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Project Title: Novel Coronavirus (2019-nCoV) IgG/IgM Antibody Joint

Detection Kit (Colloidal Gold Method)


Manufacturer:Nanjing Vazyme Medical Technology Co., Ltd.


Project Recommendation of Novel Coronavirus (2019-nCoV) IgM/IgG Antibody Joint Detection Kit (Colloidal Gold Method)


I. Project Background

The Diagnosis and Treatment Protocol for Novel Coronavirus Pneumonia (Interim, Edition 7)for 2019novel coronavirus (SARS-CoV-2)points out that suspected cases have one of the following etiological or serological evidences:

1. Showing positive test result for the novel coronavirus nucleic acid by real-time fluorescence PCR;

2. Virus gene sequencing, highly homologous with known novel coronavirus;

3. Serum novel coronavirus-specific IgM and IgG antibodies are positive; serum novel coronavirus-specific IgG antibody changes from negative to positive, or its level is increased by 4 times or more in recovery period than that in acute period.

The antibody detection result is added to the criteria for confirmed diagnosis of cases, and serves as an important indicator for dynamic detection of disease diagnosis and treatment; it complements with the nucleic acid detection and the clinical manifestations to improve detection accuracy and prevent false negatives and false positives brought by a single methodology and a single indicator, providing a more comprehensive solution for epidemic prevention and control.


II. Project Introduction

1. Significance of IgM/IgG antibody detection

IgG and IgM antibodies are immune defense proteins produced after human body is infected with novel coronavirus. IgM antibody positive can be found in some patients when nucleic acid detection shows negative result, which is one of the effective methods for early diagnosis of novel coronavirus. IgG is an antibody that appears after 14 days of infection and persists thereafter, which can be used as an indicator of previous infection. A number of studies show [Table 2] that joint detection of IgM and IgG antibodies can make the diagnosis more accurate and assist clinicians to quickly identify suspected cases of novel coronavirus.


Table 2 Evaluation of GICA Kit to detect 78 serum specimens

Item

Reagent A

Reagent B

IgM

Total antibody

Total antibody

IgM

IgG

Sensitivity (%)

90.9(30/33)

78.8(26/33)

87.9(29/33)

75.8(25/33)

66.7(22/33)

Specificity (%)

73.3(33/45)

100.0(45/45)

88.9(40/45)

88.9(40/45)

100.0(40/40)

Positive predictive value (%)

71.4(30/42)

100.0(26/26)

85.3(29/34)

83.3(25/30)

100(22/22)

Negative predictive value (%)

91.7(33/36)

86.5(45/52)

90.9(40/44)

83.3(40/48)

80.4 (45/56)

Rate of missed diagnosis (%)

9.1(3/33)

21.2(7/33)

12.1(4/33)

24.2(8/33)

33.3(11/33)

Misdiagnosed rate (%)

26.7(12/45)

0.0(0/45)

11.1(5/45)

11.1(5/45)

0.0(0/40)

Youden's index

0.64

0.79

0.77

0.65

0.67

Note: Positive number/specimen number in brackets.

In this study, the IgM antibody, IgG antibody and total antibody of SARS-CoV-2 were detected respectively. The sensitivity of IgM antibody was generally higher than that of IgG antibody and total antibody. But at the same time, the misdiagnosed rate of IgM antibody also increased significantly, with lower positive predictive value (71.4% and 83.3%). The IgG detection had good specificity and positive predictive value, but its sensitivity was lower, which is not conducive to early diagnosis. As a method for joint detection of IgM and IgG, the total antibody had higher results in all evaluation indicators, with the maximum Youden's index. In the process of clinical use of GICA reagent, the total antibody detection reagent is recommended to ensure sensitivity and specificity, and reduce missed diagnosis and misdiagnosis caused by false positive and false negative.


2. Detection advantages

1. Single card for two detection items; one card simultaneously obtains two results of IgM/IgG. The detection of IgM and IgG antibodies can not only indicate recent infection, but also indicate previous infection, which can effectively judge the onset duration and efficacy test of patients;


Test category

Advantage

Disadvantage

Single detection of
IgG and IgM

1. Indicate the infection progress respectively.
2. IgG is conductive to epidemiological investigation.

1. Sample addition test, respectively.
2.There is non-specific interference, which needs to be combined with comprehensive clinical judgment.

IgG and IgM
joint detection

1. Indicate the infection progress jointly.
2.IgG is conductive to epidemiological investigation.
3. Higher sensitivity.
4. Recommended for treatment regimen.
5. Single detection, ease of operation

1.Indirect evidence, with blood non-specific interference, which needs to be combined with comprehensive clinical judgment.

Total antibody

1. Higher sensitivity.
2. Single detection, ease of operation

1. Cannot distinguish IgG and IgM.

2. Cannot indicate the infection progress.
3. Not recommended for confirmed diagnosis.

2. The joint detection of IgM and IgG antibodies is recommended to ensure sensitivity and specificity, and reduce missed diagnosis and misdiagnosis causedby false positive and false negative;

3. The serological test is completed within 10 min through visual observation, no need for equipment; low sampling requirements, no need for special virus sampling tubes, low laboratory requirements and low biosafety risks, which can be carried out in hospitals at all levels;

4. Throat swab sample detection is easy to cause false negative due to sample contamination during sampling or operation, which can be avoided in serological detection, and joint detection of the two methods can improve the detection rate of patients.


3. Operation procedures

4. Result interpretation

I. Nucleic acid positive (+), antibody negative/positive (±) results

(1) Nucleic acid (+), IgM/IgG (-)

The patient may be in the "window period" of COVID-19 infection, which is generally 2 weeks starting from the time when the body is infected with the virus to the time when the virus antibody can be detected in the peripheral blood. During this period, the virus antibody in blood cannot be detected; therefore, IgM and IgG are both negative. This is the early stage of infection, during which the virus continuously replicates, the nucleic acid load increases exponentially, reaching the lower limit of detection of nucleic acid, and the nucleic acid detection result is positive. The advantages of nucleic acid detection over serum antibody detection lie in direct detection of pathogens, shortening the detection window period of infection, and enabling early detection of infected persons.

(2) Nucleic acid (+), IgM (+), IgG (-)

The patient may be in the early stage of COVID-19 infection, and first develops IgM antibody as the immune response of the body, but IgG or IgG content does not reach the lower limit of detection of diagnostic reagent yet.

(3) Nucleic acid (+), IgM (-), IgG (+)

The patient may be in the mid-late stage of or recurrent COVID-19 infection. When the virus first invades the human body, the immune system will first produce antibody IgM, which will reach its peak value about 1 month later. As time goes on, the virus in the human body is gradually neutralized by IgM, and IgM gradually decreases until it is below the lower limit of detection. At the same time, the immune system will produce persistent IgG antibody. In the mid-late stage of infection, IgG is the main force of body immunity, with high concentration, and can be detected. Recurrent infection can be diagnosed when the level of IgG antibody in recovery period increases 4 times or more than that in acute period.

(4) Nucleic acid (+), IgM (+), IgG (+)

The patient is in an active stage of infection, but the body has developed certain immunity to COVID-19.

II. Nucleic acid positive (-), antibody negative/positive (±) results

(1) Nucleic acid (-), IgM (-), IgG (-)

For patient having epidemiological history and relevant clinical manifestations, quarantine and dynamic monitoring are recommended to avoid false negativescaused by improper sampling methods and sample preservation;

Forpatienthavingnoepidemiologicalhistoryandrelevantclinical manifestations, release from quarantine is recommended;

(2) Nucleic acid (-), IgM (+)/weak positive, IgG (-)

Patients with positive IgM test results may be in acute stage of COVID-19 infection, and the possibility of false negative for nucleic acid should be considered in combination with clinical manifestations. It is suggested to amplify multiple nucleic acid regions to effectively avoid the influence of nucleic acid variation on detection results. Therefore, when the nucleic acid detection result is negative, only the negative result of this detection can be reported, the possibility of COVID-19 infection cannot be excluded, and repeated validation is required.

For the patients with weak positive IgM test results, on the one hand, he/she may be initially infected with COVID-19 at extremely low viral load and be at an early stage of infection, the viral load is lower than the lower limit of detection of nucleic acid. The body produces a small amount of IgM antibody, but has not yet produced IgG antibody. On the other hand, IgM false positive may be caused by interference of rheumatoid factor (RF), heterophilic antibody, hemolysis and jaundice, and some cross-reactive substances.

(3) Nucleic acid (-), IgM (-), IgG (+)

The patient may have been infected with COVID-19 in the past, but has recovered or the virus in the body has been cleared. IgG continues to be produced by immune response to provide long-lasting immunity, and therefore still detectable in blood.

(4) Nucleic acid (-), IgM (+), IgG (+)

The patient has recently been infected with COVID-19 and is in recovery period, the virus in the body has been cleared, and IgM is not decreased to the lower limit of detection; or the nucleic acid detection result is false negative, and the patient is in an active period of infection.

III. Benefit Analysis

1. Social benefits

1) The novel coronavirus IgG/IgM antibody joint detection reagent can rapidly screen infected patients without risks and improve the detection capability of hospitals.

2) Early discovery and isolation for patients with suspected novel coronavirus infection can prevent further infection of others, and reduce the harm caused by the epidemic; early hospitalization and treatment can save more patients' lives.




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