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There are six indicators for urine.

1.     Yellow stain either darker or lighter than the surrounding carpet. If the yellow stain is darker, then color has been added to the carpet. If it is lighter, then blue and/or red have been destroyed. A yellow stain by itself is not enough to say that urine is present. If it is combined with odor, then chances of it being urine are excellent; if it shines yellow under ultraviolet light, then it is likely urine. But there again, it could be a yellow high-lighter. If it has a moisture reading, then change.

2.     Urine smell can degenerate into a rotten egg odor. If there is a urine smell coming from yellow stain, then the evidence is likely good enough for small claims. Even though a urine smell is strong evidence, the lack of an odor is common, especially where some deodorants have been used.

3.     Yellow reflection under ultraviolet light. For some carpet mills, that is all they want. However, urine is not the only thing that shines yellow under ultraviolet light and sometimes old urine does not shine at all.

4.     Moisture reading will occasionally go off the scale.

5.     Acidic pH reading. Since relative readings are what is important, a reading that is 0.3 less than the surround carpet can be an indicator of urine.

6.     Ammoniated Salts can be measured with an Ammoniated Strips, Markson catalog MXR246529 for $49.00 The ammonium strips require refrigeration. Ammonium salts, (NH4+), are not found in cleaning compounds and it is not to be confused with quaternary ammonium chloride which can also turn stain resist nylon yellow. If ammoniated salts are not found, then it is very likely that the spot was not created by urine. The urine odor is an amine compound which is closely related to ammoniated salts.

If either a reflection or a smell is inconclusive, then check for moisture and/or pH. Moisture readings in excess of 17 percent are good enough to support micro- organisms. A pH reading lower than the surrounding area (generally in the low 6s) is also indicative of urine. Having both moisture and lower pH is strong evidence.

The chart below is a guideline on how to evaluate combinations of results. The + indicates that that line item combined with that number plus any other item, i.e. any combination of three.


Do more testing

Fair Chance

Good Chance

Excellent Chance

1.   Yellow stain

By itself

3 or 4 or 5

3+ or 4+ or 5+

Plus 3, 4, & 5

2.   Urine Odor



By itself

Plus any

3.   UV Reflection

By itself

1 or 4 or 5

1+ or 4+ or 5+

Plus 1, 4, & 5

4.   Acidic pH

By itself

1 or 3 or 5

1+ or 3+ or 5+

Plus 1, 3, & 5

5.   Moisture

By itself

1 or 3 or 4

1+ or 3+ or 4+

Plus 1, 3, & 4

6.   NH4 Salts

If none is found


By itself

Plus any