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Causes And Countermeasures Of False Positive Blood Culture
Jul 03, 2018

The colorimetric blood culture system uses the colorimeter sensor and reflected light principle to monitor the production and presence of carbon dioxide (CO2) dissolved in the medium. The presence of CO2 in the sample during the metabolism or decomposition of carbohydrates results in a change in the pH of the medium, resulting in a lighter color of the Liquid Emulsion Sensor (LES) at the bottom of the flask, which is continuously illuminated by the illumination sensor. After the signal is collected, a reaction curve is formed for analysis, and the initial signal method, the slope method and the acceleration method are used to obtain a positive signal in time.

In the clinical work, the instrument showed a positive alarm, while the smear stained no bacteria, and the transgenic plate was grown as a false positive. The reasons for false positives may be:

First: Environmental factors:

1. Temperature: The change of laboratory room temperature (such as the sudden change of temperature caused by switching air conditioner, air conditioner directly to the instrument, etc.) causes the reaction curve to fall or rise, causing the instrument to report yang.

2. Power supply: The power supply voltage connected to the instrument is unstable or connected to other frequently used high-power instruments under the same power supply, which will cause the reaction curve to fluctuate, causing the instrument to report yang.

3. Position: The instrument placed next to the long-term window opening, because the incubation hole is easy to absorb a large amount of dust to shield the test light source, resulting in false positive changes in the acquisition signal.

Second: The instrument or culture bottle factors:

1. Instrument: Regularly maintain the instrument and correct the background signal value (difference value) of the incubation hole to ensure the stable working state of the instrument and reduce the probability of occurrence of false positive bottles.

2. Blood culture bottle: Before use, check whether the blood culture bottle is damaged, medium or bottom discoloration, etc. Check whether the new blood culture bottle is kept at normal temperature and protected from light. Direct sunlight will cause the color of the bottom of the bottle to change.

Three: Specimen factor:

1. The cells themselves can also be fermented to produce CO2, which may lead to false positives when the blood volume exceeds the recommended blood collection volume or the recommended blood collection volume contains a large number of cells (such as leukemia, polycythemia).

2. Some rare, harsh microorganisms may be difficult to grow in blood culture flask growth media.

3. When the instrument signal shows positive, the positive bottle should be taken out quickly to avoid some bacteria (such as Streptococcus pneumoniae) from inactive culture due to self-decomposition or other reasons.

4. For some reasons (acidosis, input of a large amount of liquid), the pH value of the specimen itself is acidic, resulting in false positives.

Four: Reason for operation:

1. Anonymous bottle machine: The initial thresholds of different kinds of culture bottles are different, and the instrument uses the lowest threshold to start the calculation when processing the anonymous bottle. The algorithm that triggers the initial threshold causes the instrument to report yang.

2. After the bottling, the incubator is not closed in time to cause false positives caused by temperature changes.

3. The culture bottle was not stoppered to the bottom during the machine.

4. The bottom of the specimen bottle is covered with oil, dye, glass and other objects, causing the instrument to report yang.

After a false positive, follow the steps below to analyze the cause:

1. Check if the instrument has an alarm: check the cause of the instrument status, operating errors and temperature changes.

2. Check the color change of the bottom of the bottle: check for false positives due to environmental problems (voltage, temperature), specimen preservation or bottle bottom contamination.

3. View growth curve: Reasons for troubleshooting operations, specimens, and environmental factors through different growth curve graphs.

4. View clinical medical records, trace the collection and transportation of specimens.

Blood culture is an important method for clinical diagnosis of sepsis. Positive results have extremely high application value for definite diagnosis and symptomatic treatment. For clinically occurring false positive bottles, timely treatment should be conducted to find the cause, and relevant systems should be developed to continuously improve the quality of blood culture, and improve the positive rate and accuracy of reporting.