This is a chemical test performed on the stool for the detection of occult blood. A definitely positive test is usually interpreted as indicating the presence of bleeding somewhere in the gastrointestinal tract, subject to the reservations listed below. Cancer, especially of the stomach and cecum, is a relatively common cause of such occult bleeding, and a truly positive test should alert the clinician to this possibility.

The significance and management of the positive test is discussed more fully in Chapter VIII (Stomach And Upper Gastrointestinal Tract. Stomach), Stomach and Upper Gastrointestinal Tract, and Chapter IX (Colon And Rectum), Colon and Rectum.

The test is extremely easy to do. If the reagents are within arm's reach of the instrument table, it can be accomplished in a matter of a few moments. It is very important that the reagents be replaced every few weeks. Failure to use fresh solutions may yield incorrect results.

A number of variations of the test for occult blood are available. We recommend the guaiac test as the most generally satisfactory. The benzidine test is a highly sensitive one, and the numbers of "false alarms" may be quite high unless conditions are rigidly controlled.

Technique

1. Obtain a stool specimen prior to the use of cathartics or enemas. This may be done at the time of rectal examination, or the patient may collect the specimen at home for submission prior to the examination. It must be remembered that if the patient has been prepared with castor oil, enemas, etc., the stool specimen will be either unobtainable or unsuitable for guaiac testing.

2. Reagents for the guaiac test:

(a) Filter paper. This should be of a type that has been tested with the reagents and proved to be free of positive color reaction.

(b) Glacial acetic acid.

(c) Saturated alcoholic solution of gum guaiac. This is made by dissolving 1 Gm. of powdered gum guaiac in 5 ml. of 95% ethyl alcohol. This reagent is stable for a month.

(d) 3% fresh hydrogen peroxide. Freshness is demonstrated by a bubbling reaction when the peroxide is added to a drop of blood. Stock supply of hydrogen peroxide should be refrigerated.

3. Smear the stool on a piece of blotting paper or paper towel. Observe the color.

4. Perform the guaiac test (Fig. 31).

Spread a small amount of feces on filter paper.

Add 2 drops of glacial acotlc acid.

Add 2 drops of guaiac solution.

Add 2 drops of hydrogen peroxide.

Observe color change: Deep blue appearing in one minute or loss represents a positive 4+ test.

Lighter shades of blue represent a positive 1 + to 3+ test. No color or slight groan represents a negative test.

Fig. 31. Technique used in performing the guaiac test.

(a) Add in sequence 2 drops of each of the following:

(1) Glacial acetic acid

(2) Guaiac solution

(3) Hydrogen peroxide

(b) Interpret as follows:

(1) A deep blue color appearing within one minute represents a positive 4+ test.

(2) Lighter shades of blue represent a positive 1+ to 3+ test.

(3) No color or slight green represents a negative test.

Positive Guaiac Tests

A positive result may be encountered under the following circumstances:

1. Bleeding anywhere in the gastrointestinal tract. A wide variety of conditions may be responsible for this, ranging from cancer to a duodenal ulcer or diverticulitis to bleeding gums or hemorrhoids.

2. Ingestion of blood-containing food, such as red meat.

3. Too sensitive a test for occult blood. The benzidine and similar tests pick up minute traces of blood and are therefore unsuitable for screening purposes. The guaiac is a more gross test; although it may occasionally overlook minimal bleeding, it causes fewer "false alarms."

4. Over-reading of the guaiac test. The color change should be a real and obvious one before being recorded as positive.

Negative Guaiac Tests

A negative result may be recorded under the following circumstances:

1. No bleeding present-a true negative test, the most common result.

2. Intermittent bleeding. Significant lesions may bleed only periodically.

3. Improper performance of guaiac test (e.g., overage chemicals, improper sequence, etc.)

4. Reliance on only a single stool specimen. This is, of course, a reflection of point 2. For routine screening purposes, however, one specimen generally must suffice.

The latter three categories are often referred to as false negatives.

Management Of Positives

1. Review the situation to determine whether any nonsignificant factors listed as causes of a positive guaiac test are present. Even if they are, caution must be exercised since cancer may coexist with any of such entities.

2. Repeat the guaiac test after the patient has been on a meat-free diet for three days.

3. If a positive test persists, complete evaluation of the gastrointestinal tract must be undertaken.