False-positive Information and Courses from MediaLab, Inc.

A false positive result for blood on the reagent strip can occur when oxidizing contaminants, such as hypochlorite (bleach), remain in collection bottles after cleaning. Contamination of the urine with provodine-iodine, a strong oxidizing agent, used in surgical procedures can result in a false positive reaction. Microbial peroxide found in association with urinary tract infections may also cause false-positive results. Capoten® (Captopril) can cause decreased reactivity. The muscle tissue form of hemoglobin, myoglobin is a well-known cause of false-positive reactions on the blood portion of the reagent strip. When tissue hemoglobin is present, the urine specimen has a clear red appearance. Patients suffering from muscle-wasting disorders or muscular destruction due to trauma, prolonged coma, or convulsions or individuals engaging in extensive exertion may have myoglobin in their urine. Specific tests for myoglobin, such as immunodiffusion techniques or protein electrophoresis, are needed to confirm the presence of this substance in a urine specimen. Levels of ascorbic acid normally found in urine do not interfere with this test.

A routine reagent strip protein method, based on the principle of "protein error of indicators," produces a visible colorimetric reaction that is capable of detecting most instances of proteinuria.Traditionally, laboratories have used sulfosalicylic acid (SSA) to confirm all positive protein reagent strip results, but this practice is not as common in today's laboratories. SSA is a precipitation method that reacts with all forms of protein. However, any substance that is precipitated by acid will produce false-positive SSA results. This includes radiographic dyes, cephalosporins, penicillins, and sulfonamides. SSA may be used as a secondary protein detection method if the urine is highly alkaline (pH of 9.0 or greater) which would overwhelm the buffering capacity of the reagent on the protein reagent stick. SSA may also be used as an alternative protein detection method if the urine is highly colored so that the colorimetric reaction is masked on the reagent strip.

This phlebotomist violated hospital procedures in several ways that could adversely impact patient care: Cleaning the site only with alcohol, not iodine, could result in a false-positive contaminated blood culture. This might result in the patient receiving unnecessary intravenous antibiotics, and could prolong the patients hospital stay unnecessarily. Drawing both cultures at the same time lessens the chance of recovering a bloodstream organism.Drawing both cultures from the same site might result in both of them being contaminated, making it very difficult for the physician to distinguish contamination from a “real” bloodstream infection.Relevant topics:Blood cultures: introduction, Avoid skin contamination, Blood culture site preparation 1, Blood culture site preparation 2