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Test strips often give false negative and false positive and inaccurate results because they react with chemicals in the water, other than those they are designed to test for. Without proper chemical pretreatment of a sample, if are relying on results from test strips, you probably have a very inaccurate picture of what is in your drinking water. False negatives are probably the most dangerous, because the test strip indicates that the contaminant is absent, when in fact it is present and can present a health hazard.


There are certain circumstances under which they are useful and sufficiently accurate, but without a complete water test using proper collection methods, proper preservation of samples, and proper approved testing procedures, you will almost surely have inaccurate values for the various components you are testing for. They are not designed to substitute for a proper water test.


Once proper testing has been done by a certified laboratory, one can use test strips to monitor for changes in values. Also, in non-critical applications, such as the amount of calcium in the water, they can be used to monitor values AFTER a proper test has been done. But if you are relying on a $3.00 test strip to determine whether or not to install a $1000.00 purification system, you may want to think again.


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Test Strips: Interferences and How to Use Them Correctly

The proper use of test strips actually requires more knowledge and understanding of water chemistry than most people have. There are no detailed warnings about interferences on most store-bought test strips or home use chemical test kits. In fact, this may be the only place you ever hear about it unless you enjoy reading technical literature about water chemistry.

 Point: Take a sample of water. Dip the test strip in. Wait the prescribed period of time. Compare the color on the test strip to the manufacturer’s color chart. The color that most closely matches your test strip color has a number value that can be read from the chart. That numeric value is how much of the substance is in your water. Right? Maybe, sometimes, but more often it is incorrect and gives the tester a false sense of confidence.

 Why? Because almost every parameter that test strips are used for, is subject to interferences. These cause incorrect (false positive and false negative) readings at times, and always cause inaccurate readings. (Nitrate, Hardness, and Alkalinity are some of the worst)

So what kinds of things interfere with proper color development on test strips? Time between dipping the test strip and reading the result, pH of solution, chemicals that are present (other than the parameter being measured), temperature and the concentration of total dissolved solids, etc.. In addition to this, no two human eyes see color exactly the same way, so this can introduce some additional error.

Of these, the most insidious component is the presence of other chemicals in the water being tested. This can easily go undetected and can give you grossly incorrect readings, despite the fact that everything looks like its working properly. Every kind of test strip has some chemical agent that interferes with a proper reaction! So this is not the exception, instead it is the rule that test strips are basically inaccurate and unreliable unless you know exactly how and when to use them.

For example, I may want to do what the EPA suggests and test my water for nitrates at least once a year, but I don’t want to spend $10.00 - $25.00 to have it done professionally. So I buy a nitrate test strip from the local home improvement store (for $7.00), run some water into a glass, dip the strip, read the color and determine (based on color matching) that there are less than 2 ppm (parts per million) of Nitrate in my drinking water. The EPA says anything under 10 ppm is safe to drink, so I conclude that my family and I will be OK drinking this water for another year.

If there were nothing else in my drinking water and the pH were 7.0, then this might be true. But, if my drinking water has a pH of 6.0 or 8.5, or if it has one of the very common drinking water chemicals such as amines, chloride, chlorine (can give false positives), copper, iron (can give false positives), sulfides etc. in it, then this result may be way off. They can be in error as much as 10 to 100 fold or more, without any warning that these results are incorrect. Interferences may indicate that nitrate is present, when there is none. It may indicate that there is a small amount of nitrate present, when there is really an unsafe level.

Imagine that I first ran a test to determine the pH, ammonia, chloride, chlorine, copper, iron, and sulfide levels at the same time that I test for nitrates. If that testing showed that the interfering substances are not present in significant amounts, and my nitrate test showed that there are 2ppm of Nitrate in my drinking water, then I can be much more confident that the result is accurate. If I had the testing done professionally, and simultaneously used a test strip, and the values were close to the same, then I could feel even more confident that the test strip results are reasonably reliable and accurate. And this is the true, intended purpose for these “quick and dirty” little devices.

So what is the solution? Test the water at least once for all the parameters that may cause interferences along with the parameter of interest. Once you have established that interfering substances are not usually present in your water, then – and only then, can you rely on the test strip results as being reasonably accurate and reliable. That’s the proper use of test strips. To monitor a water supply of known chemical composition for a specific parameter, once it has been established that there are no significant interferences.

You can find this article for free at along with other very helpful information. Dr. Essich has a free “ask the pro” link at that site and will be happy to answer any questions you may have FOR FREE, by email as he has time to get to them.

Happy testing.

Eberhard Essich, Ph.D.




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Last modified by Dr. Eberhard Essich 03/22/17