Limulus amoebocyte lysate test via an open-microcavity optical biosensor

Almost since its discovery, Limulus amoebocyte lysate (LAL) testing has been an important part of the pharmaceutical quality control toolkit. It allows for in vitro endotoxin testing, which has replaced tests using animals, such as using rabbits' thermal response to judge pyrogenicity of test samples, thus leading to a less expensive and faster test of parenteral pharmaceuticals and medical devices that contact blood or cerebrospinal fluid. However, limited by the detection mechanisms of the LAL assays currently used in industry, further improvement in their performance is challenging. To address the growing demand on optimizing LAL assays for increased test sensitivity and reduced assay time, we have developed an LAL assay approach based on a detection mechanism that is different from those being used in industry, namely, gel-clot, turbidimetric, and chromogenic detection. Using a unique open-microcavity photonic-crystal biosensor to monitor the change in the refractive index due to the reaction between LAL regents and endotoxins, we have demonstrated that this approach has improved the LAL assay sensitivity by 200 times compared with the commercial standard methods, reduced the time needed for the assay by more than half, and eliminated the necessity to incubate the test samples. This study opens up the possibility of using the significantly improved LAL assays for a wide range of applications.

A rapid highly-sensitive endotoxin detection system

This paper presents a rapid, highly-sensitive, and low-cost method of endotoxin quantification based on the use of stress-responsive magnetoelastic sensors, that monitor the gel formation (viscosity change) of the Limulus Amoebocyte Lysate (LAL) assay in response to endotoxin. Ribbon-like magnetoelastic sensors, 12.7 mm x 6 mm x 28 microm, were immersed in a LAL assay after mixing with test samples of variable endotoxin concentration, and the decrease in resonance amplitude of the sensor was recorded as a function of time. Experimental results show excellent correlation between endotoxin concentration and the maximum clot rate, determined by taking the minimum point of the first derivative of the amplitude-time curve, as well as the clotting-time, defined as the time that corresponds to the maximum clot rate. Using a LAL gel-clot assay with a sensitivity of 0.06 EU/ml (EU: endotoxin unit), the magnetoelastic sensor based technology can detect the presence of endotoxin at 0.0105 EU/ml in test requiring approximately 20 min. Unlike optical methods used for determining endotoxin concentration, the color of the test solution does not impact the magnetoelastic sensor measurement. Due to the small size of the sensor reader electronics and low cost, the magnetoelastic sensor based endotoxin detection system is ideally suited for wide-spread use in endotoxin screening for sepsis prevention.

Bacterial endotoxin testing: a report on the methods, background, data, and regulatory history of extraction recovery efficiency

Since the mid-1970s the Limulus Amebocyte Lysate (LAL) assay has been used to test medical devices for bacterial endotoxins. The Association for the Advancement of Medical Instrumentation (AAMI) recently published a standard designated ANSI/AAMI ST 72: 2002, Bacterial Endotoxins--Test methodologies, routine monitoring, and alternatives to batch testing, which addresses LAL testing and associated issues. In order to perform the bacterial endotoxins test (BET), the test article must be extracted in an aqueous medium, with the extract being used as the test solution. In the early years of testing, and periodically throughout LAL test history, questions have arisen about validation of the extraction efficiency of endotoxins from medical devices. The AAMI Microbiological Methods Committee appointed a Task Group to thoroughly research the issue of extraction efficiency and to recommend whether validation of extraction efficiency is necessary for LAL testing of medical devices.

An automated sequential injection analysis system for the determination of trace endotoxin levels in water

A new automated instrument based on the Limulus amebocyte lysate (LAL)-chromogenic substrate kinetic assay for the determination of bacterial endotoxins is reported. A computer controlled syringe pump-multiport valve combination was used to aspirate the sample and other reagents into a holding coil. The syringe was always filled with air; liquid did not enter the syringe. The valve could address up to four individual radial paths, fully referenced optical fiber LED-based absorbance detectors that were housed collectively in a single, small (20 x 20 x 30 mm) metal block with a heater and temperature sensor, and maintained at 37 degrees C. Assay mixtures containing sample or standards were delivered to the individual detector cells for the simultaneous collection of the time based absorbance data. The automated system determined endotoxins with good accuracy and reproducibility in the range of 0.005-0.5 endotoxin units (EU)/mL (r2 > or = 0.99). Based on three times the standard deviation of the blank and the slope of the calibration curve, the lower limit of detection was < or = 0.003 EU/mL. The variability of the assay method is less than 5% (n = 10). Analysis time required for a 0.005 EU/mL standard was <100 min. Appropriately diluted tap water samples were simultaneously analyzed by the present system and a manually loaded commercial microplate reader based instrument; the data were statistically indistinguishable at the 95% confidence level.

Disposable TSM-biosensor based on viscosity changes of the contacting medium

The thickness shear mode (TSM)-sensor responds to changes of mechanical properties of the material contacting the surface of the sensor. One of the material properties is the viscosity of a liquid. Abiosensor based on the TSM-resonator for the detection of endotoxin has been developed. It exploits the viscosity-density change during the reaction of endotoxin with limulus amebocyte lysate (LAL). The effect of surface properties of the sensor has been investigated to achieve better output signals. It is shown that the sensor requires a hydrophilic surface to get a better coupling between the sensor and the LAL-endotoxin solution. The TSM biosensor is able to detect an endotoxin concentration as low as 100 fg/ml by using only 50-microl standard LAL solution. The disadvantages of reusable sensors, such as the contamination from previous measurement of endotoxin and the cost of the regeneration or reclining processes of the sensor, have been eliminated by using a cost effective disposable TSM-sensor.

Application of a novel apparatus, the quartz chemical analyzer, to the determination of endotoxin in blood

A novel apparatus called a quartz chemical analyzer (QCA) has been developed using a quartz crystal resonator. This apparatus measures sample viscosity changes based on resonant frequency changes of the quartz crystal. The apparatus was used to determine bacterial endotoxin concentrations by monitoring the gelation reaction of Limulus amebocyte lysate. The QCA determined endotoxin concentrations with good accuracy and reproducibility in the range of 0.001-3 EU/ml for endotoxin standard (JP XII). For endotoxin determination in human whole blood and plasma samples, the inhibitory reaction was eliminated by pretreatment of a fourfold dilution at 60 degrees C and incubation for 30 min. There are many advantages of the QCA method compared with the turbidimetric and chromogenic methods. For example, QCA can measure sample viscosity changes with high sensitivity and accuracy because QCA detects minor resonant frequency changes and the frequency data give a numerical value for easy quantitation. QCA can examine turbid samples, and the required quantities of samples and reagents are small, since the quartz crystal detects sample viscosity changes directly. The endotoxin determination time may be shortened by raising the reaction temperature, and QCA can detect other types of coagulation reactions.

Automation and computerization of chromogenic LAL assay method for bacterial endotoxin using 96-well microtiter plate

A high degree of automation was achieved for a chromogenic substrate Limulus amebocyte lysate (LAL) assay method for bacterial endotoxin using a Cetus Pro/Group Liquid Handler and 96-well microtiter plates. A Titertek Multiskan Reader was interfaced with an IBM PC using a LOTUS MEASURE software to capture optical density values of samples in a LOTUS 1-2-3 spreadsheet. A password protected, menu-driven macro programmed in LOTUS 1-2-3 automates the calculation, evaluation of assay parameters, documentation, and generation of a formatted three-page report suitable as a primary record. All assay operations, including testing 19 samples against a four-point standard curve in replicates of four each, the calculation of results, and generation of a report, are completed in less than 40 minutes. The relative standard deviation (RSD) of the assay is approximately 7%, which compares favorably with the robot automated system.

Endotoxaemia: an early predictor of septicaemia in febrile patients

In 473 consecutive febrile patients a sensitive and rapid chromogenic limulus assay was used to assess the value of endotoxaemia versus bacteraemia for predicting development of the syndrome of septicaemia. In each patient three blood specimens for culture and endotoxin testing were obtained at the onset of fever. Blood pressure, urinary output, and the occurrence of thrombocytopenia and metabolic acidosis were recorded prospectively during three days of follow-up. Septicaemia developed in 19 patients (4%). The sensitivity, specificity, and likelihood ratio for a positive result with the endotoxin assay were 79%, 96%, and 20, respectively. The corresponding indices for bacteraemia were 89%, 78%, and only 4. The results suggest that endotoxaemia is a clinically valid indicator for impending gram-negative septicaemia (positive predictive value 48%) and that the absence of endotoxaemia virtually rules out the risk that septicaemia will ensue (negative predictive value 99%).

Automation of chromogenic substrate Limulus amebocyte lysate assay method for endotoxin by robotic system

The chromogenic substrate Limulus amebocyte lysate (LAL) assay method for the detection of endotoxin was automated by a Zymate robotic system. The software developed enables the robot to automatically dilute a stock reference endotoxin standard (20,000 endotoxin units per ml) for the construction of a five-point standard curve, make sample dilutions to the proper testing concentration, and perform chromogenic substrate LAL assays in duplicate. The linearity of the standard curve and the endotoxin concentration in each sample are calculated and results are printed automatically. In 48 min the automated system assays three samples and a reference standard in duplicate along with a water blank. Sensitivity of the assay is a function of incubation time. The assay is linear (r greater than 0.99) in the region of 0 to 1.0 endotoxin units per ml or 0 to 0.2 endotoxin units per ml with incubation times of 10 or 16 min, respectively. The method can be made very sensitive, detecting as low as 0.003 endotoxin units per ml with 30 min of incubation. The precision of the assay method, determined by assaying an endotoxin reference solution eight times, is ca. 6%. The LAL reagent designed for gel-clot assay was modified for the chromogenic substrate assay. We describe the optimum conditions for the performance of the chromogenic substrate LAL assay and stability of the LAL reagent.

Limulus lysate assay for gonorrhea

The Limulus amebocyte lysate (LAL) assay is currently being studied for use in diagnosing gonococcal urethritis/cervicitis syndromes and as a screening test for gonococcal cervicitis. Experience shows that the test can be adapted to office use by private physicians, providing rapid, accurate results in situations where laboratory facilities are unavailable. Further modifications are improving the speed and interpretation of results.

Modified micromethod of the Limulus amoebocyte lysate assay for endotoxin

A Limulus amoebocyte lysate microtechnique performed in petrolatum wells on a microscope slide is described. Injection of a dye solution in ethanol directly into the wells leads to an unambiguous interpretation of the results. Twelve samples can be tested on a single slide, and compact storing of the samples is possible.

Application of a Limulus test device in rapid evaluation of gonococcal and nongonococcal urethritis in males

A test device incorporating Limulus amoebocyte lysate (Mallinckrodt, Inc., St. Louis, Mo.) was developed for the rapid, presumptive diagnosis of gonococcal and nongonococcal disease in males. The device, which was evaluated in 550 men with exudative urethritis, consisted of a specimen collection syringe, a dilution reservoir containing 10 ml of pyrogen-free water, and a Limulus amoebocyte lysate single-test vial. After specimen collection, the syringe was affixed to the dilution reservoir for rapid, accurate dilution of the clinical sample. Contamination of the specimen and potential biohazards to the user were prevented. The diluted sample was then transferred (via the collection syringe) to the lysate test vial for assay of endotoxin. Various incubation times at 37 degrees C were also studied in an additional 301 male patients, and time was reduced from the standard 60 to 30 min while still retaining equivalent predictability of culture results (P less than 0.05). Of the 550 males evaluated with the test device, 366 had positive cultures for Neisseria gonorrhoeae, and 184 were negative. A sensitivity of 99.2% and a specificity of 96.7% were obtained with the test device. Overall ability to predict culture results was 98.4%. Gram-stain sensitivity and specificity were 96.4% and 99.5%, respectively, with an overall accuracy of 97.5%. There were no statistical differences between the Limulus amoebocyte lysate test and Gram stain in predicting cultures (P less than 0.05). Thus, use of the Limulus amoebocyte lysate test device would enable the private physician to make an accurate, presumptive diagnosis of gonococcal and nongonococcal disease in males with exudative urethritis within 30 min without the need of a microscope and to initiate proper therapy during the patient's initial evaluation.

Automation of the Limulus amoebocyte lysate test by using the Abbott MS-2 microbiology system

A rapid, automated method for the performance of the Limulus amoebocyte lysate endotoxin assay has been developed by using the Abbott MS-2 Microbiology System. This instrument automatically determines sequential changes in the optical density of up to 176 samples at 1- or 5-min increments during a 1-h assay period. Graphic representation of optical density changes can be viewed on a cathode-ray tube or reproduced by using a hard-copy printer. Limulus amoebocyte lysate preparations that were obtained from different commercial producers and that had similar endotoxin sensitivities by the conventional gelation method varied somewhat in reactivity when determinations were based upon rate changes in optical density. Lysates from Associates of Cape Cod, Difco Laboratories, and M. A. Bioproducts were the most readily adaptable to the MS-2 System. Use of the MS-2 system increased the sensitivity of these preparations from 60- to 250-fold, and as little as 1 pg/ml was detected. Adaptation of the MS-2 instrument for this purpose provides an objective, reproducible, automated method for the performance of Limulus amoebocyte lysate tests on a variety of fluids.