Investigation of enzyme immunoassay time courses: development of rapid assay systems

One- and two-step non-competitive avidin-biotin immunoassays for monomeric and heterodimeric antigen

In this study of one-step and two different two-step non-competitive avidin-biotin assays (NABAs) were developed for the measurement of a monomeric antigen (lactoferrin, LF) using polyclonal antibodies and the detection of a heterodimeric antigen (lutropin. LH) using monoclonal antibodies. The assays were based on the use of performed complexes of biotinylated antibody and avidin-peroxidase conjugate. The detection limits and intra-assay CVs of the one- and two-step NABAs were 0.1-0.5 mg/ml and 2.6-5.1% for LF, and 0.1-0.2 IU/l and 2.3-3.7% for LH, respectively. The working range was 1-100 ng/ml for the LF assay and 1-100 IU/l for the LH assay. A linear relationship with high correlation coefficients (0.979-0.992 for LF-NABAs: 0.949-0.990 for LH-NABAs) and good agreement was observed between the one- and two-step assays and the corresponding three-step NABAs used as reference methods. However, under stringent conditions the one-step assay for heterodimeric antigen was found to be sensitive to interference. The results indicate that it is possible to perform the multistep NABAs using convenient one- and two-step protocols. The one- and two-step assays also retained the advantages of the avidin-biotin system: rapidity and good sensitivity.

A dot-ELISA for the detection of IgG antibodies to mumps and varicella viruses

An enzyme-linked immunosorbent assay using nitrocellulose strips (dot-ELISA) for the routine laboratory detection of IgG antibodies to mumps and varicella viruses is described. The virus antigens are dotted onto nitrocellulose strips, and the dotted strips are incubated with the sera to be tested. The bound antibodies are revealed using enzyme-labeled antihuman IgG antibodies. Reliable results are obtained when the assay is carried out at 37 degrees C. The reported data indicate that the dot-ELISA can reliably be used for the detection of IgG antibodies to mumps and varicella viruses in human sera.

Comparison of different Fasciola hepatica antigens and their use to detect infection in sheep by an enzyme immunoassay

Soluble antigens of adult Fasciola hepatica were extracted from homogenized parasites and purified with gel chromatography on Sephadex G-200. Six peaks were obtained, and the second highest in molecular weight showed highest correspondence in a microplate enzyme immuno assay with parasite metabolite antigens. Optimal coating antigen concentration was 5 micrograms/ml and sample dilution 1:20. Total running time of assay was 3 hours. Positive and negative sheep sera was obtained from a group of experimentally infected tracer lambs.

Optimizing the solid-phase immunofiltration assay. A rapid alternative to immunoassays

The technical variables of the solid-phase immunofiltration assay (SPIA) for the detection of antibodies bound to antigens on a solid-phase filter have been investigated. The binding to solid-phase filters of 125I-labelled axial filament proteins derived from Treponema phagedenis and the optimal conditions for blocking non-specific protein binding were analysed. Axial filament was applied to nitrocellulose, Hybond Nylon and Zeta Probe. After extensive rinsing, the highest amount (68%) of axial filament was observed bound to Zeta Probe. However, blocking non-specific protein binding by pre-wetting the filter with rinsing buffer containing 0.5% Tween 20, prevented the binding of protein to the filter only when nitrocellulose was used as solid phase. Tween 20 (0.5%) in the rinsing and incubation solutions was found to be necessary for the reduction of non-specific binding of contaminants in turbid sera. However, the use of such solutions resulted in a substantial leakage of antigen (47%) during rinsing procedures. Binding of antigen-specific antibody was analysed using 125I-labelled protein A. The maximal possible binding of the antibody occurred within 5 min when the antibody solution was filtered. For optimal binding of 125I-labelled protein A an incubation time of 1 h was needed. It is suggested that solid-phase immunofiltration may provide a rapid alternative for radioimmunoassays or enzyme immunoassays for the detection of specific antibodies.

Evaluation of performance parameters of a membrane-based dot immunoassay for meningococcal polysaccharide

Increasingly, membrane-based enzyme immunoassays are being developed as the preferred solid-phase enzyme immunoassay format. We describe the rate kinetics of a polyvinylidene difluoride membrane-based dot immunoassay for meningococcal group A polysaccharide. Antigen detection sensitivity decreased logarithmically with linear decreases in incubation time. The sensitivity of a 30-min assay (5-min incubation steps) was increased to nearly the level of the standard assay (1-h incubation steps) by increasing the concentration of assay reagents fourfold. These results support the idea that existing microtiter plate assays can be transferred to rapid dot immunoassay formats with little or no loss of sensitivity.

Critical appraisal of the clinical relevance of rapid diagnosis in pediatrics

To assess the potential impact of rapid diagnostic tests on the care of the pediatric patient a critical evaluation of the antigen detection tests currently available for two important pediatric diseases--Haemophilus influenzae type b meningitis and group A beta-hemolytic streptococcal pharyngitis--is presented. Antigen detection tests for Haemophilus influenzae type b meningitis are of value primarily as an adjunct to traditional procedures, whereas antigen detection tests for the rapid diagnosis of group A beta-hemolytic streptococcal pharyngitis could, in the near future, replace the blood agar culture. This comparison demonstrates that the evaluation of a new diagnostic test should include not only a determination of its accuracy, but, in addition, a determination of its advantages over current methods, the frequency with which the test will be performed, the consequences of a false-negative or false-positive test result, and the potential impact on patient care resulting from use of the test.

Antigen capture ELISA for the identification of dengue viruses

We developed a simple antigen capture enzyme immunosorbent assay (AgC-ELISA) for identifying dengue (DEN) virus. The method employed serotype-specific monoclonal antibodies as capture antibodies and an enzyme conjugate of a flavivirus-reactive monoclonal antibody as a detecting antibody. Most DEN virus strains, representing all 4 serotypes isolated from various parts of the tropics, were identical either visually of spectrophotometrically when infected cell culture fluids were used as sources of antigen. The shelf life of the solid phase presensitized with monoclonal antibodies was 4 mth at -15 degrees C. DEN prototype viruses were still identified after storage at -15 degrees C for 1 yr or at room temperature for 1 mth.

Application of a timing protocol to the reduction of inter-plate variability in the indirect enzyme immunoassay for detection of anti-Brucella antibody

A simple timing protocol was developed to monitor chromogen conversion in an enzyme immunoassay, performed in microtiter plates, for the detection of antibody to Brucella abortus in bovine serum. Application of this protocol decreased the inter-plate coefficient of variation from 28.6% to 6.8% when optical density (OD) values, subsequent to the reaction of a standard antibody reagent, were compared to a static development time. Substantial reductions in variation were also observed for low titered seropositive and for seronegative control reagents. The timing protocol was based on the mathematical relationship of the OD value at 4 minutes of development to a predetermined target OD value (1.0) for a standard antibody reagent. Application of this relationship to the calculation of a variable, final development time eliminated the need for extensive data manipulation and assay calibration.

Comparison of a new, rapid enzyme-linked immunosorbent assay with latex particle agglutination for the detection of Haemophilus influenzae type b infections

A new, rapid enzyme-linked immunosorbent assay (ELISA) for the detection of polyribosylribitol phosphate of Haemophilus influenzae type b was compared with a commercially available latex particle agglutination (LPA) system (Bactigen; Wampole Laboratories, Cranbury, N.J.). By adding specimens and the anti-polyribosylribitol phosphate immunoglobulin-enzyme conjugate to the solid phase in a single step, it was possible to complete the ELISA procedure in 30 min. The ELISA was capable of detecting 0.3 ng of polyribosylribitol phosphate per ml in cerebrospinal fluid, 0.6 ng/ml in urine, and 1.2 ng/ml in serum; the in vitro sensitivity of LPA in these body fluids was 0.6, 0.3, and 0.3 ng/ml, respectively. Both procedures detected polyribosylribitol phosphate in specimens from 25 patients with bacteriologically confirmed H. influenzae type b infections. The specificity of ELISA appeared to be superior to that of LPA. ELISA was positive in only one of seven patients who had a positive LPA test and a clinical illness that was not compatible with haemophilus infection. Moreover, five patients with bacteriologically confirmed infections due to other pathogens (Streptococcus pneumoniae type 14 [two patients], Neisseria meningitidis group C, Escherichia coli K100, and Staphylococcus aureus) had false-positive LPA tests; only two (E. coli and S. aureus) were positive by ELISA. A total of 108 samples from 61 patients who had no evidence of haemophilus infections were negative by both procedures. The ELISA is a rapid, sensitive, and specific alternative to LPA for the detection of haemophilus polyribosylribitol phosphate.

Rapid detection of bovine herpesvirus type 1 antigens in nasal swab specimens with an antigen capture enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) was developed for the detection of bovine herpesvirus type 1 (BHV-1) antigens in nasal swab specimens collected from infected animals. Development of the ELISA involved screening and selection of BHV-1-specific monoclonal antibodies for their ability to capture BHV-1 antigens and for their stability and activity after conjugation to horseradish peroxidase. Forty combinations of capture-conjugate monoclonal antibody pairs were screened for detection of nanogram amounts of purified BHV-1 by using a double-antibody-sandwich ELISA in which antigen and conjugated antibody were simultaneously added to antibody-coated wells. Of the 40 monoclonal antibody pairs, 4 were analyzed further and 1 was selected for routine application to clinical specimens. Of 129 nasal swab specimens collected during the first 10 days after experimental infection with BHV-1, 66 were found to be positive by both virus isolation and ELISA and 34 were positive for infectious virus but negative by ELISA. One specimen was positive by ELISA but negative by virus isolation, and the remaining 28 specimens were negative by both tests. Quantitation of the virus-containing specimens showed that the ELISA had a lower detection limit of 10(3.5) median tissue culture infective doses. The ELISA was judged to be highly useful for diagnosis of BHV-1 infections, since all of the nasal swab specimens that were collected from 12 animals during the first 5 days of the infection, when the clinical signs were the most apparent, were positive.

Enhanced luminescence enzyme immunoassay for factor VIII related antigen

A sandwich enzyme immunoassay for plasma factor VIII related antigen has been developed which exploits a para-iodophenol enhanced chemiluminescent reaction to detect the horseradish peroxidase label. The assay entailed 15 min incubations with sample and with conjugate and had a detection limit of 0.12 mU. It showed good within batch precision (coefficient of variation = 2.95-5.8%) and results on a series of 57 specimens agreed with results obtained by immunoelectrophoresis (correlation coefficient = 0.97).

Use of an enzyme-linked immunosorbent assay to measure antigenaemia during acute plague

An enzyme-linked immunosorbent assay (ELISA) was developed to measure concentrations of the specific F1 antigen of the plague bacillus in biological fluids. The assay employed a monoclonal antibody to capture the antigen. Sensitivity of the assay was 0.4 ng of F1 antigen. ELISA-inhibition was used to confirm the specificity of the reactions.This assay detected F1 antigen in two of ten sera from patients with acute bubonic plague and indicated that antigenaemia in man during plague may reach levels of 4-8 mug of F1 antigen per ml of serum.The probability for a correct serodiagnosis of plague was improved when the patients' sera were tested for both antibody and antigen. Two patients with antigenaemia did not have antibody, while two patients with antibody lacked antigenaemia.

Rapid enzyme-linked immunosorbent assay of whole blood for detection of antibodies to Japanese encephalitis virus

The application of the rapid system of enzyme-linked immunosorbent assay (ELISA) was studied to quantify antibodies to Japanese encephalitis virus in large-scale epidemiological surveys, especially by testing under field conditions. The assay system, with 15 min for the first reaction and 30 min each for the second and the third reactions, was highly reproducible (coefficients of variation with swine positive sera were less than 5.8%) and was significantly correlated with the routine assay system with 1 h for each reaction (correlation coefficient was 0.960). Compared with the haemagglutination inhibition test, the rapid system gave a correlation coefficient of 0.916 and qualitative agreement of 96.1%. The substitution of whole blood for serum in the first reaction was also examined not only to avoid serum separation but also to apply this system to antibody quantification in animals from which sufficient amounts of sera cannot be easily obtained: only 2 microliters were needed for the test. The results obtained with 51-fold diluted whole blood had a linear relationship to those obtained with 100-fold diluted sera in swine and humans.

Comparison of Phadebact coagglutination, Bactogen latex agglutination, and counterimmunoelectrophoresis for detection of Haemophilus influenzae type b antigens in cerebrospinal fluid

Cerebrospinal fluid specimens from patients with suspected meningitis were screened with the Phadebact Haemophilus Test (Pharmacia Diagnostics), with Bactogen (Wampole Laboratories), and by counterimmunoelectrophoresis. With culture-positive fluids, Phadebact coagglutination detected 95%, Bactogen latex agglutination detected 91%, and counterimmunoelectrophoresis detected only 79%. Both agglutination techniques were 25-fold more sensitive than counterimmunoelectrophoresis when tested with dilutions of positive fluids. To obtain specific reactions with the Phadebact reagents it was necessary to heat treat (95 degrees C, 5 min) the fluid; with Bactogen and counterimmunoelectrophoresis this was not necessary.