The urinalysis test strip, often called a dipstick, is a simple but powerful diagnostic tool used worldwide to screen for various health conditions. This plastic strip has several small, chemically-impregnated pads affixed to it. Each pad is designed to react with a specific component in the urine, such as glucose, protein, or blood. When dipped into a urine sample, these pads undergo a color change that is compared to a reference chart to determine the substance concentration. The accuracy of this initial assessment relies heavily on the correct technique used during the brief testing procedure. A single step—removing excess urine—is essential for a reliable result.
Why Removing Excess Urine is Essential
The physical removal of excess fluid is necessary to prevent a phenomenon known as chemical runover or leaching, which compromises the test’s integrity. Reagent strips are designed to keep each chemical pad isolated, but too much liquid acts as a bridge. This allows chemicals to wash from one pad to an adjacent one, causing cross-contamination and leading to highly inaccurate results, such as false positives or false negatives.
For example, the acidic buffer on the protein test pad can easily run onto the neighboring pH pad, artificially lowering the measured pH result. Excess urine can also dissolve the dry reagents and leach them out prematurely, leading to a weak or false-negative result for high-sensitivity tests like glucose or protein. Controlling the amount of residual urine confines the necessary chemical reactions to their intended areas, ensuring the accuracy of the color-change comparison and maintaining chemical isolation.
The Correct Technique for Excess Urine Removal
The standard procedure for using a reagent strip is quick and requires precise movements to ensure the pads are adequately saturated but not oversaturated. Begin by briefly immersing the reagent pads completely into a well-mixed urine specimen for no more than one to two seconds. Immediately upon withdrawing the strip, draw the edge of the strip firmly against the rim of the specimen container. This action removes the gross excess of urine that would otherwise drip down the strip.
Next, the edge of the strip must be gently blotted onto a clean, absorbent material, such as a paper towel or specialized blotting paper. Hold the strip horizontally during this step and only allow the non-reagent side or edge to touch the absorbent surface. Blotting the edge wicks away any remaining liquid without physically touching or disturbing the reagent pads themselves. This ensures that just enough urine remains absorbed within the pads to facilitate the chemical reaction without causing fluid leakage or cross-contamination.
Common Procedural Errors to Avoid
Several common errors related to excess urine removal can quickly invalidate a test result, even if the dipping technique was perfect. A frequent mistake is vigorously shaking the strip to remove excess urine, which causes the liquid to splatter and run between adjacent reagent pads, leading to runover. Another error is mistakenly pressing the entire strip surface onto the absorbent material, known as over-blotting. This forceful action can remove the necessary reagents from the pad, causing a false-negative result due to insufficient chemical reaction.
Prolonged immersion of the strip in the urine sample is another error that interferes with the excess removal process. Soaking the strip for more than a few seconds can prematurely leach the reagents out of the pads, especially the buffers, which are designed to maintain the reaction’s chemical environment. Furthermore, failing to hold the strip in a horizontal position after blotting encourages residual fluid to run down the strip’s length, leading to mixing and color distortion.
Interpreting Results and Reaction Timing
Once the excess urine has been correctly removed, the chemical reactions on the reagent pads begin immediately, making accurate timing the next crucial step. Each chemical test on the strip has a specific, manufacturer-defined reaction time, often ranging from 30 to 120 seconds. For example, the test for blood may require a reading at 60 seconds, while the leukocyte pad needs a longer interval, sometimes up to 120 seconds, for complete color development.
Reading the results too early or too late is a major source of error after poor technique in excess removal. Reading the strip before the specified time means the color change has not fully stabilized, leading to a falsely low reading. Conversely, reading the strip after the maximum recommended time, typically two minutes, allows for non-diagnostic color shifts to occur, which can lead to a false positive. Therefore, using a timer to compare each pad to the color chart at its exact moment is essential for a reliable diagnosis.