alzheimer 's disease ( ad ) is the most common neurodegenerative disorder . because the disease is often difficult to detect and diagnose at an early stage , a tremendous need exists for the identification and characterization of biomarkers that can be used to diagnose early - stage ad , or for monitoring new therapies for ad in clinical trials . much interest has been generated regarding the use of csf a 
 42 as a biomarker for diagnosing and tracking ad progression [ 1 , 2 ] . several different commercially available assays for measuring a 
 42 are currently employed across laboratories . these assays give diverse values for the levels of csf a 
 42 [ 3 , 4 ] . the relationships between the reported csf a 
 42 values from these different assays are unclear , but researchers agree in the importance of standardizing assays for csf a 
 42 . compared the a 
 42 csf measurements from two widely used a 
 42 elisa assays using the same csf sample . our data extends their findings by directly comparing human csf samples from both diseased and non diseased subjects with two different widely - used assay platforms , one of which uses colorimetric detection , while the other employs electrochemiluminescence ( ecl ) detection . comparisons were made to determine if the assays gave similar values and were able to distinguish csf from alzheimer 's subjects or age - matched controls based . ( san diego , ca ) , which is in compliance with all applicable rules and regulations for human sample collection and dissemination . csf samples from 23 individuals with ad ( 14 males , 9 females , average age 78 , average mmse score 17 ) and 22 age - matched controls ( 11 males , 11 females , average age 77 ) were tested . to avoid any effects from multiple freeze - thaw cycles , 60  l aliquots of each csf sample were placed in randomly assigned wells in a lo - bind polypropylene tray ( micronic north america , mcmurray , pa ) . all samples were assayed in duplicate in both a 
 42 assays , conducted by the same experienced scientist . innotest -amyloid142 ( innogenetics , alpharetta , ga ) is a solid - phase elisa for measuring the levels in human csf utilizing colorimetric detection with peroxidase - labeled streptavidin . absorbance at 450  nm was measured on the spectramax m2 ( molecular devices , sunnyvale , ca ) , and analysis was preformed using softmax 5.2 software . the limit of detection ( lod ) of 50  pg / ml was calculated by the manufacture as the mean of 8 determinations of the sample diluent . msd 96-well multi - array human ( 6e10 ) a 
 x-42 assay ( mesoscale discovery , gaithersburg , md ) was preformed as outlined by the manufacturer 's instructions . analysis was performed using msd workbench version 3.0.17.3 ( msd , gaithersburg , md ) . an lod of approximately 20  pg / ml was calculated by the manufacture based on data obtained from 4 different product lots ( table 1 ) . louis , mo ) , and csf samples were diluted in 96-well u - bottom polypropylene plates ( costar , lowell , ma ) . all statistical analyses and graphics were performed using either sas jmp version 8 or r version 2.9.0 . a p value < 0.05 was taken to indicate statistical significance . statistical testing for differences in mean a 
 42 level among the two diagnosis groups ( ad and amc ) was made using a two - sided , two - sample t - test . the a 
 142 elisa standard curve showed a dynamic range of 125 to 2000  pg / ml , with an lod of 50  pg / ml , and the average cv based on sample duplicates was 3.9% . the a 
 x-42 ecl assay standard curve showed a dynamic range of 12 to 3000  pg / ml , with an lod of approximately 20  pg / ml , and the average cv based on sample duplicates was 7.1% ( data not shown ) . the a 
 42 levels in the 45 csf samples determined by elisa and ecl assay are shown in figure 1(a ) . the a 
 42 levels of all the csf samples were above lods in both assays . the means of a 
 142 measured by elisa were significantly lower in ad patients compared with amc ( 500.4 versus 848.3  pg / ml , resp . ) , ( p < 0.0001 * ) . mean a 
 x-42 measured by ecl assay were also significantly lower in ad patients compared with amc ( 1235.8 versus 2280.5 although absolute concentrations varied between the elisa and ecl , the correlation coefficient for csf a 
 42 was r = 0.819 and highly significant ( p < 0.0001 ) ( figure 1(b ) ) . we provide here a direct comparison between two commonly used assays , elisa and ecl assay , in measuring a 
 42 levels in human csf . both the elisa and ecl assay showed that a 
 42 was higher in the amc than the ad group . these observations in subject differentiation are similar to other published reports [ 1 , 46 ] . we observed that in general a 
 42 levels were 2.6-fold higher in the ecl assay relative to the elisa . the observed differences may be related to a number of factors including the matrix ( i.e. , different assay dilution buffers and reagents ) , the purity of the calibrators , and differences in the affinity of the capture and detection antibodies [ 3 , 7 ] . the elisa assay uses monoclonal antibody 21f12 as the capture antibody , which recognizes a 
 142 . in contrast , ecl assay uses an undisclosed antibody , which recognizes a 
 x-42 as the capture antibody . there is also a lack of synchronization between the two assays due to the difference between the two calibrators . both assays detected the other kits ' calibrator ; however , the % recovery was not at an acceptable level ( data not shown ) . although both are plate - based methods , the detection technologies for measuring the csf a 
 42 concentrations are different and could be a contributing factor to the underlying difference . the elisa signal is detected with a peroxidase - labeled streptavidin antibody and the result is colorimetric , which provides the desired sensitivity but less dynamic range . the ecl signal is detected by incorporating a sulfo - tag labeled antibody that emits light upon electrochemical stimulation initiated at the electrode , which provides sensitivity similar to the elisa but a broader dynamic range . currently , quality control efforts are under investigation to evaluate interlaboratory variance components and to aid in the standardization of csf a 
 42 measurements . in addition , larger harmonization studies are needed that include the assays studied here as well as other manufactures assays for the measurement of a 
 42 in human csf . the present study provides an important first step by comparing and establishing the relationship between two widely used platforms for measuring a 
 42 .
alzheimer 's disease ( ad ) , which is characterized by a degeneration of neurons and their synapses , is one of the most common forms of dementia . 
 csf levels of amyloid  
 42 ( a 
 42 ) have been recognized as a strong candidate to serve as an ad biomarker . 
 there are a number of commercial assays that are routinely employed for measuring a 
 42 ; however , these assays give diverse ranges for the absolute levels of csf a 
 42 . in order to employ csf a 
 42 as a biomarker across multiple laboratories , 
 studies need to be performed to understand the relationship between the different platforms . 
 we have analyzed csf samples from both diseased and nondiseased subjects with two different widely used assay platforms . 
 the results showed that different values for the levels of csf a 
 42 were reported , depending on the assay used . 
 nonetheless , both assays clearly demonstrated statistically significant differences in the levels of a 
 42 in csf from ad relative to age - matched controls ( amc ) . 
 this paper provides essential data for establishing the relationship between these assays and provides an important step towards the validation of a 
 42 as a biomarker for ad .