Pocket Tips: Go with the flow
Like any diagnostic equipment, there are right and wrong ways to use Pocket Diagnostic tests. In this article we look at some of the problems you might face when running the tests – and give suggestions on how to improve your results.
Flooding
Adding too much sample to the device will cause ‘flooding’, and any result obtained can not be relied upon. Flooding of the test device causes the extraction buffer and sample to pass over the surface of the membrane inside the plastic housing, rather than flowing through the membrane. Unless the buffer flows through the pores inside the membrane, the reactions which produce accurate test results can not take place.
Signs of flooding include visible liquid in the sample well, and the membrane appearing to be wet through the viewing window. The C (control) line may not appear (although it is present in the images shown here).
This image also shows a high level of debris in the sample, and a large air bubble. Air bubbles can be removed by gently squeezing the test strip, or tapping the end gently on a firm surface.
To solve the problem: discard the flooded test and apply 2-3 drops of sample to a new test device.
Debris and foreign objects
One of the most common causes of test failure is the debris and foreign bodies in the sample. Root samples are the worst for this problem, since growing medium is often attached to the roots when put in the extraction bottle. The real problem here are the fine particulates which can be loaded into the sample well then block the membrane flow.
The test in the image (right) shows clear signs of fine particles in the sample which have caused staining of the membrane. Even though the control line is visible, results from tests looking like this should be treated with caution.
Solve the problem by washing growing media off root samples, and by being careful to avoid picking up debris in the dropper pipette.
Weak or missing lines
Faint T line
If the C (control) line appears normal, then a weak T (test) line most commonly indicates a low level of target pathogen in the sample. Even faint lines do indicate a positive result. However, if the lines are any other colour than blue, the result should not be read as positive (some sample types can produce weak brown or green lines which indicates non-specific binding to the T line and a negative result).
The image shows a weak blue T line, which can be interpreted as a weak positive result.
Faint or missing C line
If the C line is weak or missing, the test result should be ignored. The most common cause of a missing or weak C line is the presence of a high concentration of sample in the buffer. This can inhibit flow of the reagents along the membrane and prevent lines from forming. Samples taken from plants with bacterial diseases are more likely to show this problem, because bacterial cells can clog the membrane.
The test on the left has been opened after running to show a failure of the C line to appear due to blockage of the memberane with the highly concentrated plant extract used as a sample (shown by the green colouration). The test on the right in this figure has been run with a sample containing high levels of a bacterial pathogen. The T line has fomed, but the C line is very faint. This result should be considered as invalid.
To overcome this problem, try using a smaller amount of sample in the extraction buffer and loading onto a new test strip.
Technical advice on the use of Pocket Diagnostic tests is always available to users. If you are uncertain about any results you obtain, please get in touch via email or phone.