CultureStat—Distinguishing Log Phase and Lag Phase Bacteria and Reducing False Positive Results

The American Medical Association defines an infection as “the invasion and multiplication of microorganisms in the body tissues.”¹ A urinary tract infection is defined as “bacterial infections affecting any part of the urinary tract.”²

The only life phase of bacteria in which multiplication of bacteria takes place is the “logarithmic” or “exponential” phase, commonly known as “log phase.” In order to accurately determine if a person is suffering from a urinary tract infection, it is therefore essential that the test conducted can determine not only the presence of bacteria, but whether such bacteria are in log phase.

Current test methods on agar plates are “presence” tests and cannot distinguish between log phase bacteria and bacteria which are not in log phase (such bacteria could be in “lag phase”, during which bacteria are not able to divide, or “stationary phase”, which is the phase prior to death). This is a primary reason why plating results in a large percentage of false positives—bacteria that are not infectious (i.e., not in log phase) are identified as being present on the plate and the test is called a “positive”, even though the bacteria are not causing an infection.

CultureStat determines whether or not a sample contains log phase bacteria, which is essential in determining whether a patient in fact has a urinary tract infection. It does this by conducting two reads in a 3 hour timeframe and identifying respiration and an increase in cell mass (the two primary characteristics of bacteria in log phase). Only those samples with bacteria that demonstrate respiration and a statistically relevant increase in cell mass are declared positive. By properly identifying log phase bacterial growth as positive and lag phase bacteria and other contaminates as negative, CultureStat produces more accurate results and significantly reduces the percentage of false positives compared to current plating testing methods.

¹ American Medical Association Complete Medical Encyclopedia, Jerrold B. Leikin, MD and Martin S. Lipsky, MD, Editors, Random House Reference, New York, 2003, ISBN 0-8129-9100-1, page 719. 
² Ibid., page 1257.