Lactate testing can help identify and monitor patients with severe sepsis and septic shock, but clinicians and laboratorians have to be on guard for practices and factors that can affect test accuracy.
Luckily, using point-of-care testing for the analyte isn’t one of them.
Mayo Clinic did a study a few years ago in which the laboratory compared three whole blood point-of-care methods with two central laboratory methods using plasma, says Brad Karon, MD, PhD, a Mayo Clinic pathologist. “Analytically, the methods all compared fairly well throughout the clinical decision range for sepsis,” which he notes typically runs from 3 mM/L to 5 mM/L.
“The two central lab plasma methods produced nearly identical values. Some of the POC applications showed a negative bias at high (>6 mM/L) lactate [levels], but concordance around a cutoff of 5 mM was greater than 90 percent for the POC methods.”
Methodist North Hospital in Memphis has also found that the central lab’s lactate testing results correlate with point-of-care results, says Jane Nichols, supervisor of the lab. “We didn’t think they would at first, but they do.”
Inaccurate lactate results tend to stem from preanalytical rather than analytical issues, Dr. Karon says. For example, a difficult blood draw with prolonged tourniquet time or prolonged transport times to the lab can cause an elevated lactic acid level in the specimen, producing a false-positive. “That means someone might unnecessarily get antibiotics and fluids. If followup doesn’t suggest the person has a serious illness, then the lactate will probably be repeated or ignored—probably repeated.”
For central lab testing, a gray-top tube should be used; it can be maintained at room temperature for up to an hour without lactate levels rising, Dr. Karon says. But “if you’re going to use a normal serum or plasma tube or a blood gas syringe, you have to put those on ice and do analysis in 20 to 30 minutes to get accurate values.”
Another thing that can produce a false-positive, says Marc Zubrow, MD, director of critical care medicine at Christiana Care Health System, Wilmington, Del., is drawing the blood sample peripherally from an ischemic limb. Thus, Christiana Care’s nurses are instructed not to draw blood for lactate testing from anywhere that appears ischemic.
Dr. Zubrow says his institution doesn’t find a discrepancy between the central lab and point-of-care lactate values. But he doesn’t get too caught up in small differences in numbers between the two testing modes. His motto: “The trend is your friend.”
If a patient’s lactate starts out at 10 “and we get it down to 2 when it’s really 2.5, because the point-of-care result is .5 off, I don’t care. It’s still coming down and I can see the patient is getting better.”