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Covid 19 Serological Rapid Test

Introduction

A screening of SARS-CoV-2 infection was conducted in the health workers of the Tuscan regional health service, carried out with tests rapid serology in lateral flow immunochromatography. Some field validation studies were also conducted to verify the sensitivity and specificity of the aforementioned rapid tests in the detection of anti-SARS-CoV-2 antibodies. The representatives of the hospital-university companies (AOU) of Careggi, Pisa, Siena, Meyer, of the Toscana Gabriele Monasterio Foundation (FTGM), of the Institute for Cancer Study and Prevention (ISPRO) and of the three USL companies of Tuscany have transmitted to the Regional Health Agency aggregate data in tabular form on the screening activities carried out at their companies and on the field validation activities of the rapid tests used.

The results of the aforementioned screening and validation activities are summarized below, referring for further details to complete report .


Validation of rapid tests

The rapid serological tests used for the screening of healthcare workers in the AOUs of Careggi, Pisa, Siena, Meyer, FTGM and ISPRO – Test SCREEN and ORIENT GENE – were validated in terms of sensitivity and specificity on sera considered true positive and true negative due to the presence of antibodies against SARS-CoV-2.

The sensitivity of both tests was high, and the negative predictive value (probability that a subject with a negative test is really serologically negative) for serum prevalence up to 3% is practically 100% for both tests .

Considering cautiously both immunoglobulin classes and the results obtained on winter sera collected in the pre-Covid era, the specificity is mediocre for both tests, with positive predictive values, for example serum prevalence of 2%, around 10%. The specificity considerably improves considering the summer serums and, regardless of seasonality, considering only the IgG . In other words, the major specificity problems are found with IgM in winter sera, presumably due to cross-reactions with other human coronaviruses and/or influenza and parainfluenza viruses. Regardless of seasonality, the specificity considering only the frankly positive IgG is around 95% for both tests, with positive predictive values, again for serum prevalence, for example 2 %, which reach 31 and 25% respectively for the test SCREEN and ORIENT.


Screening of health workers from Tuscan hospital-university companies, the FTGM and ISPRO
At 24 April , 17,098 health workers from the AOUs of Careggi, Pisa, Siena, Meyer, FTGM and ISPRO had performed the rapid screening test . The Careggi AOU used the SCREEN test, while the Pisa and Siena AOUs, FTGM and ISPRO used the ORIENT GENE test. A total of 406 operators (equal to 2.6% ) tested positive in the test and 538 (equal to at 3.4% ) were positive or dubious . The percentages of positive tests and even more so of doubtful tests are higher in Careggi than in other companies.

On the same date, all healthcare professionals who tested positive or doubtful in the rapid test at the AOU di Careggi, Pisa, Meyer, FTGM and ISPRO did the nasopharyngeal swab for the search for SARS-CoV-2. At the Careggi AOU 20 health workers tested positive, equal to 7% of the positive/doubtful rapid test and 3.7% of those subjected to the screening test; at the AOU of Pisa 6 health workers, equal to 4.4% of the positive/doubtful rapid test and 1% of those subjected to the screening test; to AOU Meyer 5 health workers, equal to 10.9% of the positive/doubtful rapid test and 4.2% of those subjected to the screening test; to FTGM 2 health workers, equal to 15.4% of the positive/doubtful rapid test and 3% of those subjected to the screening test. No healthcare worker at ISPRO tested positive for the swab. At the Siena AOU, confirmation swabs were performed only on operators with confirmed seropositivity with ELISA test (24 out of 56) and only one operator tested positive, equal to 1.8% of the positive/doubtful rapid test and 0.3 ‰ of those subjected to the screening test.

Screening of healthcare workers of USL companies

As of 5 May 2020 , 35,091 health workers from the three USL companies in Tuscany had performed the rapid screening test [1] . A total of 1,111 operators (equal to 3.2%) tested positive.

The percentages of positive tests were highest in the Southeast ASL and lowest in the Northwest ASL.
On the usual date, 580 healthcare workers who tested positive for the rapid test had swabbed and 94 were positive, equal to 16.2%, with limited variability between the three ASLs.

RSA operators of the South-East ASL

For the South-East ASL, the results of the screening carried out among the RSA operators were also made available, for which both the rapid test and, practically all, were performed also the tampon. 4% of operators tested positive or doubtful in the rapid test . Nineteen of the 62 operators who were positive on the rapid test tested positive on the swab, or more than 30%. It should be noted that 23 operators tested positive for the swab who tested negative for the rapid test were also : the screening protocol would therefore have identified 45% of operators with shedding viral .


Complete case summary of all health agencies

As of May 5, 2020, 52,189 health workers from the Tuscan Health Authorities had performed the rapid screening test, A total of 1,649 operators (equal to 3.2% ) tested positive . The percentages of positive tests are higher in the Southeast ASL and lower in the AOU of Siena. As of the usual date, 1,086 healthcare workers who tested positive for the rapid test had carried out the swab and 128 were positive , equal to 11.8% , with higher percentages at the South-East and North-West ASLs and lower at the Siena AOU (apart from the case of ISPRO, with no positive swab operator).

Discussion of results

The validation of the two tests used for screening of healthcare professionals was carried out on a limited number of samples considered seronegative or seropositive.

Sensitivity is high for both tests with negative predictive values ​​equal to or close to one hundred percent [2] . Having analyzed only sera from symptomatic patients, the sensitivity of the tests to detect seropositivity following asymptomatic infections remains not evaluated. However, there is no evidence in the literature to indicate that the antibody response may be different in asymptomatic infection.

However, it is essential to distinguish between the sensitivity of each serological test in diagnosing as positive a subject who has actually produced an antibody response against the virus by the sensitivity of the test in identifying a subject with ongoing infection and therefore potentially contagious.

According to some studies, the antibodies in the blood become detectable in the blood median starting from the 10th day from the infection [3] and the shedding of the virus in asymptomatic subjects lasts medianly from the 5th to the 15th day from the infection [4] . According to other studies, virus shedding can last much longer, even in paucisymptomatic patients, with median times of about 30 days [5] . In any case, an unspecified percentage of subjects with ongoing SARS-CoV-2 infection will result negative at each serological test (and not just the rapid test).

On the other hand, the positive predictive values ​​are less encouraging estimated on the basis of the results on specificity given the low serum prevalence levels. In the situation of considering the results of both immunoglobulins (IgM and IgG) and also the doubtful cases positive, on the basis of the specificity values ​​obtained on the winter sera, for serum prevalence of 2%, the PVPs are around 10% for both test. The situation improves considering the results obtained on the summer sera , with VPP around 20%. This means that, even in this case, for these assumed serum prevalence levels, every 5 subjects with a positive serological test , 4 can be false positives .

On the over 52 thousand health workers who took the rapid test, 1.649, equal to 3, 2%, were positive or doubtful. As we have said, the true seroprevalence of antibodies against SARS-CoV2 in this population group is probably lower than the percentage of test positives , but it is not possible to estimate it precisely because given these test positive percentages and the predictive values ​​of these tests, the true prevalence confidence intervals include zero [6] . The ability to interpret these data is also limited by the unavailability to date of individual data and data on the occupational and anamnestic characteristics of healthcare workers undergoing screening.

As of May 5, 2020, 1,086 HIV-positive operators had swabbed and 128, equal to < strong> 11.8% , were positive [7] . This percentage is probably lower than the real percentage of HIV-positive healthcare workers with ongoing infection because, as we have said, many operators who are positive on the rapid test could be false positives. </ span>


Conclusions

Screening of high-risk asymptomatic population groups has two objectives:

1) identify and isolate cases with ongoing infection

and in perspective:

2)  readmitting cases with previous and immune infections to work and social activities, without restrictions and without danger to them and to others

Thanks to the screening, 128 health workers who were swab positive and therefore potentially contagious were identified. However, the possibility of identifying these cases with serological tests is limited by the fact that the appearance of antibodies in the blood occurs after the beginning of the viral shedding period: a part of healthcare workers with infection in progress and therefore potentially contagious at the time of the test have not been identified , as demonstrated by the case of health workers in the RSA of the South-East ASL where only 19 of the 42 positive swab operators were also positive in the rapid test. Also, to achieve this, the test should be repeated periodically.

On the other hand, 1,521 healthcare workers were identified as positive for rapid and negative for swab </ strong >. Unfortunately, however, we cannot be sure that all these health workers are really immune to SARS-CoV2 data i low positive predictive values ​​of tests with these prevalence levels. However, the presence of antibodies does not necessarily indicate that the person is protected against reinfection by SARS-Cov-2. A large proportion of healthcare professionals who have positive rapid test and negative swab test results may be false positive rapid test , and therefore may not have contracted the infection and don’t be immune . This problem can be limited by considering the positivity only for IgG, but with low seroprevalence levels the problem of the low positive predictive value remains.

By: Paolo Francesconi for the regional rapid tests working group

Gian Maria Rossolini, with coordinator functions
Maria Grazia Cusi; Mauro Pistello; Danilo Tacconi; Spartaco Sani; Alessandro Bartoloni; Luisa Galli; Fabio Voller; Paolo Francesconi, and with the collaboration of Francesca Carozzi and Marco Zappa.

Thanks to all the Health Authorities for their cooperation.


Covid 19 test, how it works, Igg Igm result.

【INSTRUCTIONS FOR USE】

Allow the test, sample, buffer and / or controls to reach room temperature (15-30 ° C ) before taking the test.

  1. Remove the test from the foil pouch and use it within one hour. The best results are obtained if the test is performed immediately after opening the foil package.
  2. Place the test cassette on a clean and flat surface.
    For serum or plasma samples:
    • Transfer 10 UL of sample to sample collection well (S) with a dropper or pipette, then add 2 drops of buffer (approximately 80 UL) and start the timer.
    For venipuncture whole blood or fingerstick whole blood samples:
    • Transfer 20 UL of sample to sample collection well (S) with a dropper or pipette, then add 2 drops of buffer (approximately 80 UL) and start the timer.
  3. Wait for the colored line (s) to appear. Read the result after 10 minutes. Do not interpret the result after 20 minutes.

【INTERPRETATION OF RESULTS】

IgG POSITIVE: * Two colored lines appear. One colored line should always appear in the control line area (C) while the other should appear in the IgG test line area.
IgM POSITIVE: * Two colored lines appear. One colored line should always appear in the control line area (C) while the other should appear in the IgM test line area.
IgG and IgM POSITIVE: * Three colored lines appear. One colored line should always appear in the control line area (C) while two lines should appear in the test line areas, IgG and IgM respectively.

* NOTE: The intensity of the color in the test line areas may vary depending on the antibody concentration of the 2019 virus -nCoV present in the analyzed sample. Therefore, any shade of color in the areas of the test lines is to be considered a positive result.
NEGATIVE: A colored line appears in the control line area (C). No lines appear in the areas of the test lines, respectively IgG and IgM.
INVALID: The control line (C) does not appear. Insufficient sample volume or incorrect procedural techniques are the most likely reasons for control line failure. Read the instructions again and repeat the procedure with a new test. If the result is still not valid, please contact your distributor.

【QUALITY CONTROL】

Internal procedural controls are included in the test. A colored line appearing in the control region (C) is an internal procedural control. Confirm sufficient sample volume and correct procedural technique. Check standards are not provided with this kit; however, it is recommended that positive and negative controls be tested as a good laboratory practice to confirm the test procedure and to verify test performance.

【PRINCIPLE】

The rapid cassette test 2019-nCoV IgG / IgM (Whole blood, serum or plasma) is a sumembrane immunoassay for the detection of IgG and IgM 2019-nCoV antibodies in human whole blood, serum or plasma samples. This test consists of two components, an IgG component and an IgM component. In the IgG component, anti-human IgG is coated in the area of ​​the IgG test line. During the test, the sample reacts with the 2019-nCoV antigen coated particles inside the test cassette. The mixture then migrates upward on the membrane chromatographically by capillary action and reacts with anti-human IgG in the IgG test line area, if the sample contains IgG antibodies to 2019-nCoV, a colored line will appear in the test line area IgG. Similarly, the anti-human IgM component coated in the IgM test line region, so if the sample contains 2019-nCoV IgM antibodies, the sample-conjugate complex reacts with the anti-human IgM, so a colored line will appear in the IgM test line area. Therefore, if the sample contains 2019-nCoV IgG antibodies, a colored line will appear in the IgG test line area, while if the sample contains 2019-nCoV IgM antibodies, a colored line will appear in the IgM test line area. If the specimen does not contain antibodies to the 2019-nCoV virus, no colored lines will appear in any of the areas of the respective test lines, indicating a negative result. With the internal procedural control feature, a colored line will always appear in the control line area, indicating that the correct volume of sample has been added and that the membrane has been soaked.

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