Test Result Interpretation

On all laboratory reports you will see a variety of units and symbols. It is important to understand what these symbols are and what they mean for result interpretation. You may be required to convert your symbols and understanding what they are will make that process easier. The chart below are the common (but not all) units the SELS uses for reporting.

UNITS & CONVERSIONS

ppb/ppm Solids Liquids Microbiology Radiochemistry Temperature Symbols Qualifiers
ppm: Parts Per Million

The number of parts of solute per 1 million parts of total solution. This can be applied to liquid and solid measurement.

Also Known As: mg/L; mg/kg; μg/g

In one Olympic size swimming pool, 1 ppm equals 1.25 liter bottles

Example: 5 ppm of lead

5 grams of lead in 1 million grams of solution OR 5 milligrams of lead in 1 million milligram of solution OR 5 pounds of lead in 1 million pounds of solution

ppb: Parts Per Billion

The number of solute per 1 billion parts of total solution.This can be applied to liquid and solid measurement.

Also Known As: μg/L; μg/kg

In one Olympic size swimming pool, 1 ppb equals 0.5 teaspoon

Example: 5 ppb of lead

5 grams of lead in 1 million grams of solution OR 5 milligrams of lead in 1 million milligram of solution OR 5 pounds of lead in 1 million pounds of solution

μg/g: Microgram Per Gram

Also Known As: ppm

mg/kg: Milligram Per Kilogram

Also Known As: ppm

μg/kg: Microgram Per Kilogram

Also known As: ppb

Helpful Conversions
  • μg/g → mg/kg
    • 1 μg/g = 1 mg/kg
mg/L: Milligram Per Liter

Also known as: ppm

μg/L: Microgram Per Liter

Also known as: ppb

Helpful Conversions
  • μg/L → mg/L
    • n μg/L ÷ 1,000 = mg/L
  • mg/L → μg/L
    • n mg/L x 1,000 = μg/L
MPN: Most Probable Number

Estimate  of concentration of viable microorganisms in a sample by means of replicate liquid broth growth in ten-fold dilutions.

Example: 50 MPN/100 mL

50 MPN/100 mL means that the Most Probable Number of viable cells in 100 mL of sample is 50

Present or Absent

The presence or absence of microorganisms

Also known as: P/A OR +/-

For public water supplies’ drinking water (water that has been chemically treated). By law, EPA states that the presence of any bacteria in drinking water must be treated (regardless of bacteria quantity).

CFU: Colony Forming Units

Number of viable bacterial cells in a sample per unit of volume

Example: 50 CFU/100mL

50 CFU/100 mL means 50 Colony Forming Units per 100 mL of sample

pCi: Picocuries

The curie is a standard measurement for the intensity of radioactivity contained in a sample. It’s not a measurement of volume or mass like in liquids or solids but the measurement of decay of radioactive atoms per minute. The basis for the curie is radioactivity of one gram of pure radium. Radium decays at a rate of apporxiametly 2.2 trillion disintegrations per minute. A picocurie is equal to the decay of two radioactive atoms per minute.

Example: 4 pCi/L

4 pCi/L means 8 atoms decaying every minute in every liter of water

DPM: Disintegrations Per Minute

The number of atoms that decay in one minute

C: Celsius

Measurement of temperature

0 °C is the freezing point of water

100 °C is the boiling point of water

Helpful Conversion
  • Celsius to Fahrenheit:
    • (n °C x 1.8) + 32
< : Less Than

Denotes a value being smaller than the number in comparison.

When this symbol appears on a SELS report, it means the value was less than the analytical reporting limit.

J Qualifier

Indicates an estimated value

MI Qualifier

Indicates a matrix interference and results could be biased high or low

HT Qualifier

Indicates hold time was exceeded before sample receipt or sample analysis

IP Qualifier

Indicates sample was improperly preserved

BR Qualifier

Indicates sample was received broken

LA Qualifier

Indicates a lab accident occurred and the sample can’t be analyzed

 

 

Even though some units look different, they can actually be equivalent. This can get confusing when you begin comparing different laboratory reports or looking at EPA’s MCLs. Be prepared to convert and understand what units are equivalent.

Example: Result is 6 mg/l

6 mg/L = 6 ppm = 6,000 ppb = 6,000 μg/L

 

HOW TO READ RESULTS

LIQUID

For concentrations of chemicals in water, they are measured in units of the mass of chemical (mg or µg) per volume of water (L). This is written as mg/L or µg/L. For example, your SEL results show 51.1 µg/L for Lead. This means for every 1 liter of water sample, there is 51.1 µg of Lead (or 0.0511 mg/L).

SOLIDS

For concentrations of chemicals in soil samples, they are measured in units of the mass of chemical (mg or µg) per mass of soil (kg). This is written as mg/kg or µg/kg. For example, your SEL results show 51.1 µg/kg for Lead. This means for every 1 kilogram of soil sample, there is 51.1 µg of Lead (or 0.0511 mg/kg).

TEST RESULTS

Understanding your results is an easy three step process;

  1. Compare
  2. Convert (if applicable)
  3. Conclude
EXAMPLE 1

COMPARE

Once you have your results, compare your number to the EPA’s regulated Maximum Contaminant Levels (MCLs). For public water supplies, these are numbers that are legally enforceable standards that are applied to drinking water. For private citizens, the MCLs can serve as a great guideline for evaluating your results.

In this example, the “Component Name” is the analyte that you requested; Chlorite. Your result is 636 µg/L. Now, we look at the EPA’s MCLs and see that the Maximum Contaminant Level (MCL) amount of Chlorite that you can have in your result is 1.0 mg/L.

 

EPA’s MCL for Chlorite

CONVERT

When comparing the numbers, it may seem alarming to see such a large number; 636 µg/L. However you will notice that the EPA MCLs are reported in mg/L unlike SELS (SELS reported in µg/L). The next step to interpreting results is to ensure all the units from SELS and EPA are the same. This is especially import for Public Water Supplies needing to report for compliance.  First convert your test result that was reported in µg/L to the same unit as EPA’s mg/L.

CONCLUDE

You have 0.636 mg/L of Chlorite per 1 Liter of water which is below the regulated 1.0 mg/L MCL established by EPA for public drinking water supplies.

EXAMPLE 2

COMPARE

Once you have your results, compare your number to the EPA’s regulated Maximum Contaminant Levels (MCLs). For public water supplies, these are numbers that are legally enforceable standards that are applied to drinking water. For private citizens, the MCLs can serve as a great guideline for evaluating your results.

In this example, the “Component Name” is the analyte that you requested; E. Coli and Total Coliform. Your result for both analysis is Absent. Now, we look at the EPA’s MCLs and see that the Maximum Contaminant Level (MCL) amount of E. Coli and Total Coliform that you can have in your result is 5.0%. This 5.0% is in reference to the number of samples a Public Water Supply is required to collect during a compliance period (click here for Compliance Guidance). A private citizen should compare to the MCLG (Maximum Contaminant Level Goal); zero.

EPA’s MCL for Total Coliforms and E. Coli

 

CONVERT

When comparing these two results, Absent and zero, it easy to understand that they are equivalent.

CONCLUDE

Your water is Absent of E. Coli and Total Coliforms which is the same as EPA’s MCLG of zero.

EXAMPLE 3

COMPARE

Once you have your results, compare your number to the EPA’s regulated Maximum Contaminant Levels (MCLs). For public water supplies, these are numbers that are legally enforceable standards that are applied to drinking water. For private citizens, the MCLs can serve as a great guideline for evaluating your results.

In this example, the “Component Name” is the analyte that you requested; Arsenic. Your result is <1.00 µg/L. Now, we look at the EPA’s MCLs and see that the Maximum Contaminant Level (MCL) amount of Arsenic that you can have in your result is 1.0 mg/L.

CONVERT

In this example, there is nothing to convert. When a report shows a result of <1.00, that means the actual value of the sample was less than the analytical reporting limit.

CONCLUDE

Your sample was less than the MCL of arsenic.

 

NEED A LITTLE MORE ASSISTANCE

Still unsure about your results? Give us a call at 405-702-1000 or email us at selsd@deq.ok.gov

Make sure you have your results on hand when contacting us. Below is the information you will need in order for us to better serve you.

  1. Sample Number (yellow): This is a unique ID that SELS gives to your specific sample
  2. Facility ID & Sampling Point (green): For Public Water Supplies only
  3. Component Name (red): This is what your sample was analyzed for
  4. Result (purple): What your analysis concluded