What is a Sudan spot?

A Sudan stain highlights fats present in a sample such as blood or stool. The test uses a family of dyes known as Sudan stains that bind to fats when applied to a microscope slide. Various dyes are available for medical testing, allowing care providers to select the most appropriate for a given sample. The same dyes are also used in the manufacture of some products.

Staining is common practice for pathologists and other care providers who want to evaluate samples as effectively and reliably as possible. They use stains to highlight certain structures in a sample or to label specific compounds. In some cases, a stain reveals the presence of something that shouldn't be there, while in other cases, stains make it easier to find specific phenomena of interest in a sample. In the case of a stain in Sudan, the goal is to highlight the fats present with the dye to allow the pathologist to identify their presence and estimate their concentrations.

These dyes are lysochromatic, meaning that they attach to and color fats when added to a sample. Some Sudan dyes are indiscriminate and can work on various types of fats. Others are more concentrated and may highlight a particular fat, such as a triglyceride. Pathologists evaluating a specimen must consider their options when selecting a dye for testing to determine the best option. This may depend on why a Sudan stain was ordered and what the pathologist hopes to find on the test.

Fecal stain tests in Sudan are used to check for signs of steatorrhea or fat in the stool. This may indicate that a patient has a malabsorption problem or a disorder of the pancreas. The tests can also be used with blood samples in patients with suspected blood disorders. The technician performing a Sudan stain can look at the dye used and the findings, including a positive or negative indicator that fats are present and what types of fats were found. This is a qualitative test only, providing no hard measurements, although the technician can provide information on apparent concentrations in the sample.

Pathology texts and reference guides for laboratories may include images of different types of stain tests in Sudan. This can help technicians identify findings of interest or concern by comparing them to known samples. Control images are taken from healthy individuals and show what the test should look like if the results are negative. Other sample images show what's going on with different types of conditions, highlighting the findings associated with them so pathologists know what to look for and how to accurately identify it.

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