How long can a specimen remain to be viable for laboratory examination? What is the basis of this timing?

The viability of a specimen for laboratory examination largely depends on the type of specimen and the specific test to be performed. Here are some general guidelines:

1. Blood Specimens: Whole blood for most tests can be stored at 2-8°C for up to 48 hours. However, some tests such as blood gases need to be analyzed immediately.

2. Urine Specimens: For routine urinalysis, the sample should be examined within 2 hours of collection. If this is not possible, the sample can be refrigerated at 2-8°C for up to 24 hours.

3. Stool Specimens: For routine tests, the sample should be examined within 2 hours of collection. If this is not possible, the sample can be refrigerated at 2-8°C for up to 24 hours. For culture, the sample should be processed immediately.

4. Tissue Specimens: These should be processed immediately. If this is not possible, they can be stored in formalin for later examination.

The basis for these timings is the stability of the analytes (substances being measured) in the specimen. Over time, biological processes can alter the concentration of these analytes, leading to inaccurate results. For example, bacteria in urine can multiply if the sample is not refrigerated, leading to a falsely high count. Similarly, blood cells can break down over time, altering the results of a complete blood count. Therefore, it's important to process specimens as quickly as possible to ensure accurate results.