The colorimetric blood culture system employs a colorimeter sensor and the principle of reflected light to monitor carbon dioxide dissolved in the culture medium ( CO ₂ ) of their generation and existence. The microorganisms present in the sample produce [substances] during metabolic processes or the breakdown of carbohydrates. CO ₂ Leading to the pH of the culture medium ( pH ) changes, thereby causing the semi-permeable silicone material at the bottom of the culture flask ( Liquid Emulsion Sensor ， THESES ) The color becomes lighter, and after continuously acquiring signals via the irradiation sensor, a response curve is generated for analysis. Positive signals are promptly detected using three computational methods: the initial threshold method, the slope method, and the acceleration method.

In clinical practice, the instrument indicates a positive alarm, yet bacterial organisms are not observed on stained smears, and subculturing onto agar plates yields sterile growth. False positive . That leads to the occurrence of false positives The reasons may include ：

I. Environmental Factors:

1. Temperature: Variations in laboratory room temperature—such as abrupt temperature changes caused by turning the air conditioner on or off, or direct airflow from the air conditioner onto the instrument—can lead to a downward or upward shift in the response curve, triggering positive alarms on the instrument.

2. Power Supply: Unstable power supply voltage at the instrument’s connection, or the simultaneous operation of other high‑power devices on the same circuit, can cause fluctuations in the response curve, resulting in positive‑error alarms.

3. Location: Instruments placed adjacent to a window that is frequently open are prone to accumulating substantial dust in the incubation wells, which can obscure the test light source and lead to spurious positive results in signal acquisition.

II. Instrument or Culture Flask Factors:

1. Instrument: Perform regular instrument maintenance and calibrate the background signal values of the incubation wells ( difference Value) Ensures the instrument operates in a stable state, thereby reducing the likelihood of false-positive results.

2. Blood culture bottle: Before use, inspect blood culture bottles for any signs of damage, discoloration of the medium or the bottom, and verify that new bottles have been stored at room temperature away from light; direct sunlight can cause changes in the color of the bottle bottom.

3. Specimen Factors:

1. Cells themselves can also produce it through fermentation. CO ₂ In cases where the blood volume drawn exceeds the recommended amount, or when the sample contains a high number of cells—such as in conditions like leukemia or polycythemia—a false-positive result may occur.

2. Certain rare, fastidious microorganisms may be difficult to grow in blood culture broth.

3. When the instrument indicates a positive result, the positive vial should be promptly removed to prevent certain bacteria (such as Streptococcus pneumoniae) from becoming non‑viable due to autolysis or other factors.

4. Certain factors (such as acidosis or the infusion of large volumes of fluid) can alter the specimen itself. pH A slightly acidic pH can lead to false-positive results.

4. Operational Reasons:

1. Anonymous Bottle on the Machine: Different types of culture flasks have different initial thresholds, and when the instrument processes anonymous flasks, it begins its calculations using the lowest threshold. The algorithm for triggering the initial threshold causes the instrument to report a positive result.

2. False positives caused by temperature fluctuations resulting from the incubation chamber not being promptly sealed after bottling.

3. During the procedure, the culture flask was not sealed tightly.

4. The bottom of the specimen vial is covered with oil, dye, glass fragments, and other foreign matter, causing the instrument to report a positive result.  

After a false positive occurs, the following steps can be followed to analyze and identify the cause:

1. Check whether the instrument has triggered an alarm: investigate potential causes such as instrument status, operator error, and temperature fluctuations.

2. Examine changes in the color at the bottom of the vial to rule out false positives caused by environmental factors (voltage, temperature), specimen storage conditions, or contamination at the vial base.

3. View the growth curve: Use various types of growth‑curve plots to identify potential causes related to operational procedures, samples, environmental conditions, and other factors.

4. Review clinical records to trace the specimen collection, transportation, and other related processes.

Blood culture is an essential diagnostic tool for sepsis, and a positive result is of great clinical value for establishing a definitive diagnosis and guiding targeted therapy. False‑positive bottles encountered in clinical practice should be promptly addressed to identify the underlying causes, and appropriate protocols should be implemented to ensure continuous quality improvement in blood culture testing, thereby enhancing both the positivity rate and the accuracy of reporting.