Published in Environmental Protection Magazine, 10, 37-41, (May, 1999)
?!? PQLs, MDLs, EDLs, MQLs, SQLs, EQLs, LLDs, MDCs, IDLs, LOQs, LODs, CRQLs, CRDLs ?!? With the number of terms in use, it's easy to understand the confusion over laboratory reporting limits. Many of these terms are used interchangeably and several refer to the same thing. However, some have significantly different meanings. When making decisions based on laboratory results, it is critical that the engineer or scientist understands these terms. Then they know exactly what the lab is saying when a compound is reported as not present. This is especially true for detailed site assessments, risk assessments, and statistic-based site monitoring.
HMACTTO (HOW MANY ACRONYMS CAN THEY THINK OF)?
Before trying to define these terms, let's first look at the words behind the acronyms.
CRDL Contract Required Detection Limit
CRQL Contract Required Quantitation Limit
EDL Estimated Detection Limit
EQL Estimated Quantitation Limit
IDL Instrument Detection Limit
LLD Lower Limit of Detection
LLQ Lower Limit of Quantitation
LOD Limit Of Detection
LOL Limit Of Linearity
LOQ Limit Of Quantitation
MDC Minimum Detectable Concentration
MDL Method Detection Limit
MQL Method Quantitation Limit
PQL Practical Quantitation Limit
SDL Sample Detection Limit
SQL Sample Quantitation Limit
UCL Upper Calibration Limit
UQL Upper Quantitation Limit
Looking at the expressions above, you may notice that there are two distinct classes of reporting limits: detection limits and quantitation limits. A detection limit refers to the minimum concentration of an analyte that can be measured above the instrument background noise. Thus, when detection limits are used as reporting limits, the laboratory is saying that the analyte is not present at or above the value given. It may be present at a lower concentration, but cannot be "seen" by the instrument. The different types of detection limits arise from different methods of calculation and from taking different factors into account. For example, is the limit based on a statistically significant signal-to-noise ratio or a statistically determined confidence level? Is there a correction for the effects of sample handling and preparation? How about for sample matrix and dilution? In any case, detection limits are only a measure of the ability of the test procedure to generate a positive response and have nothing to do with the accuracy of that response.
On the other hand, a quantitation limit refers to the minimum concentration of an analyte that can be measured within specified limits of precision and accuracy. They are generally 5-10 times the detection limit. Thus, when quantitation limits are used as reporting limits, the laboratory is saying that the analyte is not present in a sufficient amount to be reliably quantified (i.e., at a concentration above the quantitation limit). It may be present and even positively identified or "seen" at a lower concentration. There are different ways (discussed below) of handling analytes that are positively identified at a concentration below the reported quantitation limit. Like detection limits, the different types of quantitation limits arise from different methods of calculation and from taking different factors into account. Unlike detection limits, there are both lower and upper quantitation limits. These can be thought of in terms of the calibration range of the instrument. In fact, even though quantitation limits are defined in terms of precision and accuracy, they are often arbitrarily set at the lowest (or highest) non-zero standard in the calibration curve. Alternatively, they may be set at some multiple of the detection limit.
Whether detection or quantitation, the reporting limits used by a laboratory may be derived specifically for that laboratory, instrument, sample matrix, etc. Often though, they are taken from the analytical method. Every analytical method gives either detection or quantitation limits that are typically attained under routine laboratory operating conditions. For detection limits, if the laboratory uses the limits given in the analytical method, they must maintain documentation to show they are able to attain these limits. For many analytes, the laboratory can attain a lower detection limit which can be reported if specifically requested for your project. Since quantitation limits are generally more arbitrary, they do not normally require as thorough documentation by the laboratory.
Definitions
Now let's look at the specific definitions to get a better grasp on all this. The definitions presented here are based on text in the analytical methods. They may not agree with the laboratory's definition. If you are unsure, it is always best to check with the lab.
CRDL
Minimum level of detection acceptable under the contract Statement of Work (SOW). The inorganic SOW for the Contract Laboratory Program gives CRDLs, but laboratory-derived IDLs (adjusted for sample size, dilution and moisture) are used for reporting limits. The CLP CRDLs are based on typical instrument capabilities and should be attained by the laboratory. Inorganic analytes reported at a concentration above the laboratory's IDL but below the CLP CRDL are flagged with a "B".
CRQL
Minimum level of reliable quantitation acceptable under the contract Statement of Work (SOW). The organic SOW for the Contract Laboratory Program gives CRQLs, and they are used for reporting limits (after adjustment for %moisture and dilution). The CLP CRQLs are arbitrarily set at the concentration of the lowest non-zero standard in the calibration curve. Organic analytes that are positively identified below the CLP CRQL are reported as present, but at an estimated concentration (with a "J" flag).
EDL
Minimum concentration required to produce a specified signal-to-noise (S/N) ratio. The SW-846 Method 8290 for dioxins/furans by GCMS requires that EDLs be used for reporting limits. The EDLs are explicitly determined by the laboratory for each analyte in each sample. The noise in the vicinity of the absent analyte is measured then multiplied by a S/N ratio of 2.5. This labor-intensive procedure is used in order to obtain the lowest possible reporting limits for these highly toxic compounds. It could be specially requested for other GCMS analyses as well.
EQL
Lowest concentration that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating conditions. EQLs normally are arbitrarily set rather than explicitly determined. Most organic SW-846 methods give EQLs. The SW-846 EQLs are arbitrarily set at some multiple of typical MDLs for reagent water. Multiplying factors are given for various matrices such as groundwater, wastewater, soil and sludge, etc. Generally, laboratories use the SW-846 EQLs (adjusted for sample size, dilution, and %moisture) for reporting limits, but they may use EQLs that they have generated. SW-846 does not stipulate how to handle organic analytes that are positively identified at a concentration below the SW-846 EQL. Generally, laboratories DO NOT report these as present.
IDL
Lowest concentration that can be detected by an instrument without correction for the effects of sample matrix or method-specific parameters such as sample preparation. IDLs are explicitly determined and generally defined as three times the standard deviation of the mean noise level. This represents 99% confidence that the signal is not random noise. The inorganic methods in CLP, SW-846, EPA 200 series, and Standard Methods all give typical IDLs, but laboratory-derived IDLs (adjusted for sample size, dilution, and %moisture) are used for reporting limits. The IDL does not include the upward correction necessary to account for the effects of sample matrix or handling/ preparation (minimal for inorganic water analyses). This is important to remember especially for risk assessments and highly contaminated samples.
LLD
Generally the same as IDL
LLQ
Generally the same as EQL
LOD
Generally the same as IDL
LOL
Concentration at or above the upper end of the calibration curve at which the relationship between the quantity present and the instrument response ceases to be linear. The organic SOW for the Contract Laboratory Program defines the calibration range in terms of the region of demonstrated linearity. In other words, the LOL is set at the concentration of the highest standard analyzed even though it could extend beyond this. Organic analytes that are positively identified at a concentration above the LOL are flagged with an "E".
LOQ
Generally the same as EQL
MDC
Generally the same as MDL
MDL
Lowest concentration that can be detected by an instrument with correction for the effects of sample matrix and method-specific parameters such as sample preparation. MDLs are explicitly determined as set forth in 40 CFR Part 136. They are defined as three times the standard deviation of replicate spiked analyses. This represents 99% confidence that the analyte concentration is greater than zero. The organic methods in the EPA 500 series, EPA 600 series, and Standard Methods all give typical MDLs for clean water samples. Generally these clean-water MDLs (corrected for %moisture, sample size, and dilution) are used for reporting limits, but the laboratory may use MDLs that they have generated. MDLs generated by the laboratory using the sample matrix of interest are the most reliable. If the clean-water MDLs are used, remember that they do not include all of the upward correction necessary to account for the effects of sample matrix. As with IDLs, this is important to remember especially for risk assessments and highly contaminated samples.
MQL
Generally the same as EQL
PQL
Generally the same as EQL. This term was used in SW-846 prior to 1994.
SDL
The MDL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis.
SQL
The EQL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis. One exception applies for the TRRP (Texas Risk Reduction Program) Guidelines for Review and Reporting of COC Concentration Data (TRRP-13). The term SQL, as defined in this document and the relevant rule, is analogous to the Sample Detection Limit (SDL).
UCL
Highest concentration that can be reliably measured within specified limits of precision and accuracy during routine laboratory operating conditions. Specifically defined as the concentration of the highest calibration standard in the laboratory's initial calibration curve adjusted for initial sample volume or weight.
Picture This
The following illustrates the relationship between the various reporting limits.
Defining reporting limits is no simple matter. There are several unique approaches among the major analytical methods. Each approach may be flexible and thus result in different limits. A clear understanding of these terms will help in deciding exactly how to handle analytes that are reported as not present. Additionally, it can help in communicating with the laboratory in order to obtain the most appropriate reporting limits for your project.
HMACTTO (HOW MANY ACRONYMS CAN THEY THINK OF)?
Before trying to define these terms, let's first look at the words behind the acronyms.
CRDL Contract Required Detection Limit
CRQL Contract Required Quantitation Limit
EDL Estimated Detection Limit
EQL Estimated Quantitation Limit
IDL Instrument Detection Limit
LLD Lower Limit of Detection
LLQ Lower Limit of Quantitation
LOD Limit Of Detection
LOL Limit Of Linearity
LOQ Limit Of Quantitation
MDC Minimum Detectable Concentration
MDL Method Detection Limit
MQL Method Quantitation Limit
PQL Practical Quantitation Limit
SDL Sample Detection Limit
SQL Sample Quantitation Limit
UCL Upper Calibration Limit
UQL Upper Quantitation Limit
Looking at the expressions above, you may notice that there are two distinct classes of reporting limits: detection limits and quantitation limits. A detection limit refers to the minimum concentration of an analyte that can be measured above the instrument background noise. Thus, when detection limits are used as reporting limits, the laboratory is saying that the analyte is not present at or above the value given. It may be present at a lower concentration, but cannot be "seen" by the instrument. The different types of detection limits arise from different methods of calculation and from taking different factors into account. For example, is the limit based on a statistically significant signal-to-noise ratio or a statistically determined confidence level? Is there a correction for the effects of sample handling and preparation? How about for sample matrix and dilution? In any case, detection limits are only a measure of the ability of the test procedure to generate a positive response and have nothing to do with the accuracy of that response.
On the other hand, a quantitation limit refers to the minimum concentration of an analyte that can be measured within specified limits of precision and accuracy. They are generally 5-10 times the detection limit. Thus, when quantitation limits are used as reporting limits, the laboratory is saying that the analyte is not present in a sufficient amount to be reliably quantified (i.e., at a concentration above the quantitation limit). It may be present and even positively identified or "seen" at a lower concentration. There are different ways (discussed below) of handling analytes that are positively identified at a concentration below the reported quantitation limit. Like detection limits, the different types of quantitation limits arise from different methods of calculation and from taking different factors into account. Unlike detection limits, there are both lower and upper quantitation limits. These can be thought of in terms of the calibration range of the instrument. In fact, even though quantitation limits are defined in terms of precision and accuracy, they are often arbitrarily set at the lowest (or highest) non-zero standard in the calibration curve. Alternatively, they may be set at some multiple of the detection limit.
Whether detection or quantitation, the reporting limits used by a laboratory may be derived specifically for that laboratory, instrument, sample matrix, etc. Often though, they are taken from the analytical method. Every analytical method gives either detection or quantitation limits that are typically attained under routine laboratory operating conditions. For detection limits, if the laboratory uses the limits given in the analytical method, they must maintain documentation to show they are able to attain these limits. For many analytes, the laboratory can attain a lower detection limit which can be reported if specifically requested for your project. Since quantitation limits are generally more arbitrary, they do not normally require as thorough documentation by the laboratory.
Definitions
Now let's look at the specific definitions to get a better grasp on all this. The definitions presented here are based on text in the analytical methods. They may not agree with the laboratory's definition. If you are unsure, it is always best to check with the lab.
CRDL
Minimum level of detection acceptable under the contract Statement of Work (SOW). The inorganic SOW for the Contract Laboratory Program gives CRDLs, but laboratory-derived IDLs (adjusted for sample size, dilution and moisture) are used for reporting limits. The CLP CRDLs are based on typical instrument capabilities and should be attained by the laboratory. Inorganic analytes reported at a concentration above the laboratory's IDL but below the CLP CRDL are flagged with a "B".
CRQL
Minimum level of reliable quantitation acceptable under the contract Statement of Work (SOW). The organic SOW for the Contract Laboratory Program gives CRQLs, and they are used for reporting limits (after adjustment for %moisture and dilution). The CLP CRQLs are arbitrarily set at the concentration of the lowest non-zero standard in the calibration curve. Organic analytes that are positively identified below the CLP CRQL are reported as present, but at an estimated concentration (with a "J" flag).
EDL
Minimum concentration required to produce a specified signal-to-noise (S/N) ratio. The SW-846 Method 8290 for dioxins/furans by GCMS requires that EDLs be used for reporting limits. The EDLs are explicitly determined by the laboratory for each analyte in each sample. The noise in the vicinity of the absent analyte is measured then multiplied by a S/N ratio of 2.5. This labor-intensive procedure is used in order to obtain the lowest possible reporting limits for these highly toxic compounds. It could be specially requested for other GCMS analyses as well.
EQL
Lowest concentration that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating conditions. EQLs normally are arbitrarily set rather than explicitly determined. Most organic SW-846 methods give EQLs. The SW-846 EQLs are arbitrarily set at some multiple of typical MDLs for reagent water. Multiplying factors are given for various matrices such as groundwater, wastewater, soil and sludge, etc. Generally, laboratories use the SW-846 EQLs (adjusted for sample size, dilution, and %moisture) for reporting limits, but they may use EQLs that they have generated. SW-846 does not stipulate how to handle organic analytes that are positively identified at a concentration below the SW-846 EQL. Generally, laboratories DO NOT report these as present.
IDL
Lowest concentration that can be detected by an instrument without correction for the effects of sample matrix or method-specific parameters such as sample preparation. IDLs are explicitly determined and generally defined as three times the standard deviation of the mean noise level. This represents 99% confidence that the signal is not random noise. The inorganic methods in CLP, SW-846, EPA 200 series, and Standard Methods all give typical IDLs, but laboratory-derived IDLs (adjusted for sample size, dilution, and %moisture) are used for reporting limits. The IDL does not include the upward correction necessary to account for the effects of sample matrix or handling/ preparation (minimal for inorganic water analyses). This is important to remember especially for risk assessments and highly contaminated samples.
LLD
Generally the same as IDL
LLQ
Generally the same as EQL
LOD
Generally the same as IDL
LOL
Concentration at or above the upper end of the calibration curve at which the relationship between the quantity present and the instrument response ceases to be linear. The organic SOW for the Contract Laboratory Program defines the calibration range in terms of the region of demonstrated linearity. In other words, the LOL is set at the concentration of the highest standard analyzed even though it could extend beyond this. Organic analytes that are positively identified at a concentration above the LOL are flagged with an "E".
LOQ
Generally the same as EQL
MDC
Generally the same as MDL
MDL
Lowest concentration that can be detected by an instrument with correction for the effects of sample matrix and method-specific parameters such as sample preparation. MDLs are explicitly determined as set forth in 40 CFR Part 136. They are defined as three times the standard deviation of replicate spiked analyses. This represents 99% confidence that the analyte concentration is greater than zero. The organic methods in the EPA 500 series, EPA 600 series, and Standard Methods all give typical MDLs for clean water samples. Generally these clean-water MDLs (corrected for %moisture, sample size, and dilution) are used for reporting limits, but the laboratory may use MDLs that they have generated. MDLs generated by the laboratory using the sample matrix of interest are the most reliable. If the clean-water MDLs are used, remember that they do not include all of the upward correction necessary to account for the effects of sample matrix. As with IDLs, this is important to remember especially for risk assessments and highly contaminated samples.
MQL
Generally the same as EQL
PQL
Generally the same as EQL. This term was used in SW-846 prior to 1994.
SDL
The MDL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis.
SQL
The EQL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis. One exception applies for the TRRP (Texas Risk Reduction Program) Guidelines for Review and Reporting of COC Concentration Data (TRRP-13). The term SQL, as defined in this document and the relevant rule, is analogous to the Sample Detection Limit (SDL).
UCL
Highest concentration that can be reliably measured within specified limits of precision and accuracy during routine laboratory operating conditions. Specifically defined as the concentration of the highest calibration standard in the laboratory's initial calibration curve adjusted for initial sample volume or weight.
Picture This
The following illustrates the relationship between the various reporting limits.
Defining reporting limits is no simple matter. There are several unique approaches among the major analytical methods. Each approach may be flexible and thus result in different limits. A clear understanding of these terms will help in deciding exactly how to handle analytes that are reported as not present. Additionally, it can help in communicating with the laboratory in order to obtain the most appropriate reporting limits for your project.
