Sustainable Hospitals

Interpreting Analytical Results for Mercury and Other Substances

In many hospitals, purchased products, laboratory reagents, or waste samples are tested to determine their mercury content for regulatory compliance. To get the most out of testing, it is important to understand how to read and interpret the results.

Q. How does one start to read a lab report?
The first step is to look for the "Lowest Detection Limit" (LDL) for the test. This tells you the lowest value at which the test is known to be accurate and precise. What this means is that if the sample contains mercury below this level, the test cannot accurately measure it.
   The detection limit may be hard to find on a report form. It may be at the bottom of the test results page, the beginning of the report, or at the end of the report. If you cannot find the detection limit for the test run, always contact the laboratory that performed the analysis. You cannot interpret results without knowing the LDL.

Q. What is the next step?
Look for the section where results are reported. There may be several columns, such as:
Component     Results Concentration Units
Mercury Examples: Examples:

BQL (below quantitative limits)     %

ND (not detectable) mg/L

<0.2 mg/kg

0.222 g/L

To interpret the results, you will need to know 1) the result, 2) the concentration units, and 3) the detection limit (LDL) for the test. You cannot interpret ND, DQL, or < results without a detection limit for the test run.

Q. Does a result of "Not Detectable" (ND) mean the sample is mercury free?
No. ND should never be interpreted as a concentration of 0. This is where the LDL (lower detection limit) comes in. ND means that the test cannot tell if mercury is present if its concentration is below the LDL. Therefore, mercury may be present. Any one of the following things may be happening, but without a more sophisticated test one can't tell which is true:

  • There may truly be no mercury present.
  • The test method is not sophisticated enough to measure mercury which is actually present at low levels (below the LDL).
  • The chemical makeup of the solution interferes with the testing so that mercury is actually present but wasn't detected. This is called the matrix effect and what it means is that other components in the liquid throw off the results.
Q. What does "Below Quantitative Limits" (BQL) mean?
This is another way of saying that the results were below the lowest detectable limits of a particular test. This should not be interpreted as 0.

Q. What about a result of "<" (less than)?
The results were less than the lowest detection limit. Again, this should not be interpreted as 0. For example, result <2.0 mg/L. The detection limit for the test run was 2.0 mg/L. The analysis of the sample detected no mercury at this concentration. It could still contain mercury below this level.

Here are some examples of how to interpret results:


Test Detection Limit (LDL)

What this means
<2.0 mg/L
2.0 mg/L No mercury was detected at a concentration of 2.0 mg/L. However the sample could still contain mercury below this level. (A test with a lower LDL could determine lower levels of mercury in the sample).
2.2 mg/L 2.0 mg/L     Mercury is present at a concentration of 2.2 mg/L

Q. What is matrix interference?
In some cases, the chemical makeup of the solution interferes with the testing. This is called the matrix effect and what it means is that testing results are thrown off by other components in the liquid. What you are looking for in the sample (the analyte, e.g. mercury) will determine potential interferences. Good lab analysts will know what interferences are common for a given analyte and if you ask prior to testing, you might ascertain whether those interferences are likely to be present. Many interferences can be managed by slightly altering the analysis technique to yield more accurate results. Therefore, addressing the possible interferences prior to analysis will yield better results.

Q. How are results compared with regulatory standards?
In most cases, the purpose of testing samples is to determine whether a waste discharge will be in regulatory compliance. To compare sample results with a regulatory standard or limit, two pieces of information are needed: the concentration of mercury in the sample and the allowable concentration standards. Test results for the waste sample must be compared with that standard to ensure that the sample is below the limit. It is important that units of concentration are the same for the standard and the sample, otherwise the comparison is invalid.

For more informationů

You can find out more about alternative products at the Sustainable Hospitals Project (SHP) website, or by contacting the SHP at 978-934-3386 or email

back to factsheet listing page


R E G I S T E R     G L O S S A R Y     F E E D B A C K     S I T E  M A P     H O M E

Copyright © 2003 Sustainable Hospitals / Lowell Center for Sustainable Production
All rights reserved.
Images copyright © 1998 PhotoDisc, Inc.