Solvent System for Peroxidase Inhibitor Reagent for Fecal Occult Blood Determinations

This invention relates to a method of avoiding interferences which cause false-positive fecal occult blood determinations in stool samples when employing a method based on the reaction of guaiac with hemoglobin.

The avoidance of a false-positive test has, in the past, often relied on control of the dietary intake of the patient, when tests which rely on guaiac-peroxidase reagents are applied. An improved method of applying such tests is described in U.S. Pat. No. 4,333,734 by the same inventor hereof, wherein false-positive tests, due to the presence of peroxidase from ingested foods, are avoided. The basis of this method is a combination of the partial denaturing of peroxidase enzymes and the removal of necessary metals for efficient peroxidase enzyme activity. This combination of actions eliminates the interfering effect of peroxidase without requiring extreme conditions that might effect hemoglobin reactively.

During extensive clinical testing of the method and device of U.S. Pat. No. 4,333,734, it has been found that certain of the samples taken from patients still produce false-positive tests. It is believed that these false positive tests may be caused by drugs or drug metabolites, or a large variation in pH in the excrement of the patients.

The present invention has as its primary object the reduction or elimination of false-positive tests which occur due to interfering drug or drug metabolites when using the guaiac-peroxidase type reaction scheme for determination of hemoglobin in various samples.

Briefly, the present invention provides an aqueous alcohol solvent system which, due to solubility differences, retards the extraction of drugs from the sample into the reaction mixture, thereby practically eliminating the effect of drugs on the reactants when applying the guaiac-peroxidase test scheme. The inventive method is particularly useful in conjunction with, and is therefor an improvement over, the method described in U.S. Pat. No. 4,333,734. This prior method requires the application of an inhibitor reagent prior to the addition of the reaction initiator reagent, that is, a two step procedure. This leads to possible extraction of interfering drugs into the reaction zone by migration analogous to paper-chromatographic separations.

In the most preferred embodiment, the addition of mild acid to the reaction mixture enhances the rate of peroxidase inhibition and insures uniformity of reaction conditions. Color intensity for positive tests, is also enhanced.

The present invention improvement for avoiding a false-positive test due to drug metabolites or other water soluble interfering substances is based on the use of solvent wherein the solubility of these substances is reduced, viz an alcohol containing solvent mixture. It has been found that a water-alcohol mixture, preferably about 50% alcohol to 50% water-depending on the alcohol used and the solubility of the various test system reagents in the resulting mixture-can be used to great advantage to improve the reliability of the prior art test procedure. Lower alcohols that are miscible over a wide range of compositions with water, such as methanol, ethanol, propanol, isopropanol and n-butanol, are useful in the practice of this invention. Of these, ethanol and isopropanol are preferred for their reasonable cost coupled with general availability and relative safety in handling.

The present invention also provides a method wherein the color intensity which results from applying the inventive test, is increased, by assuring that the reaction mixture remains slightly acid even in the presence of alkaline samples. This is accomplished by adding one or two drops of weak acid, especially acetic acid, to the sample smear on the slide. The acid also speeds up the rate of peroxidase inhibition when using the method of U.S. Pat. No. 4,333,734, from 21/2-3 hours normally encountered under non-acid conditions to 5-10 minutes with the acid addition and thereby provide a tremendous savings in time in running the procedure.

In the tables presented in this application, color intensity is scored as follows:

Negative--no color response

Trace--color response barely visible to naked eye

+1--slight color response

+2--moderate color response

+3--strong color response

+4--very strong color response

Because higher alcohol to water ratios are expected to give the best results by minimizing interfering substance solubility, reagent solubility test were run to determine maximum alcohol concentration that can be used with the peroxidase denaturing reagents to be applied to the sample.

It was found that 6M guanidine hydrochloride and the EDTA reagent used for the inhibition of vegetable and fruit peroxidase activity in the known method, was not completely soluble in 70% aqueous solution of methanol, ethanol, isopropanol or n-butanol; The solubility of guanidine hydrochloride.EDTA in a 50% alcohol solution is effective except for n-butanol which showed solubility problem signs in the form of a fine suspension. Thus 50% aqueous alcohol was selected for the following experiment.

Following the procedure of Fleisher, U.S. Pat. No. 4,333,734 the reagents were modified to include a water-alcohol solvent, as follows:

The effective concentration of this alcohol is about 35% because of the high specific volume of guanidine hydrochloride. Methanol, ethanol, isopropanol and n-butanol were tested.

Acid Reagent: 5M Acetic Acid

Experiments were run:

Prepared fecal occult test slides (guaiac impregnated) for detecting occult blood based on phenolic oxidation of guaiac by hemoglobin in the presence of hydrogen peroxide as a color developer (see U.S. Pat. No. 3,996,006), were used.

A 25 .mu.l aliquot of horseradish peroxidase (0.36 mg/ml) was applied to each slide. Two drops of 5M acetic acid and 2 drops of alcohol inhibitor were added prior to addition of developer (H.sub.2 O.sub.2). Developer was added at the time intervals indicated in the table below.

A number of advantages of the use of modified peroxidase inhibitor reagent, according to the invention, have been observed and include the following:

1. The time for inhibition of peroxidase interferences is effectively reduced from about two-three hours to about 10 minutes. This can be seen from Table I (above), wherein only a trace of reaction is left after 10 minutes when acid is added to the reaction with modified reagent. Without the acid addition, the inhibition time is still only about 30 minutes to reduce the reaction to "trace".

2. Not only does the acetic acid help reduce the inhibition time, it also insures uniform test conditions for each sample. That is, the addition of mild acid such as acetic acid insures that the reaction mixture remains slightly acid even in the presence of alkaline samples.

3. The alcoholic matrix is more effective in testing patients exhibiting steatorrhea a condition in which the fat content of the stool is significantly increased. Prior treatment of the specimen with alcoholic inhibitor reagent reduces the hydrophobic consistency of the specimen and thus renders more effective the inhibition of peroxidase activity and reactivity of hemoglobin.

4. The color of positive guaiac reaction was found to be more vivid in positive tests. The latter may result from the uniformity of test acidity reducing the otherwise relatively rapid fading of color exhibited under positive test condition (i.e. presence of hemoglobin).

It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.