Kantaro COVID-SeroKlir Kit: Precise Measurement of COVID-19 IgG Antibodies

The Kantaro COVID-SeroKlir kit available from EKF Diagnostics is a direct ELISA designed for the quantitative detection of human IgG antibodies to the SARS-CoV-2 virus in samples of serum and plasma (K2-EDTA/Li-Heparin). The COVID-SeroKlir obtained FDA Emergency Use Authorization (EUA) in November 2020 and was earlier awarded a CE mark in October 2020.

COVID-SeroKlir has shown a sensitivity of 97.8% and a specificity of 99.6% to detect SARS-CoV-2 specific IgG antibodies against two virus antigens—the full-length spike protein as well as its receptor-binding domain. The COVID-SeroKlir kit is a two-step enzyme-linked immunosorbent assay (ELISA) that can be used by any CLIA-certified lab without requiring proprietary equipment.

The SeroKlir ELISA kit includes components for testing 630 patient samples by using standard ELISA protocols and equipment. The test includes a two-phase ELISA—the initial plate is used to screen for RBD positive or negative samples, while the second plate offers a quantitative result of the antibody titer (concentration) with full-length Spike protein.

The SeroKlir ELISA kit was created by clinicians from the Icahn School of Medicine at Mount Sinai Health System in New York.1 Trial for the test have been performed on a cohort of over 70,000 patients, including more than 30,000 who were diagnosed with COVID-19.2

Key features

  • A sensitivity of 97.8% and a specificity of 99.6%
  • Peer-reviewed in the Science and Nature journals
  • Reduces false negatives and false positives
  • Includes 30,000 COVID-19 patient test data set
  • Has been awarded FDA EUA and CE mark

Kit specifications

Table 1. Source: EKF Diagnostics

Kit component Specification
7 x RBD plates 96 well polystyrene microplate coated with recombinant SARS-CoV-2 Spike protein RBD antigen sufficient for 630 screening tests
3 x spike plates 96 well polystyrene microplate coated with full length recombinant SARS-CoV-2 Spike protein sufficient for 228 quantitative tests
Controls RBD Positive Control
RBD Negative Control
Spike Low Control
Spike Mid Control
Spike High Control
Calibrators 8 calibrators (range 0-200 AU/mL)
Consumables RBD conjugate concentrate - IgG ELISA
Spike conjugate concentrate - IgG ELISA
Conjugate buffer - IgG ELISA
Sample buffer - IgG ELISA
TMB substrate - IgG ELISA
Stop solution - IgG ELISA
Wash buffer - IgG ELISA


Product Code: CVD2-2019

Performance data

Table 2. Source: EKF Diagnostics

  Performance specifications
Assay type Solid phase sandwich ELISA
Format 96 well one-piece plate
Assay length 3.5 hours (RBD ELISA)
3.5 hours (spike ELISA)
Sample types Serum (20 uL)
EDTA plasma (20 uL)
Heparin plasma (20 uL)
Assay range 3.2 – 160 AU/mL
Specificity 99.6%
Sensitivity 97.8%



Two phase ELISA investigates the full length Spike protein as well as its Receptor Binding Domain (RBD):

  • RBD is used as the first phase to detect antibody negative samples
  • In the second phase, full-length Spike protein is used to confirm positive samples and obtain a quantitative antibody titer
Kantaro COVID-SeroKlir Kit: Precise Measurement of COVID-19 IgG Antibodies

Image Credit: EKF Diagnostics

Analytical sensitivity

Analytical sensitivity: The recommendations outlined in CLSI guideline EP17-A2 were used to establish Limit of Blank (LoB), Limit of Detection (LoD) and Limit of Quantitation (LoQ). Spike ELISA and RBD ELISA summary data is presented opposite.

Positive Percent Agreement: PPA for the positive samples confirmed using a known EUA-authorized molecular test was found to be 97.8%. Two samples that tested negative on an existing EUA-approved serology test also tested negative while using the COVID-SeroKlir Kantaro Quantitative SARS-CoV-2 IgG Antibody Kit. This suggests that these were true negative samples.

Negative Percent Agreement: The NPA for the negative samples was 99.6%. A total of 14 samples tested positive on the RBD ELISA. Of these, 13 samples further tested negative on the Spike ELISA. Thus, the number of negative samples out of 282 was 281.

Table 3. Source: EKF Diagnostics

Sensitivity RBD ELISA
0.70 0.82 ---
1.98 2.61 3.20


Video Credit: EKF Diagnostics

Clinical utility

An initial qualitative (screening) ELISA is carried out against the recombinant RBD of the SARS-CoV-2 virus. A quantitative ELISA performed against the full-length SARS-CoV-2 Spike protein helps analyze positive samples from this screen.

The assay helps determine the quantitative levels of neutralizing antibodies that suggest an adaptive immune response to SARS-CoV-2 in patients presumed to have acquired previous SARS-CoV-2 infection, or to detect IgG seroconversion in patients after known recent SARS-CoV-2 infection.

Establishing the number of individuals shown to have developed specific antibodies to SARS-CoV-2 helps determine seroprevalence in any geographic region or clan of exposed individuals and may be suggestive of the prospective risk of reinfection. The assay results correlate with the in vitro neutralization of the SARS-CoV-2 virus.

It is not advisable to use the COVID-SeroKlir Kantaro Quantitative SARS-CoV-2 IgG Antibody Kit results as the sole basis for diagnosis and the diagnosis of patients with severe COVID-19 infection.

The results are intended to detect the presence of SARS-CoV-2 IgG antibodies. In general, IgG antibodies to SARS-CoV-2 can be detected 10–14 days following infection but may be found even later. The existence of IgG antibodies, after previously negative testing, denotes IgG antibody seroconversion after infection with the SARS-CoV-2 virus.

Negative results do not rule out severe SARS-CoV-2 infection and should never be used as the sole basis for decisions related to patient management. IgG antibodies may not exist for over two weeks after infection and patients might remain infectious during severe infection even if IgG antibody exists.

It is advisable to combine the results with patient history, clinical observations and epidemiological information. The COVID-SeroKlir Kantaro Quantitative SARS-CoV-2 IgG Antibody Kit’s sensitivity early following infection remains unknown.


  1. Amanat et al. Nature Medicine 26 2033-1036 (2020).
  2. Wajnberg et al. Science 10.1126/science.abd7728 (2020).