Reducing substances in faeces: Keys to accurate diagnosis

sustancias reductoras en heces

Table of contents

Digestive disorders are a common concern in today’s population. Fortunately, we have some markers that can help us understand the origin of some common intestinal conditions. Reducing substances in faeces are one such marker. As health professionals, we can help to identify certain intolerances, such as lactose intolerance.

Therefore, today we explain from a clinical point of view exactly what these substances are, how they influence a diagnosis and how they should be used in a clinical laboratory.

What are reducing substances in faeces?

Reducing substances in faeces are chemical compounds present in human faeces. They have the main ability to reduce certain chemical reagents, such as Benedict’s reagent or Fehling’s reagent (which will be explained later as case studies). These reagents contain copper salts which are produced in the presence of specific substances in order to reduce their number.. The appearance of these elements in the faeces could indicate the the presence of certain gastrointestinal disorders, such as carbohydrate (lactose) malabsorption.

They are therefore an exceptional marker for detecting this type of pathology. and other gastrointestinal disorders. If the laboratory is able to identify the cause of the problem in the shortest possible time by means of reducing substances in faeces, it is possible to offer the patient a much more specific diagnosis and treatment at an earlier stage. In addition to to assist in the identification, monitoring of the number of these markers may also help to assess the effectiveness of treatment in patients with these disorders.

An analysis can also give us information about the patient’s diet. and their ability to digest certain foods. With these data it will be possible to suggest a series of dietary changes to improve the patient’s gastrointestinal health. As can be seen, reducing substances in the faeces are not only useful for detecting the problem, but also for monitoring the problem and providing information on what to improve in the patient.


If the laboratory detects the presence of reducing substances in a patient’s stool samples, one of the following underlying causes can usually be concluded:

Carbohydrate malabsorption

If the digestive system is unable to break down carbohydrates into their simpler components such as glucose, galactose, fructose, etc. Or, on the contrary, due to a deficiency of digestive enzymes such as lactase. Unabsorbed carbohydrates are produced and pass into the large intestine. There, intestinal bacteria ferment them and produce short-chain fatty acids and other metabolites that have reducing properties.

Intestinal inflammation

If the intestine has an inflammatory condition (Crohn’s disease, ulcerative colitis…), the damaged intestinal tissue can affect the proper absorption of nutrients, including carbohydrates. The most common finding is an increase of reducing substances in the faeces.

Gastrointestinal infections

Some bacterial and parasitic infections of the gastrointestinal tract can disrupt the normal absorption of nutrients and lead to the presence of reducing substances in the faeces. The cause would be, as in other cases we have discussed, abnormal fermentation of unabsorbed carbohydrates.


This is one of the most common reasons for the increase of reducing substances in faeces. Intake of certain carbohydrate-rich foods, such as fermentable sugars found in fruits, dairy products and legumes, may temporarily increase the presence of these markers. The main reason is that bacterial fermentation occurs in the colon.

Sugars and faeces

To understand the problem of reducing substances in faeces, it is necessary to know which sugars cause the increase in these markers. All of them would not have been completely absorbed during digestion and therefore would not have been absorbed by the small intestine. The most common are usually:

  • Lactose. It is one of the sugars that causes most digestive problems. It is present in many of the foods we eat, such as milk and dairy products in general. If not absorbed correctly, they may appear in the faeces. Especially in lactose intolerant people. In this precise case it is worse because lactase, the enzyme that breaks down lactose, is absent or in reduced quantities.
  • Fructose. Present in many fruits and some vegetables, this sugar can also show up in a stool test if it is not absorbed properly. In this case the situation is more likely if the patient has fructose intolerance or its derived syndrome.
  • Sucrose. Regular table sugar, which is naturally present in most processed foods and in fruits and vegetables, can also appear in the faeces if malabsorption occurs. However, this is much less common than in the case of lactose or fructose.

All of the above-mentioned sugars can remain undigested because of a deficiency in the digestive enzymes that cause breakdown or because of disorders affecting their absorption in the small intestine.. Their presence in the faeces may indicate problems of malabsorption. Reducing substances in the stool can therefore help the laboratory to detect such changes and give an accurate diagnosis.

How does their presence influence analysis?

The presence of sugars such as those discussed above (lactose, fructose and/or sucrose) in the faeces may alter the results of a faecal reducing substances test. All of these have the ability to ferment in the colon thanks to intestinal bacteria. During this fermentation process, various metabolic products are produced, including short-chain fatty acids and gases (hydrogen and methane).

The result of the process can act as reducing substances, i.e. they have the ability to reduce certain chemical reagents.such as Benedict’s or Fehling’s. Both are used in tests for reducing substances in faeces. If the test is carried out and the reagent is added, it is easy to tell whether or not reducing substances are present in the sample, as a characteristic colour change occurs. There is no further evidence of a positive result in this case.

Specific cases

A common example of a clinical test where reducing substances are detected in faeces is the Benedict’s or Fehling’s test. . These are commonly used to diagnose carbohydrate malabsorption.

In order to perform the first step, it is necessary to add a solution of Benedict’s reagent to a stool sample. If reducing substances are present in the faeces, the solution will change colour and confirm a positive result. The variation in shade is caused by the reduction of Benedict’s reagent by the fermentation products of unabsorbed carbohydrates.

The second, the Fehling test is similar. It is also based on the ability of the fermentation products in the faeces to reduce copper salts in the reagent. If the presence of reducing substances is detected, the Fehling’s solution will turn brick red.

Both tests are useful in diagnosing carbohydrate malabsorption disorders and aid in the diagnosis of gastrointestinal diseases. The presence of reducing substances in the faeces indicates that carbohydrates are not being properly digested and absorbed in the small intestine. This may require further assessment and specific treatment.

The importance of the laboratory

In cases of analysis of any kind of substance or component, the laboratory with which you collaborate has a fundamental role to play. It will be in charge of detecting and quantifying the tests, in this case reducing substances in faeces. This requires the following steps accurate, checked and monitored at all times:

  • Tests. The first action will be to perform a series of tests to help detect the presence of reducing substances in faecal samples, such as those discussed in the previous section. Both involve the addition of chemical reagents to samples and their observation for changes indicative of reducing substances.
  • Interpretation of results. It is time to analyse the results. A positive result will indicate the presence of reducing substances. In this regard, it is very important to communicate this to the responsible doctor, whether positive or negative. In this way, the patient can be informed and treatment can be provided accordingly.
  • Quantification. There are cases where the laboratory can quantify the amount of reducing substances. This can be done by titration methods or by using spectrophotometry to measure the intensity of the colour change in the tests. Their quantification will provide additional information on the severity of the underlying disorder.
  • Report of results. With as much information as possible, the laboratory shall issue a report with the results including the interpretation of the results. If required, quantification of reducing substances. This report shall be sent to the requesting physician for use in diagnosis and subsequent treatment.

Correct treatment of samples

In procedures of this type, in which it is necessary to collect a stool sample from the patient or any other type of fluid or liquid, it is essential to follow a series of steps . They will allow for the ‘cleanest’ possible subsequent analysis, meaning that the sample has not been contaminated and the results can be trusted:

Key points

  1. Patient preparation is crucial to ensure the accuracy of any test, not only for reducing substances in faeces. It is necessary to inform the patient about the procedure and the importance of following the detailed instructions. It may be necessary for the patient to temporarily stop taking certain medications or adjust their diet on occasion. However, any such modifications must be made under the supervision and approval of the patient’s attending physician.
  1. Once informed, you will be provided with a clean, sterile container in which to collect the sample, in this case faeces. It is recommended to use atoilet seat cover or a plastic sheet to avoid contamination by toilet water. The patient shall sample different areas of the stool.
  1. Once the sample has been collected, it should be labelled on the container with the patient’s information. It shall include the name and the date and time of collection. This shall be stored in an airtight container at room temperature or according to the specific instructions of the laboratory. It is crucial to transport it to the laboratory as soon as possible, preferably within 24 hours of collection, to avoid decomposition of the components.
  1. Once in the laboratory, the sample will be subjected to a series of tests for reducing substances in faeces using the relevant methods.. Chemical reagents shall be added to the sample to observe any colour change or formation of precipitates, which would indicate the presence of reducing substances.
  1. Finally, the results of the test shall be recorded and reported to the physician, together with any necessary notes.

In addition to following the procedure to ensure the quality of the analysis, it is also essential that there is communication between the laboratory and the clinical team about any questions or data.. This ensures better interpretation of results and a good working environment in which all health professionals coordinate to deliver the best possible work.


Ambar Lab

That is our philosophy at Ambar Lab . Our team is made up of a group of professionals ready to attend to the needs of any of our patients. Nuestro equipo está formado por un grupo de profesionales dispuestos a atender las necesidades de cualquiera de nuestros pacientes.

However, our services go beyond this, ranging from the performance of laboratory diagnostic tests to the development of new projects and advice on laboratory management issues. If you want us to become your partner, you can contact our team so that we can start working together.

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