The NT-proBNP immunoassay is a 3.5 hour, 96-well sandwich enzyme-linked immunosorbent assay (ELISA) for the quantitative determination of NT-proBNP in human serum and EDTA plasma.
Principle of the Assay
This kit is a sandwich enzyme immunoassay for the quantitative determination of NT-proBNP in human serum and EDTA plasma.
The figure below explains the principle of a sandwich ELISA:
In a first step, sample and conjugate (sheep anti-human NT-proBNP-HRP) are pipetted into the wells of the microtiter strips, which are pre-coated with polyclonal sheep anti-NT-proBNP antibody. NT-proBNP present in the sample binds to the pre-coated antibody in the well and forms a sandwich with the conjugate (detection antibody). In the washing step all non-specific unbound material is removed. In a second step, the substrate (Tetramethylbenzidine; TMB) is pipetted into the wells. The enzyme catalyzed color change of the substrate is directly proportional to the amount of NT-proBNP present in the sample. This color change is detectable with a standard microtiter plate ELISA reader.
The NT-proBNP ELISA kit uses highly purified, epitope-mapped antibodies. The antibodies utilized in the NT-proBNP ELISA (BI-20892) are as follows:
Capture antibody: AA 32-57 (polyclonal sheep anti-human NT-proBNP) Detection antibody: AA 8-29 (HRP-labeled polyclonal sheep anti-human NT-proBNP)
Typical Standard Curve
The figure below shows a typical standard curve for the human NT-proBNP ELISA. The immunoassay is calibrated against recombinant NT-proBNP (1-76) peptide:
ELISA Kit Components
Contents
Description
Quantity
PLATE
Polyclonal sheep anti-human NT-proBNP antibody pre-coated microtiter strips in a strip holder, packed in an aluminum bag with desiccant
12 x 8 tests
WASHBUF
Wash buffer concentrate 20x, clear cap
1 x 50 ml
STD
Standards 1-5, recombinant human NT-proBNP (0; 10; 40; 160; 640 pmol/l), white caps, lyophilized
5 vials
CTRL
Controls, yellow caps, lyophilized, exact concentrations see labels
2 vials
CONJ
Conjugate (polyclonal sheep anti-human NT-proBNP-HRP), red dye, brown cap, ready to use
1 x 22 ml
SUB
Substrate (TMB solution), amber bottle, blue cap, ready to use
1 x 22 ml
STOP
Stop solution, white cap, ready to use
1 x 7 ml
Storage instructions: All reagents of the human NT-proBNP ELISA kit are stable at 4°C (2-8 °C) until the expiry date stated on the label of each reagent.
Serum and EDTA plasma are suitable for use in this assay. Do not change sample type during studies. We recommend duplicate measurements for all samples, standards and controls. The sample collection and storage conditions listed are intended as general guidelines.
Serum & Plasma
Collect venous blood samples in standardized serum separator tubes (SST) or standardized blood collection tubes using EDTA as an anticoagulant. For serum samples, allow samples to clot for 30 minutes at room temperature. Perform separation by centrifugation according to the tube manufacturer’s instructions for use. NT-proBNP is stable in whole blood at room temperature (18-26°C) for several hours. Nevertheless, we recommend separating serum or EDTA plasma by centrifugation as soon as possible. Assay the acquired samples immediately or aliquot and store at -25°C or lower. Lipemic or hemolyzed samples may give erroneous results. Do not freeze-thaw samples more than three times.
Reagent Preparation
Wash Buffer
1.
Bring the WASHBUF concentrate to room temperature. Crystals in the buffer concentrate will dissolve at room temperature.
2.
Dilute the WASHBUF concentrate 1:20, e.g. 50 ml WASHBUF + 950 ml distilled or deionized water. Only use diluted WASHBUF when performing the assay.
The diluted WASHBUF is stable up to one month at 4°C (2-8°C).
Standards
1.
Pipette 500 µl of distilled or deionized water into each standard (STD) and control (CTRL) vial. The standard and control concentration are printed on the label of each vial.
2.
Leave at room temperature (18-26°C) for 10 min. Swirl gently.
Reconstituted STDs and CTRLs are stable at -25°C or lower until expiry date stated on the label. Avoid freeze-thaw cycles.
Sample Preparation
Bring samples to room temperature and mix samples gently to ensure the samples are homogenous. We recommend dublicate mesurements for all samples. Samples for which the optical density (OD) value exceeds the highest point of the standard range (STD5, 640 pmol/l) can be diluted with STD1 or NT-proBNP negative human serum.
Assay Protocol
Read the entire protocol before beginning the assay.
1.
Bring reagents and samples to room temperature (18-26°C).
2.
Mark position for STD/SAMPLE/CTRL (Standard/Sample/Control) on the protocol sheet.
3.
Take microtiter strips out of the aluminum bag. Store unused strips with desiccant at 4°C in the aluminum bag. Strips are stable until the expiry date stated on the label.
4.
Add 50 µl STD/CTRL/SAMPLE in dublicates into the respective wells.
5.
Add 200 µl CONJ (Conjugate) into each well. Swirl gently.
6.
Cover the plate tightly and incubate for 3 hours at room temperature.
7.
Aspirate and wash wells 5 x with 300 µl diluted WASHBUF. After the final wash, remove remaining WASHBUF by strongly tapping plate against a paper towel.
8.
Add 200 µl SUB (Substrate) into each well.
9.
Incubate for 30 min at room temperature in the dark.
10.
Add 50 µl STOP (Stop solution) into each well, swirl gently
11.
Measure absorbance immediately at 450 nm with reference 630 nm, if available.
Calculation of Results
Read the optical density (OD) of all wells on a plate reader using 450 nm wavelength (reference wavelength 630 nm). Construct a standard curve from the absorbance read-outs of the standards using commercially available software capable of generating a four-parameter logistic (4-PL) fit. Alternatively, plot the standards’ concentration on the x-axis against the mean absorbance for each standard on the y-axis and draw a best fit curve through the points on the graph. Curve fitting algorithms other than 4-PL have not been validated and will need to be evaluated by the user. Obtain sample concentrations from the standard curve. If required, pmol/l can be converted into pg/ml by applying a conversion factor (1 pg/ml = 0.118 pmol/l (MW: 8.475 kDa)). Respective dilution factors have to be considered when calculating the final concentration of the sample.
The quality control (QC) protocol supplied with the kit shows the results of the final release QC for each kit at production date. Data for OD obtained by customers may differ due to various influences and/or due to the normal decrease of signal intensity during shelf life. However, this does not affect validity of results as long as an OD of 1.50 or more is obtained for STD5 and the values of the CTRL are in range (target ranges see labels).
Background & Therapeutic Areas
INFORMATION ON THE ANALYTE
NT-proBNP Protein
BNP is mainly expressed by ventricular myocardium in response to volume overload and increased filling pressure. The BNP gene encodes a 134-amino acid preproBNP precursor, which is converted to 108-amino acid proBNP by the proteolytic cleavage of a 26-amino acid signal peptide (26). ProBNP is further cleaved into the the N-terminal fragment of proBNP (NTproBNP, 1-76) and the C-terminal region biologically active BNP hormone (77-108) (30) also named BNP-32 and additional C-terminal fragments (cf. http://www.uniprot.org/uniprot/P16860#PRO_0000001532; (Kambayashi et al., 1990; Nakagawa et al., 2019; Sudoh et al., 1989).
BNP has a key role in cardiovascular homeostasis with biological actions including natriuresis, diuresis, vasorelaxation, and inhibition of renin and aldosterone secretion. A high concentration of BNP in the bloodstream is indicative of heart failure.
Heart and Cardiovascular Disease
Acute myocardial infarction
Cardiac impairment
Risk assessment of rheumatoid arthritis patients under nonsteroidal anti-inflammatory agents/analgesics (NSAIAs)
All Biomedica ELISAs are validated according to international FDA/ICH/EMEA guidelines. For more information about our validation guidelines, please refer to our quality page and published validation guidelines and literature.
Validation literature
1. ICH Q2(R1) Validation of Analytical Procedures: Text and Methodology. 2. EMEA/CHMP/EWP/192217/2009 Guideline on bioanalytical method validation. 3. Bioanalytical Method Validation, Guidance for Industry, FDA, May 2018
Calibration
This immunoassay is calibrated against recombinant human NT-proBNP (1-76) peptide.
Detection Limit & Sensitivity
To determine the sensitivity of the NT-proBNP ELISA, experiments measuring the lower limit of detection (LOD) and the lower limit of quantification (LLOQ) were conducted.
The LOD, also called the detection limit, is the lowest point at which a signal can be distinguished above the background signal, i.e. the signal that is measured in NT-proBNP free sample (three independent measurements) plus three times the standard deviation of the measurements.
The LLOQ, or sensitivity of an assay, is the lowest concentration at which an analyte can be accurately quantified. The criteria for accurate quantification at the LLOQ are an analyte recovery between 75 and 125% and a coefficient of variation (CV) of less than 25%. To determine the LLOQ, standard 2, i.e. the lowest standards containing NT-proBNP, is diluted, measured three times and its concentration back calculated. The lowest dilution, which meets both criteria, is reported as the LLOQ.
The following values were determined for the NT-proBNP human ELISA:
LOD
3.0 pmol/l
LLOQ
3.3 pmol/l
Precision
The precision of an ELISA is defined as its ability to measure the same concentration consistently within the same experiments carried out by one operator (within-run precision or repeatability) and across several experiments using the same samples but conducted by several operators at different locations using different ELISA lots (in-between-run precision or reproducibility).
Within-Run Precision
Two samples of known concentration were tested three times to assess within-run precision, also called intra-assay precision. The experiment was performed by one operator using a single plate.
ID
Within-Run Precision
n
Mean NT-proBNP [pmol/l]
SD [pmol/l]
CV [%]
Sample 1
3
60.2
2.0
4
Sample 2
3
35.5
0.9
3
In-Between-Run Precision
Two samples of known concentration were tested eight times to assess in-between-run precision, also called inter-assay precision. The experiment was performed by multiple operators using plates and kit components from two lots.
ID
In-Between Run Precision
n
Mean NT-proBNP [pmol/l]
SD [pmol/l]
CV (%)
Sample 1
8
52.1
1.7
3
Sample 2
8
108.1
7.9
7
Accuracy
The accuracy of an ELISA is defined as the precision with which it can recover samples of known concentrations.
The recovery of the NT-proBNP ELISA was measured by adding recombinant NT-proBNP to human samples containing a known concentration endogenous NT-proBNP. The % recovery of the spiked concentration was calculated as the percentage of measured compared over the expected value.
This table shows the summary of the recovery experiments in the NT-proBNP human ELISA in different sample matrices:
% Recovery
Sample Matrix
n
+ 80 pmol/l
+ 320 pmol/l
Mean
Range
Mean
Range
Serum
4
99
93 - 109
108
105 - 113
EDTA plasma
4
94
91 – 97
93
87 - 101
Data showing recovery of NT-proBNP in human serum samples:
NT-proBNP [pmol/l]
% Recovery
Sample Matrix
ID
Reference
+ 80 pmol/l
+ 320 pmol/l
+ 80 pmol/l
+ 320 pmol/l
Serum
s1
1.6
88.5
347.2
109
108
Serum
s2
0.1
74.2
360.2
93
113
Serum
s3
1.7
80.9
338.7
99
105
Serum
s4
4.4
81.6
346.5
96
107
Mean
99
108
Min
93
105
Max
109
113
Data showing recovery of NT-proBNP in human EDTA plasma samples:
NT-proBNP [pmol/l]
% Recovery
Sample Matrix
ID
Reference
+ 80 pmol/l
+ 320 pmol/l
+ 80 pmol/l
+ 320 pmol/l
EDTA plasma
e1
0.8
73.4
276.8
92
87
EDTA plasma
e2
7.7
72.8
279.0
91
87
EDTA plasma
e3
9.3
77.2
324.1
97
101
EDTA plasma
e4
19.1
74.9
309.5
94
97
Mean
94
93
Min
91
87
Max
97
101
Dilution Linearity & Parallelism
Tests of dilution linearity and parallelism ensure that both endogenous and recombinant samples containing NT-proBNP behave in a dose dependent manner and are not affected by matrix effects. Dilution linearity assesses the accuracy of measurements in diluted human samples spiked with known concentrations of recombinant analyte. By contrast, parallelism refers to dilution linearity in human samples and provides evidence that the endogenous analyte behaves the same way as the recombinant one. Dilution linearity and parallelism are assessed for each sample type and are considered acceptable if the results are within 20% of the expected concentration.
The table below shows the mean recovery and range of serum and plasma samples spiked with recombinant human NT-proBNP and serially diluted with assay buffer:
% Recovery of recombinant NT-proBNPin diluted samples
Sample
Matrix
n
1+1
Mean
Range
Serum
3
117
112 - 119
EDTA plasma
2
84
83 - 84
Data showing dilution linearity of recombinant human NT-proBNP in human serum samples:
NT-proBNP [pmol/l]
Sample matrix
ID
Reference
1+1
% Recovery
Serum
s1
338.7
202.2
119
Serum
s2
360.2
202.4
112
Serum
s3
278.5
165.7
119
Mean
117
Min
112
Max
119
Data showing dilution linearity of recombinant NT-proBNP in human EDTA plasma samples:
NT-proBNP [pmol/l]
% Recovery
Sample matrix
ID
Reference
1+1
1+1
EDTA plasma
e1
347
145
84
EDTA plasma
e2
277
115
83
Parallelism was assessed by serially diluting human samples containing endogenous NT-proBNP with assay buffer.
The table below shows the mean recovery and range of serially diluted endogenous NT-proBNP in several human sample matrices:
% Recovery of endogenous NT-proBNPin diluted samples
Sample
Matrix
n
1+1
Mean
Range
Serum
3
82
79 - 86
EDTA plasma
2
84
83 - 85
Data showing paralellism of endogenous NT-proBNP in human serum samples:
NT-proBNP [pmol/l]
% Recovery
Sample matrix
ID
Reference
1+1
1+1
Serum
s1
44.2
17.4
79
Serum
s2
51.3
20.4
80
Serum
s3
61.7
26.5
86
Mean
82
Min
79
Max
86
Data showing paralellism of endogenous NT-proBNP in human EDTA plasma samples:
NT-proBNP [pmol/l]
% Recovery
Sample matrix
ID
Reference
1+1
1+1
EDTA plasma
e1
44.2
17.4
79
EDTA plasma
e2
51.3
20.4
80
Mean
84
Specificity
This assay recognizes endogenous (natural) and recombinant human NT-proBNP (1-76).
Sample Stability
The stability of endogenous NT-proBNP was tested by comparing NT-proBNP measurements in samples that had undergone 3 freeze-thaw cycles.
For freeze-thaw experiments, samples were collected according to the supplier's instruction using blood collection devices and stored at -80°C. Reference samples were freete-thawed once. The mean recovery of sample concentration after 3 freeze-thaw cycles is 94%.
NT-proBNP [pmol/l]
% Recovery
3 F/T vs ref
Sample ID
Serum
Reference
1x
2x
3x
#S1
70
67
67
69
98
#S2
15
13
15
14
98
#S3
215
185
191
188
87
#S4
141
129
129
126
90
#S5
49
47
48
47
96
#E1
203
191
199
187
92
#E2
282
257
271
248
88
#E3
68
64
79
69
101
Mean
94
Samples can undergo at least up to 3 freeze-thaw cycles.
Sample Values
NT-proBNP Values in Apparently Healthy Donors
NT-proBNP reference ranges were established using serum and plasma samples from apparently healthy donors. No medical histories were available for the volunteers.
Sample Matrix
n
Median [pmol/l]
Median [pg/ml]
Range [pmol/l]
Serum
70
5.8
49.1
1.1 – 541.5
EDTA plasma
28
5.6
47.4
0.6 – 32.0
We recommended establishing the normal range for each laboratory.
NT-proBNP Values in an Unselected Hospital Panel
NT-proBNP was measured in samples of patients from an unselected hospital panel:
Cohort
Sample Matrix
n
Median NT-proBNP [pmol/l]
Range [pmol/l]
Unselected hospital panel
Serum
117
46.5
0.0 – 621.1
Unselected hospital panel
EDTA plasma
40
28.6
0.3 – 489.2
NT-proBNP Values in Heart Failure Patients
NT-proBNP was measured in serum samples of patients from a heart failure cohort:
Cohort
Sample Matrix
n
Median NT-proBNP [pmol/l]
Range [pmol/l]
NYHA classification II
Serum
41
25.5
1.1 - 75.6
NYHA classification III
Serum
14
63.2
9.2 – 264.3
NYHA classification IV
Serum
7
166.8
84.1 – 399.3
The graph below shows a comparison of a heart failure cohort and apparently healthy donors:
Matrix Comparision
To assess whether plasma and serum matrix behave the same way in the NT-proBNP ELISA, concentrations of NT-proBNP were measured in EDTA-plasma and serum from apparently healthy donors. Each individual donated blood in all tested sample matrices.
Serum and plasma samples showed a correlation of R2=0.98.
The graph below shows the correlation of serum and plasma samples from apparently healthy individuals:
Comparison with other Assays
Comparison Between NT-proBNP Assays: Biomedica SK-1204 Versus Roche Elecsys 2nd Generation Assay
Performance of the NT-proBNP ELISA was compared with the Roche Elecsys Assay.
Data on serum values of a heart failure panel NYHA classification II-IV measured with the Biomedica NT-proBNP ELISA (#SK-1204) are shown below:
n
63
mean
54.0 pmol/l
max
399.3 pmol/l
min
1.1 pmol/l
The figure below shows the correlation between the NT-proBNP ELISA and the Roche Elecsys Assay in a heart failure panel NYHA classification II-IV panel:
Data on serum values of a heart failure panel NYHA classification IV measured with the Biomedica NT-proBNP ELISA (#SK-1204)
n
7
Mean
166.8 pmol/l
Max
399.3 pmol/l
Min
84.1 pmol/l
The figure below shows the correlation between the NT-proBNP ELISA and the Roche Elecsys Assay in a heart failure panel NYHA classification IV panel:
Data on serum values of a heart failure panel NYHA classification III measured with the Biomedica NT-proBNP ELISA (#SK-1204) are shown below:
n
14
Mean
63.2 pmol/l
Max
264.3 pmol/l
Min
9.2 pmol/l
The figure below shows the correlation between the NT-proBNP ELISA and the Roche Elecsys Assay in a heart failure panel NYHA classification III panel:
Data on serum values of a heart failure panel NYHA classification II measured with the Biomedica NT-proBNP ELISA (#SK-1204) are shown below:
n
41
Mean
25.5 pmol/l
Max
75.6 pmol/l
Min
1.1 pmol/l
The figure below shows the correlation between the NT-proBNP ELISA and the Roche Elecsys Assay in a heart failure panel NYHA classification II panel:
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