Quantitative ATP tests vs qualitative ATP tests
You need to monitor the microbial quality of your water? You need an ultra-fast, easy-to use, sensitive, reliable and accurate method…
ATPmetry is a technique that measures ATP quantity. It is known to evaluate instantaneously and efficiently the quantity of microorganisms of a water sample.
However, ATP tests performances vary strongly depending on the kit.
This study compares different commercial ATP tests for the monitoring of bacteria in water. ATPmetry is a very fast analysis method that is used on the field. Results are obtained in less than 2 minutes, thus allowing early-responsiveness. The DENDRIDIAG® kit, 2nd generation ATPmetry, includes sample concentration and internal calibration. It is compared to several first-generation ATP kits:
- AquaSnap® (Hygiena)
- AccuPoint® (Neogen)
- LIGHTNING MVP ICON® (Biocontrol Systems)
All the kits include the sampling system, the reagents and the matching luminometer. The different ATP monitoring tests are assessed based on several criteria:
- Linearity: gives the measuring interval for each kit.
- Repeatability: gives the measurement precision by replicate measurements on the same sample by a same operator.
- Sensitivity: research of contamination traces in sterile water
- Robustness: influence of the environment on the measure
What is the measuring interval of my kit?
Analysis of the linearity gives the measuring interval. A solution of ATP is diluted in sterile water. Each solution is tested 3 times with each method. This study does not take into account the lysis efficiency of the kit.
“The straighter the line, the better it is.”
DENDRIDIAG, Clean-Trace and AccuPoint: ATP monitoring tests show great linearity from 0 to 4.00 LOG of picogram of ATP per ml.
MVP ICON and AquaSnap are saturated above 3.00 LOG of ATP. Furthermore, the curve starts to bend after 2.00 log of ATP. The measuring interval is limited. In other words, above 100-1000 pg/ml, those kits will not display accurate results.
Does the result change if the analysis is done several times by the same operator?
The measure of dispersion helps us study the repeatability of each kit. In other words, we observe the variability of the result when the measurement is done by a same operator on the same sample.
“For repeatable measurements, variations must be as low as possible.”
The dispersion is measured through the coefficient of variation (CV) which should be the lowest possible.
DENDRIDIAG®: CV = 8%
Clean-Trace: CV = 33%
MVP ICON: CV = 16%
AquaSnap: CV = 90%
Accupoint: CV = 70%
First generation ATPmetry kits analyse water from 100 µl samples. The sampling volume can vary from 80 to 160 µl depending on the operator. Furthermore, with such a small volume, it is difficult to get a representative sample of the monitored water. Consequently, result variations are very high for a same sample.
The key value of DENDRIDIAG® kit is the filtration step. 10 ml to 1 liter of water sample are concentrated in 100 µl making the analysis representative. In this way, variations due to the heterogeneity of the water are strongly limited.
Sensitivity – Research of contamination traces
The main goal is to compare the sensitivity of the different ATP monitoring tests. To do this, we are looking for the smallest volume of water which can be detected in 1 liter of sterile water.
Which volume of waste water can I detect in 1 liter of sterile water?
DENDRIDIAG: detection of 10 µl of waste water in 1 liter of sterile water.
AquaSnap and Clean-Trace: detection of 1 ml of waste water in 1 liter of sterile water.
AccuPoint: detection of 10 ml of waste water in 1 liter of sterile water.
MVP ICON: detection of 100 ml of waste water in 1 liter of sterile water.
Search for contamination traces requires high sensitivity. DENDRIDIAG® kit has a major advantage: sample filtration before analysis. Indeed, the procedure includes concentration of bacteria on a membrane. Filtration helps reach higher level of sensitivity without extending the handling time. Thereby, you can gain on the sensitivity threshold by a factor of 100 compared to first generation kit.
Are my results affected by environmental factors (temperature, pH…)?
Environmental factors are different for each water sample. Hot domestic water’s temperature is obviously very different from drinking water’s temperature for example. It also differs whether it is summer or winter. Same is true for the pH, salt content, etc…
In this study, we chose to play on the temperature and pH of a surface water. It is analysed 3 times with each ATP tests and measurement variations are assessed. Robustness evaluates the capacity of a method to maintain its performances when parameters are modified.
“For reliable results, variations must be as low as possible.”
Tested parameters: 22°C – pH8; 60°C – pH8; 4°C – pH8; 22°C – pH4; 22°C – pH10
Variations are measured by the coefficient of variation which must be the smallest possible.
DENDRIDIAG: CV = 6%
AquaSnap: CV = 58%
Clean-Trace: CV = 64%
MVP ICON: CV = 109%
Accupoint: CV = 133%
Regarding robustness, DENDRIDIAG® ATP tests is largely more efficient than other kits thanks to internal calibration. This standardisation takes into account the environmental factors. Indeed, all ATPmetry kits use an enzyme: the luciferase. It is sensitive to the environment (temperature, pH, inhibitors, activators…). For instance, if the sampled water is very cold, the enzyme will have a reduced activity. Therefore, the result obtained will be lower than reality. It then leads to high rate of false negatives.
Addition of a known quantity of ATP in each sample helps you avoid this effect and makes the analysis much more reliable.
Second generation DENDRIDIAG® kit clearly differentiates itself from first generation ATPmetry kits. It has 2 major advantages:
- Filtration: it concentrates microorganisms of the sample to reach very low level of sensitivity. In this way, it is possible to monitor the microbial quality of drinking water and ultra-pure water. Up to 10 µl of waste water can be detected in 1 liter of sterile water.
- Internal calibration: after each measurement, the operator adds a known quantity of ATP to standardized the measure. Consequently, environmental factors are considered. This internal calibration prevents high rate of false negatives. Besides, it makes it possible to compare results over time and space and for different luminometers.