Rep. Mary Franson, R-Alexandria, received a letter from the Minnesota Department of Health in response to her inquiries about cycle thresholds for COVID-19.
Staff from MDH’s Infectious Disease Epidemiology, Prevention and Control sector and Public Health Laboratory provided answers to Franson’s questions regarding cycle thresholds of virus testing.
According to the CDC, a cycle threshold is used “to improve the test’s ability to detect virus.” A PCR (most common) test “creates many copies of the same genetic material from the virus in a process called amplification. The cycle threshold (Ct value) is the point at which a reaction reaches a fluorescent intensity above background levels … There is a correlation between the Ct value and the amount of viral genetic material that was present in the specimen.”
The letter from MDH stated that Minnesota does not have consistent standards for labs to follow regarding cycle thresholds. “Many commercially available platforms only report the qualitative result (positive or negative) so no Ct value is available,” the letter said.
In response to Franson’s question of whether the state has “any checks in place to confirm that a positive test is in fact reflective of an active case, rather than reflecting dead virus in someone’s system,” the letter from the MDH stated, “Distinguishing between active or ‘dead’ virus is not the purpose of molecular tests.”
The letter said research is ongoing to find out if a consistent Ct value exists to suggest “active infection.” However, MDH continued, right now “Ct value alone has not been able to predict the viability of virus in viral culture as there are many factors that can influence the Ct value.”
When asked if the state keeps record of “the cycle at which a test comes back as positive,” MDH responded, “Minnesota does not keep track of the cycle at which a test comes back as positive because neither state nor federal reporting requirements require laboratories to report this information.” Since some tests cannot report a Ct value, some labs would not be able to acquire this information.
MDH provided Franson with this link for more information on Ct values.