-CoV-2, UW Laboratory Medicine offers Rapid Flu/RSV testing and an extended respiratory virus panel. At this time, due to staffing and reagent constraints, we discourage the ordering of extended viral panels unless clinically indicated. If multiple tests are ordered:

  • SARS-CoV-2 and Rapid Flu/RSV: One swab. The sample will first go to Microbiology, an aliquot is taken, and then the sample will be sent to Virology. Expect a 30 min delay in SARS-CoV-2 turnaround time for this extra step. Use code FABRMC for inpatient testing at UWMC. Virology would prefer to limit their own Flu/RSV testing.
  • SARS-CoV-2 and Respiratory panel: Separate swabs highly preferred; with test volume so high, this is surest way to make sure nothing gets missed and has the fastest TAT. It is technically possible to do both tests from same swab or add on a test to a post-nucleic acid extraction if necessary.
  • SARS-CoV-2, Rapid Flu/RSV, and Respiratory Panel: Either two swabs (One for SARS-CoV-2 and Rapid Flu/RSV, one for respiratory panel) or 3 swabs if the patient can tolerate it.

Should I send sputum in a cup or swab the specimen?

Either is acceptable.

What is the rate of positivity for sampling with nasopharyngeal vs. oropharyngeal swabs? One swab only vs. two swabs? NP swab vs. OP vs. sputum vs. BAL?

There is currently not enough data to assess these questions with internal data. Several are under active study. Nasopharyngeal and oropharyngeal swabs are preferred; patients with a high clinical index of suspicion and a negative NP or OP swab result may benefit from sampling of the lower respiratory tract.

RT-PCR Assay Performance

How sensitive is the SARS-CoV-2 RT-PCR test?

The UW laboratory developed test (LDT) using the CDC kit has been shown to be at least as sensitive as any other test that to which it has been compared. Side-by-side comparison with the Washington State Public Health Lab showed 100% concordance in validation. The test is highly sensitive in an analytic sense; if viral RNA is present in the sample, it is very likely to be detected. However, either because of sampling error or the biology of the disease (e.g. virus present in lower but not upper respiratory tract), there have been known cases of patients with negative RT-PCR results who later were RT-PCR positive. The frequency of this (i.e. the clinical sensitivity) has not yet been determined.

In published literature, one case series of 51 patients showed a sensitivity of RT-PCR of 71% from the first throat swab or sputum. 23% of those initial negatives were positive on the 2nd RT-PCR, 4% on the 3rd, and 2% on the 4th RT-PCR test (Fang et al. 2020). Another study conducting serial RT-PCR testing showed the mean time from an initial negative RT-PCR to subsequent positive RT-PCR was 5.1 days (± 1.5 days) (Ai et al. 2020).

In internal studies, the in-house UW laboratory developed test is slightly more sensitive than the Hologic Panther test. The Hologic Panther test has a ~95% sensitivity when compared to our laboratory developed test; that is, of 20 positive samples as detected by the UW test, 19 would be called positive by the Hologic test. All of these missed samples were of low viral load close to the limit of detection.

How specific is the COVID RT-PCR test? Does it cross-react with the “coronavirus” test on an extended respiratory panel?

All of the offered assays are highly specific for the SARS-CoV-2 virus, with no known cross-reactivity to either other human coronaviruses or to other human respiratory pathogens. The coronavirus assay on the extended respiratory panel does not detect SARS-CoV-2.

What is the positive and negative predictive value?

Because the test is highly specific, positive results have a very high positive predictive value and should be treated as true cases of COVID-19 infection. Negative results must be interpreted within the patient’s context and should not be taken as the sole determining factor in ruling out the disease. Patients with a high index of clinical suspicion may require additional testing.

What does an “inconclusive” result mean?

The UW SARS-CoV-2 Real-time RT-PCR assay targets two distinct gene regions (see SARS-CoV-2 (COVID-19) Qualitative PCR [NCVQLT] for details). When one of the two targets, but not both, is present above the threshold for positivity, the test is reported as “inconclusive”. This is usually seen with low amounts of viral DNA. In practice, “inconclusive” results should be treated as presumptive positive COVID cases.

Is confirmatory testing performed for inconclusive results?

No. The state lab is no longer requesting specimens with inconclusive results for confirmatory testing.

What gene targets are used in each assay?

The UW SARS-CoV-2 Real-time RT-PCR assay targets two distinct regions within the N gene of SARS-CoV-2. Amplification of both targets results in a presumptive positive (detectable) test result, while amplification of one of two targets results in an inconclusive result, and amplification of neither target results a negative (non-detectable) test result. Samples with inconclusive test results should be considered as positives; due to capacity constraints follow-up testing is currently not available.

The Hologic Emergency Use Authorization (EUA) SARS-CoV-2 Real-time RT-PCR assay targets two conserved regions of the of SARS-CoV-2 (the causative agent for COVID-19) ORF1ab gene. The two regions are not differentiated; amplification of either or both regions is a presumptive positive (detectable) test result and amplification of neither target results a negative (non-detectable) test result.

Serological (antibody) testing

What are the indications for serological testing for SARS-CoV-2 (COVID-19)?

Indications for serological testing include:

  • Testing patients who may have had COVID-19 or exposure to SARS-CoV-2 more than 10 days ago.
  • As part of a potential “return to work” algorithm.
  • Serological testing is NOT indicated for diagnosis of acute infection.

What COVID-19 serology test do you offer?

UW Virology is performing the Abbott SARS-CoV-2 IgG immunoassay on the ARCHITECT instrument. This is a chemiluminescent microparticle immunoassay (CMIA) used for the qualitative detection of IgG antibodies to SARS-CoV-2 nucleocapsid protein in human serum and plasma. This is a high-throughput automated system allowing for the testing of many samples each day.

What is the target antigen used in the Abbott immunoassay?

The Abbott SARS-CoV-2 IgG immunoassay detects antibodies to the viral nucleocapsid protein (NP).

How are the results reported, and what is the clinical significance?

  • The results are either “positive” or “negative” based on the manufacturer-indicated cutoff.
  • A negative result indicates that either a person has not been infected with SARS-CoV-2 or there is not a detectable level of antibody present. Explanations for this may include a very recent exposure such that not enough time has elapsed to generate an immune response, or the immune response has decreased below the detectable level. A negative result does not rule out current or past infection with SARS-CoV-2.
  • A positive result indicates previous or current infection. Recent studies examining serial plasma samples in hospitalized patients with SARS-CoV-2 infection suggest that the median time to seroconversion is about 10 days in moderately ill patients, and 14 days in severely ill patients. It is important to note that a positive serology test cannot distinguish between active or past COVID-19. If there is concern for active infection, molecular testing (PCR) with a nasopharyngeal swab is recommended.
  • At this time, it is not known whether the presence of antibodies confers protection from reinfection with SARS-CoV-2, how long the antibody response lasts, or the association between antibody response and clinical outcomes of individuals with COVID-19.

How sensitive is the serological test?

This depends on the time after infection. The Abbott product insert reports a high sensitivity, with 91% sensitivity before 14 days after symptom onset and 100% sensitivity more than 14 days after symptom onset. Our internal data from 125 patients recovered 100% sensitivity at 17 days after symptom onset and 13 days after PCR positivity. The sensitivity of the test in subjects with asymptomatic infection is unclear, and the duration of positive results is unknown at this time.

How specific is the Abbott test? Does it cross-react with other human coronaviruses?

  • This assay does not appear to cross-react with other human coronaviruses, but this type of cross-reactivity cannot be completely ruled out.
  • The product insert of the assay reports a specificity of 99.6%.
  • Our internal testing in pre-COVID-19 sera has detected 1 false positive in 1,020 specimens for a specificity of 99.90%.

What are the limitations of this test?

  • This is not intended for acute diagnosis early in the course of disease. As stated above, negative results do not rule out a SARS-CoV-2 infection. For patients in which there is a high clinical suspicion for COVID-19 is high, PCR-based testing is recommended to evaluate infection. Antibody testing should not be used alone to diagnose COVID-19.
  • False-positive results rarely may occur as the result of infection with non-COVID-19 human coronaviruses.
  • Immunocompromised patients with COVID-19 may not have detectable levels of antibodies, or have a delayed antibody response.

What is the turnaround time?

Within 1 day.

My patient has a positive serology result, and is interested in being a potential plasma donor. Where can I refer this patient for more information?

This assay is not meant for the screening of donated blood. However, if a patient is interested in being a potential convalescent plasma donor, please refer him or her to the following website: https://newsroom.uw.edu/news/plasma-donors-sought-among-those-recovered-covid-19. At the bottom of this website is the contact information patients can use to get more information about participating.

Clinical Performance and Considerations

What is the rate of co-infection with COVID-19 and other respiratory viruses?

Because UW testing guidelines have de-emphasized up front co-testing for multiple respiratory viruses, there is relatively little internal data to support a generalized conclusion at this time other than that co-infection with other viruses can occur. A positive result for another virus does not definitively rule out the presence of SARS-CoV-2.

Does UW Virology publish information about testing volumes or rates of positivity?

Yes, see our public COVID-19 Testing Dashboard

References

Associated Tests

Code Name Specimen
NCVIGG COVID-19 Antibody (IgG) Blood
NCVQLT SARS-CoV-2 (COVID-19) Qualitative PCR nasopharyngeal (NP) or oropharyngeal (OP...
NCVRPD SARS-CoV-2 (COVID-19) Qualitative Rapid PCR (special approval only) nasopharyngeal (NP) swab

Last updated 2020-04-28T00:07:31.893021+00:00