Blood tests provide valuable insights into a person’s health, helping diagnose conditions and monitor health. The accuracy of these tests relies on the quality of the blood sample. Blood samples degrade over time, compromising test reliability. Understanding the factors that influence how long a blood sample remains viable is important for accurate diagnostics.
Factors Influencing Sample Stability
Several factors determine how long a blood sample remains stable for testing. Time is a primary consideration, as the degradation of blood components increases with the duration of storage. Temperature is a key factor; colder temperatures, such as refrigeration (2-8°C or 36-46°F) or freezing (e.g., -20°C or -70°C), slow degradation, preserving the sample longer than room temperature (20-25°C or 68-77°F). Some tests, like blood gas analysis, require samples to be kept at room temperature, while others, such as glucose and ammonia tests, need refrigeration.
The type of collection tube and the anticoagulants or additives they contain impact sample stability. Tubes with anticoagulants like ethylenediaminetetraacetic acid (EDTA) prevent clotting, making them suitable for tests requiring whole blood, such as a complete blood count. Heparin tubes inhibit thrombin formation, often used for plasma chemistry tests. Tubes without anticoagulants, like serum tubes, allow the blood to clot and separate, yielding serum for chemistry and serology tests.
Proper mixing of blood with anticoagulants, by gently inverting the tube 8-10 times immediately after collection, is necessary to prevent clotting. Vigorous shaking should be avoided to prevent red blood cell damage.
Stability for Common Blood Tests
The stability windows for blood samples vary depending on the specific test. For a Complete Blood Count (CBC), parameters like white blood cell (WBC) count, platelet count, hematocrit, hemoglobin, red blood cell (RBC) count, and mean corpuscular hemoglobin (MCH) remain stable for at least 24 hours at room temperature. Refrigeration (4°C) extends stability for most CBC parameters, including WBC and platelet counts, up to 24-72 hours. Some parameters, such as mean corpuscular volume (MCV) and red cell distribution width (RDW), may show changes after 24 hours at room temperature.
Basic Metabolic Panel (BMP) and Comprehensive Metabolic Panel (CMP) tests, which measure glucose, electrolytes, kidney function, and liver enzymes (for CMP), have varying stability. Glucose degrades quickly, so samples for glucose testing should be analyzed or separated within 1 hour at room temperature or refrigerated. Most other CMP components, including liver enzymes like AST, ALT, and ALP, and electrolytes like sodium, potassium, and chloride, are stable for up to 12 hours at 4°C, but values may be less stable at room temperature. Serum and plasma samples for chemistry tests can be stored refrigerated for up to 48 hours after separation from cells.
Coagulation tests (PT, INR, APTT) are more time-sensitive. Samples for PT and INR can be stored safely for up to 24 hours when refrigerated, but APTT samples deteriorate after 12 hours even with refrigeration. At room temperature, all coagulation parameters (PT, INR, APTT) deteriorate within 12 hours. Optimal results for these tests require analysis within 4 hours if stored at room temperature.
Blood cultures, which detect bacteria or fungi, are time-sensitive. Inoculated blood culture bottles should be transported to the laboratory quickly, ideally within 2 hours, and kept at room temperature. Holding them at room temperature for up to 12 hours before placing them in an analyzer is acceptable, but testing is not possible after this period.
Hormone levels and lipid panels are more stable than some other analytes. For example, total cholesterol, follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), and free thyroxine (FT4) are stable even after long-term frozen storage. Lipid profile components, including total cholesterol, triglycerides, HDL-C, and LDL-C, remain stable for a few hours at room temperature or refrigerated.
Impact of Improper Sample Handling
Improper handling or storage of blood samples, or delays in testing, can lead to inaccurate results. This can result in misdiagnosis, delayed treatment, or inappropriate medical interventions. When analytes degrade, they can lead to falsely low or high measurements, masking conditions or indicating non-existent problems. For example, a delay in separating serum or plasma from cells can lead to changes in glucose and potassium levels.
Hemolysis is a common issue caused by rough handling, vigorous shaking, or improper collection techniques like using a needle that is too small. When red blood cells break open, they release their contents like hemoglobin and potassium into the surrounding serum or plasma. This interferes with many laboratory tests, causing a pink or red tint in the sample and leading to falsely elevated potassium levels or interference with spectrophotometric readings, invalidating the results. Hemolysis is a leading cause of sample rejection in laboratories.
Ultimately, compromised sample integrity necessitates a re-draw, causing patient inconvenience, additional costs, and delayed accurate diagnostic information. Ensuring proper collection, handling, and timely transport of blood samples to the laboratory is important for reliable test results and effective patient care.