Stool samples are used in diagnostic and screening tests to assess gastrointestinal health, examining the specimen for infectious agents, blood, and other biomarkers. These tests rely on the sample remaining as close to its original state as possible. Because a stool sample is a complex biological matrix containing living organisms, enzymes, and unstable chemical compounds, its viability is highly time- and temperature-dependent. Understanding the specific shelf life of a sample is necessary to ensure the laboratory results accurately reflect the patient’s condition.
Factors Affecting Sample Integrity
The viability of a stool sample diminishes over time primarily due to continued biological activity and the natural breakdown of its components. After collection, gut bacteria continue their metabolic processes, which can significantly alter the balance of microorganisms, especially when testing for bacterial pathogens. This overgrowth of normal flora can mask disease-causing bacteria, potentially leading to a false-negative result.
Warmer temperatures accelerate the metabolism of bacteria and the activity of digestive enzymes, speeding up degradation. Furthermore, specific analytes, such as occult blood, break down through natural chemical processes, making the sample unsuitable for accurate analysis of these unstable biomarkers.
General Storage Timelines
The time an unpreserved stool sample remains viable is short, particularly for fresh analysis. If not placed in a preservative medium, the sample must generally reach the laboratory for processing within one to two hours of collection at room temperature. After this brief window, the growth of normal bacteria begins to compromise the results of culture-based tests.
Refrigeration (2°C to 8°C) is the standard method for extending shelf life by slowing metabolic and enzymatic degradation. Under refrigeration, an unpreserved sample is typically stable for 24 to 72 hours, depending on the specific test ordered. Freezing is generally discouraged for most routine tests, as thawing can destroy parasites and damage bacterial viability, leading to inaccurate results.
Test-Specific Storage Requirements
The required storage time and temperature change significantly based on the specific analysis the laboratory will perform.
Bacterial Cultures
Tests for bacterial cultures, such as those looking for Salmonella, Shigella, or Campylobacter, are the most time-sensitive because they require the pathogenic bacteria to be alive and in their original concentration. Unpreserved samples must be processed rapidly. Placing the stool in a special transport medium, like Cary-Blair, stabilizes the sample, often extending its viability to 72 hours at room temperature or under refrigeration.
Blood and Parasite Tests
Fecal Immunochemical Tests (FIT) and Fecal Occult Blood Tests (FOBT) look for the presence of human blood, a relatively stable protein. These samples are often collected on specialized cards or in preservative vials and can remain viable for up to nine days at room temperature, provided they are protected from heat. Conversely, tests for Ova and Parasites (O&P) require chemical fixatives like formalin or PVA immediately after collection to preserve the delicate structure of protozoa or helminth eggs. Stool mixed with these fixatives is stable for several months at room temperature, making transit time less restrictive.
Biomarker and Antigen Tests
Tests for biomarkers like C. difficile toxins or specific antigens, such as H. pylori antigen, are less sensitive to time than live cultures because they detect stable molecular components. For C. difficile testing, the sample is typically stable for one day at room temperature or up to five days when refrigerated. Similarly, tests for fecal leukocytes or lactoferrin, which detect inflammatory markers, have a refrigerated stability of up to 48 hours or longer, provided the sample is not frozen.
Ensuring Proper Collection and Transport
To maintain sample integrity, the collection process must strictly follow the provided instructions, starting with using only the sterile, leak-proof container supplied by the laboratory. Preventing contamination with urine, toilet water, or other materials is essential, as this can dilute the sample or introduce interfering substances. Contamination can also destroy fragile cellular elements and protozoa, rendering the sample unsuitable for analysis.
Once collected, the container must be tightly sealed and correctly labeled with the patient’s information and the date and time of collection. If refrigeration is required, the sample should be stored in a sealed plastic bag in the refrigerator until transport. For transport to the lab, using a cooler with an ice pack is the best practice to maintain the low temperature and suppress biological activity.