Capillary tubes are slender, small-bore glass tubes designed to collect precise, small-volume liquid samples for scientific or medical analysis. They rely on the physical principle of capillarity, which allows a liquid to spontaneously climb the interior surface of the narrow tube. This mechanism enables the collection of microliter quantities of fluid, often blood, without requiring external suction devices or pumps, making them widely used in laboratory and point-of-care settings.
Safety Guidelines and Handling Preparation
Before initiating any sample collection, a thorough inspection of the capillary tube is necessary, as the tubes are typically made of fragile glass. Users must check the entire length of the tube for any chips, fractures, or fine cracks that could lead to unexpected breakage during handling or processing. Because broken glass presents a significant laceration risk, cautious handling is required throughout the procedure.
Appropriate personal protective equipment (PPE) must be donned before removing the tube from its packaging. This includes disposable, fluid-impermeable gloves and protective eyewear to shield against potential splashes or shattering glass. Handling the tube should be deliberate and firm, yet gentle, to prevent the sharp edges from causing injury or the tube from fracturing under strain.
Principles of Capillary Action and Filling Techniques
The process of drawing a sample relies entirely on capillary action, a phenomenon driven by the interplay between cohesive and adhesive forces. Adhesion is the attraction between the liquid molecules and the solid inner surface of the glass tube, while cohesion is the internal attraction linking the liquid molecules together.
Because the tube has a very narrow diameter, these adhesive forces overcome gravity and draw the fluid upward spontaneously. To initiate filling, the user simply touches one end of the tube to the surface of the liquid sample. The tube should be held horizontally or angled slightly downward to allow smooth flow and prevent the trapping of air bubbles, which can compromise the sample integrity.
Volume control is achieved by monitoring the liquid column as it climbs the tube or by using tubes marked with a specific fill line. For blood collection, gently apply pressure near the site to encourage a large drop to form, then touch the tube to the drop itself, rather than the skin. This technique prevents scraping the tissue, which can introduce tissue fluid that dilutes the sample and potentially leads to inaccurate results.
Sealing and Post-Collection Processing
Once the appropriate volume has been drawn into the capillary tube, immediate sealing is required to prevent evaporation or leakage of the collected sample. One common method utilizes a specialized sealing clay or wax, typically provided in a small tray. The open end of the tube is pressed into the clay until a secure plug forms, often penetrating a few millimeters into the opening.
In certain laboratory applications not involving biohazardous fluids, flame sealing can be used to permanently close the tube end by melting the glass. Maintaining the seal’s integrity is crucial for preserving the sample’s concentration and preventing contamination during subsequent processing. Following sealing, the tube must be accurately labeled immediately with identifying information to ensure proper sample tracking.
The sealed tubes are then prepared for analysis, often involving placement in a centrifuge or heating block depending on the required test. For example, hematocrit determination requires centrifugation to separate cellular components from the plasma. The sealed ends prevent the sample from leaking during the high-speed spinning process.
Disposal Procedures for Used Capillary Tubes
Capillary tubes must be treated as both sharp objects and potential biohazards after sample collection. They are categorized as sharps waste, regardless of whether they appear to contain residual fluid. Under no circumstances should used capillary tubes be discarded into general waste containers or regular trash bins.
All used tubes must be placed immediately into a designated puncture-proof sharps container designed for medical waste. These containers are typically rigid, leak-proof, and clearly marked with a biohazard symbol. Following this procedure ensures that the contaminated glass does not pose a risk of injury or pathogen exposure to personnel handling the waste downstream.