Frequent Questions

My laboratory reported a minimum detection level (or minimum reporting level) greater than the upper limit for bin 1 (0.075 oocysts per liter). If that is the case, how can we possibly ever have our results average to less than the bin 2 level? Shou

The approved method for LT2 sampling, method 1622/1623, does not utilize detection limits. When a laboratory analyzes an LT2 Cryptosporidium sample using Method 1622/1623, the results are entered into the LT2 Data Collection and Tracking System as the total number of oocysts found and the total volume examined.  The data system will then use the data to calculate a concentration based on the number of oocysts detected and the volume examined. Therefore, when zero oocysts are detected in LT2 monitoring, the number found is reported as zero for purposes of calculating the bin concentration.  If 10 L are collected and no oocysts are found, a value of 0 oocysts/L will be averaged with the values from other required samples. Because zeros can be included in the calculation, an average less than 0.075 oocysts per liter is possible. When the resulting average from all monitoring samples is <0.075 oocysts/L the location qualifies for bin 1.  There may be some confusion about "detection limits" because they were used for other programs in the past, such as the Information Collection Rule Cryptosporidium monitoring.  Some laboratories may continue to list this on their laboratory reporting form because they have clients that conduct monitoring for purposes other than LT2. Please be aware that the minimum detection/reporting level has nothing to do with the laboratory proficiency.  To examine a laboratory's proficiency, water systems should request the laboratory's control chart for proficiency test results.
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