5.1 This method measures the concentration of ATP present in the sample. ATP is a constituent of all living cells, including bacteria and fungi. Consequently, the presence of ATP is an indicator of total microbial contamination in metalworking fluids. ATP is not associated with matter of non-biological origin.

5.2 Test Method D4012 validated ATP as a surrogate for culturable bacterial data (Guide E1326).

5.3 This method differs from Test Method D4012 in that it eliminates interferences that have historically rendered ATP testing unusable with complex organic fluids such as MWFs.

5.4 The ATP test provides rapid test results that reflect the total bioburden in the sample. It thereby reduces the delay between test initiation and data capture, from the 36 h to 48 h (or longer) required for culturable colonies to become visible, to approximately 5 min.

5.5 Although ATP data generally covary with culture data in MWF,4 different factors affect ATP concentration than those that affect culturability.

5.5.1 Culturability is affected primarily by the ability of captured microbes to proliferate on the growth medium provided, under specific growth conditions. It has been estimated that less than 1 % of the species present in an environmental sample will form colonies under any given set of growth conditions.5

5.5.2 ATP concentration is affected by: the microbial species present, the physiological states of those species, and the total bioburden (see Appendix X1).

5.5.2.1 One example of the species effect is that the amount of ATP per cell is substantially greater for fungi than bacteria.

5.5.2.2 Within a species, cells that are more metabolically active will have more ATP per cell than dormant cells.

5.5.2.3 The greater the total bioburden, the greater the ATP concentration in a sample.

5.5.3 The possibility exists that the rinse step (11.15) may not eliminate all chemical substances that can interfere with the bioluminescence reaction (11.39).

5.5.3.1 The presence of any such interferences can be evaluated by performing a standard addition test series as described in Appendix X3.

5.5.3.2 Any impact of interfering chemicals will be reflected as bias relative to data obtained from fluid that does not contain interfering chemicals.