May08, Unitedkingdom  2021 

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Abstract Volume: 2 Issue: 1 ISSN:

Serological Tests in Screening COVID-19

Attapon Cheepsattayakorn*, Ruangrong Cheepsattayakorn1,Porntep Siriwanarangsun2

1.Department of Pathology, Faculty of Medicine, Chiang Mai University, Chiang Mai, Thailand.

2. Faculty of Medicine, Western University, Pathumtani Province, Thailand

*Corresponding Author: Dr. Attapon Cheepsattayakorn, 10th Zonal Tuberculosis and Chest Disease Center, 143 Sridornchai Road Changklan Muang Chiang Mai 50100 Thailand.

Received Date:  December 24, 2020

Publication Date:  January 01, 2021

Characteristically, after infection, antibodies are detected in the blood of individuals, particularly individuals with few or mild symptoms.  In patients with varying symptoms of COVID-19 and negative results of reverse-transcriptase-polymerase-chain reaction (RT-PCR) tests, the testing has a significant clinical role when nasopharyngeal swabs are taken more than 5 days after symptom onset. The Royal College of Pathologists (RCPath) developed seven principles for the production of a COVID-19  testing strategy. Testing is carried out for  a the purpose is one of these  RCPath’s principles.  Nevertheless, denial of requesting  SARS-CoV-2 (COVID-19)  antibody tests for reassurance should be cautioned. With a lower antibody level, whether the protective immunity will be sustained is questionable.  Several  immune-based assays  were  developed against  different  SARS-CoV-2 (COVID-19)  viral proteins  as  the followings: 1) Entire  Spike (S)  protein, IgG  antibody from  patient  serum can  cross-react  with SARS-CoV  and  MERS-CoV, 2) S1  subunit of  Spike (S)  protein, IgA, IgG  antibodies from  patient  serum can  cross-react  with SARS-CoV  only,3) Receptor-binding  domain (RBD), IgG  antibody from  patient  serum can  cross-react  with SARS-CoV  only, and  4) Nucleocapsid (N), IgG antibody from patient serum can cross-react with  SARS-CoV only. Long  et  al  demonstrated in  their  study that  IgG  antibody and  neutralizing  antibody levels  initiate  decreasing within  2-3  months after  infection  in  the  majority of  persons  with recovery from  SARS-CoV-2 (COVID-19)  infection. Nevertheless, an analytical  study of  the  dynamics of  neutralizing  antibody titers  demonstrated  reduced neutralizing antibodies  around 6-7  weeks after illness onset.  In conclusion, serological  data  greatly supplement  the  laboratory results  from  the quantitative  reverse-transcriptase polymerase-chain  reaction (qRTPCR), the  design of  virus  elimination programs (seroepidemiology), the discovery of  the  monoclonal antibodies, and  development  of SARS-CoV-2 (COVID-19) vaccines.   

 Keywords: COVID-19, SARS-CoV-2, test, serological.

Serological Tests in Screening COVID-19


COVID-19: Coronavirus Disease 2019,

IgA: Immunoglobulin A,

IgG: Immunoglobulin G,

IgM: Immunoglobulin M,

MERS-CoV: Middle-East-Respiratory-Syndrome Coronavirus,

N: Nucleocapsid,

qRT-PCR: quantitative Reverse-Transcriptase-Polymerase-Chain Reaction,

RBD: Receptor-Binding Domain, RNA: Ribonucleic Acid,

RT-PCR: Reverse-Transcriptase-Polymerase-Chain Reaction,

S: Spike protein,

SARS-CoV-2: Severe Acute Respiratory Syndrome Coronavirus type 2 


Characteristically, after infection, antibodies  are  detected in  the  blood of  individuals, particularly  individuals with  few  or  mild  symptoms. In  patients  with varying  symptoms  of COVID-19  and  negative results  of  reverse-transcriptase-polymerase-chain  reaction (RT-PCR)  tests, the testing  has  a significantly  clinical  role when  nasopharyngeal  swabs are  taken  more than  5  days after  symptom  onset (1, 2). The  Availability  and Accuracy  of  Antibody Tests Immunoglobulin  M (IgM) rises  soonest, whereas  IgA and  IgG  persist. IgG  alone.  The maximum  sensitivity  for IgM  alone, IgA  alone, and IgG  alone  appear during  the  days 15-21  after  the symptom onset  that  are 75.4 % (64.3-83.8), 98.7 % (39.0-100), and  88.2 % (83.5-91.8), respectively (3), whereas  the specificity  at  all times  for  IgM alone  and  IgG alone  are  98.7 % (97.4-99.3)  and 99.1 % (98.3-99.6), respectively (3) .  The  sensitivity and  specificity  of the  antibody  tests are  critical  due to  false  negative rates  of  RT-PCR that  are  between 2 %  and  29 % (3).  A previous  study  on immunological  assessment  of SARS-CoV-2 (COVID-19) infections  in  China revealed  that  81.1 % (30/37)  and 62.2 % (23/37)  of asymptomatic individuals  tested  positive for  IgG  and IgM, respectively  and  83.8 % (31/37)  and 78.4 % (29/37)  of  the symptomatic  patients  tested positive  for  IgG (around 3-4  weeks  after COVID-19  exposure)  and IgM, respectively (4).  In  acute phase  that  the viral  ribonucleic  acid (RNA) can  be  identified in  a  respiratory sample, IgG  levels  in symptomatic  patients  were significantly statistical  higher than  those  in the  asymptomatic  individuals (4).  

Serological  Test  Interpretation     

The pre-test probability of infection has much influence on the interpretation of the serological test results not only influenced by the accuracy  of  the test itself.  When screening suggestive symptomatic individuals, the  pre-test  probability will  be much higher, compared to  asymptomatic persons (5)


Serological  Testing  Pitfalls

COVID-19  screening is  essentially  amounted by  non-specific  indication and  population-based  policies on  testing.  Inconsequences of  testing with  uncareful  consideration, this  risks the  potential  harm. In  more affluent  populations, the  rates of  testing  will be  higher (6) that  limits the  estimates  of seroprevalence.  The  Royal College  of  Pathologists (RCPath)  developed seven  principles  for the production  of  a COVID-19  testing  strategy. Testing  being  carried out  for  a purpose  is  one of  these  RCPath’s principles (7).  Nevertheless,  denial of  requesting  SARS-CoV-2 (COVID-19)  antibody tests  for  reassurance should  be  cautioned (8, 9).   


Immunity  and  Antibody Tests 

In eliminating COVID-19, a combination  of  B and  T cell immunity  is  likely to  involve  for the production  of  protective-immunity  memory (5). Nevertheless, currently, several longitudinal  studies  demonstrated waning  of  antibody levels (10). With a lower  antibody level, whether  the  protective immunity  will  be sustained  is  questionable (5). A recent  study  revealed that  produced  antibodies can  provide  long-term immunity, whereas non-neutralizing  antibodies  can be  generated.  Antibody enhancement, a  phenomenon  that can  facilitate  a more  severe-secondary  infection (11).  This

phenomenon  is  not to  date  with SARS-CoV-2 (COVID-19), but  it  has been  demonstrated  in other  coronaviruses (11).  Immune-based  Assays  developed against  different  SARS-CoV-2 (COVID-19)  Viral Proteins Several  immune-based assays  were  developed against  different  SARS-CoV-2 (COVID-19)  viral proteins  as  the followings : 1) Entire  Spike (S)  protein, IgG  antibody from  patient  serum can  cross-react  with SARS-CoV  and  MERS-CoV (12), 2) S1  subunit of  Spike (S)  protein, IgA, IgG antibodies from  patient  serum can  cross-react  with SARS-CoV  only (12), 3)Receptor-binding  domain (RBD), IgG  antibody from  patient  serum can  cross-react  with SARS-CoV  only (12), and  4) Nucleocapsid (N),IgG antibody  from  patient serum  can  cross-react with  SARS-CoV  only (12)


IgG  antibody responses  sustained  for at  least  34 months  after outbreak in  persons  with laboratory-confirmed MERS-CoV infection (13), whereas  IgG  levels in  SARS-CoV-infected  individuals were  sustained  for more  than  two years (14, 15).  Neutralizing  antibodies that  associate  with the  numbers  of virus-specific  T  cells have  been detected in most  COVID-19 convalescent patients (16-19).  Long  et al  demonstrated  in their  study  that IgG  antibody  and neutralizing  antibody  levels initiate  decreasing  within 2-3  months  after infection  in  the majority  of  persons with  recovery  from SARS-CoV-2 (COVID-19)  infection (4).  Nevertheless, an  analytical study  of  the dynamics  of  neutralizing antibody  titers  demonstrated reduced  neutralizing  antibodies around  6-7  weeks after  illness  onset (20).


Serological data greatly supplement  the  laboratory results  from  the quantitative reverse-transcriptase-polymerase-chain reaction (qRT-PCR), the design  of  virus elimination  programs (seroepidemiology), the discovery  of  the monoclonal  antibodies, and  development of  SARS-CoV-2 (COVID-19)  vaccines.



1.Watson  J, Whiting  PF, Brush  JE.  “Interpreting  a  COVID-19  test  result”.  BMJ  2020; 369 : m1808.

2.Kucirka  LM, Lauer  SA, Laeyendecker  O, Boon  D, Lessler  J.  “Variation  in  false-negative  rate  of  reverse  transcriptase  polymerase  chain  reaction-based  SARS-CoV-2  tests  by  time  since  exposure”.  Ann  Intern  Med  2020; 173 : 262-267.  

3.Arevalo-Rodriguez  I, Buitrago-Garcia  D, Simancas-Racines  D, et  al.  “False-negative  results  of  initial  RT-PCR  assays  for  COVID-19 :a  systematic  review”.  MedRxiv (Preprint)  2020.  DOI : 10.1101/2020.04.16.20066787%.

4.Long  Q-X, Tang  X-J, Shi  Q-L, Li  Q, Deng  H-J, Yuan  J, et  al.  “Clinical  and  immunological  assessment  of  asymptomatic  SARS-CoV-2  infections”.  Nature  Medicine  2020; 26 : 1200-1204.    

5.Watson  J, Richter  A, Deeks  J.  “Testing  for  SARS-CoV-2  antibodies”.  BMJ  2020; 370 : m3325.  DOI : 10.1136/bmj.m3325  

6.Hart  JT.  “The  inverse  care  law”.  Lancet  1971; 1 : 405-412.  

7.Martin  J.  COVID-19  testing : a  national  strategy.  Royal  College  of  Pathologists  2020.  Available  at :  (accessed  on  December  5, 2020).    

8.Petrie  KJ, Sherriff  R.  “Normal  diagnostic  test  results  do  not  reassure  patients”.  Evid  Based  Med  2014; 19 : 14.

9.Rolfe  A, Burton  C.  “Reassurance  after  diagnostic  testing  with  a  low  pretest  probability  of  serious  disease : systematic  review  and  meta-analysis”.  JAMA  Intern  Med  2013; 173 : 407-416.     

10.Ibarrondo  FJ, Fulcher  JA, Goodman-Meza  D, et  al.  “Rapid  decay  of  anti-SARS-CoV-2  antibodies  in  persons  with  mild  COVID-19”.  N  Engl  J  Med  2020.  DOI : 10.1056/NEJMc2025179  

11.Fierz  W, Walz  B.  “Antibody  dependent  enhancement  due  to  original  antigenic  sin  and  the  development  of  SARS”.  Front  Immunol  2020; 11 : 1120.  

12.Okba  NMA, Muller  MA, Li  W, Wang  C, GeurtsvanKessel  CH, Corman  VM, et  al.  “SARS-CoV-2  specific  antibody  responses  in  COVID-19  patients”.  Emerg  Infect  Dis  2020; 26.  DOI : 10.1101/2020.03.18.20038059

13.Payne  DC, et  al.  “Persistence  of  antibodies  against  middle-east-  -respiratory-syndrome  coronavirus”.  Emerg  Infect  Dis  2016; 22 : 1824-1826.    

14.Guo  X, et  al.  “Long-term  persistence  of  IgG  antibodies  in  SARS-CoV  infected  healthcare  workers”.  Preprint  available  at  : (2020) 

15.Wu  LP, et  al.  “Duration  of  antibody  responses  after  severe  acute  respiratory  syndrome”.  Emerg  Infect  Dis  2007; 13 : 1562-1564.  

16.Ni  L, et  al.  “Detection  of  SARS-CoV-2-specific  humoral  and  cellular  immunity  in  COVID-19  convalescent  individuals”.  Immunity  2020.  DOI : 

17.Thevarajan  I, et  al.  “Breadth  of  concomitant  immune  response  prior  to  patient  recovery : a  case  report  of  non-severe  COVID-19”.  Nature  Medicine  2020; 26 : 453-455.  

18.Wu  F, et  al.  “Neutralizing  antibody  response  to  SARS-CoV-2  in  a  COVID-19  recovered  patient  cohort  and  their  implications”.  Preprint  available  at :  (2020). 

19.Suthar  MS, et  al.  “Rapid  generation  of  neutralizing  antibody  responses  in  COVID-19  patients”.  Preprint  available  at : (2020). 

20.Wang  X, et  al.  “Neutralizing  antibodies  responses  to  SARS-CoV-2  in  COVID-19  patients  and  convalescent  patients”.  Preprint  available  at : (2020). 


Volume 2 Issue 1 January 2021

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