Q & A

May 2006

Q: Is it critical that sodium fluoride tubes be used to collect specimens for lactate? With increasing use of lactate as a risk marker for sepsis, can serum that is collected for analysis of other markers of sepsis also be used to measure lactate? Also, how stable is lactate at room temperature?

A. Measurement of blood lactate concentrations is important for detecting and monitoring impaired circulatory and tissue oxygenation in critically ill patients. Hyperlactemia is also a fairly common side effect of antiretroviral therapy with nucleoside reverse transcriptase inhibitors in patients with HIV.

Lactic acid is produced during glycolysis as an alternative to pyruvate entering the Krebs cycle. Formation of lactate from pyruvate enables metabolism of glucose via glycolysis to continue. The lactic acid that is produced is converted back into glucose in the Coricycle, which takes place in the liver. The capacity of the liver to uptake lactate becomes saturated when lactate concentrations exceed 2 mmol/L.

Many reports have shown that changes in blood lactate concentrations are superior to oxygen delivery or oxygen consumption at predicting survival in patients with septic shock. However, some investigators have warned against using lactate as a prognostic indicator in patients with sepsis because lactate has a limited ability to identify tissue hypoxia.¹

While lactate will be increased in the presence of anaerobic metabolism, it can also be increased in the absence of anaerobic metabolism due to a variety of factors, such as if patients have decreased activity of the enzyme pyruvate dehydrogenase. This enzyme controls entry of substrate into the Krebs cycle. Deficiency of this enzyme leads to an increase in blood lactate concentrations. Another factor is impaired clearance of lactate as a result of hepatic dysfunction.²

Lactic acid concentrations are determined in vivo by the rate of production in various tissues, such as skeletal muscle, brain, skin, renal medulla, and erythrocytes, and the rate of metabolism in the liver and kidneys.³ While there is no universally accepted lactate concentration that defines hyperlactemia, lactate concentrations between 2 mmol/L and 5 mmol/L have been used to signify hyperlactemia. Lactic acidosis has been defined as a plasma lactate concentration greater than 5 mmol/L and a venous pH of less than 7.35.⁴

Lactate may be measured in whole blood or plasma. Various methods can be used to stabilize lactate in blood. Early methods involved the use of precipitating agents to denature the cellular glycolytic enzymes followed by filtration or centrifugation of the sample. Collecting blood in tubes containing sodium fluoride/potassium oxalate, or F/Ox, is the preferred method for obtaining specimens for lactate measurements. Blood samples collected with no additive, or with EDTA or heparin as an anticoagulant, are subject to increases in lactate concentrations over time because blood cells continue to metabolize glucose following collection, resulting in the production of lactic acid.

The metabolism of glucose that occurs in blood samples collected without any additive or in whole blood anticoagulated with EDTA or heparin is time and temperature dependent. In addition, the presence of leukocytosis can significantly alter lactate concentrations in whole blood. One recent study evaluated the effect of storage temperature, glucose concentrations, and blood cell counts on the stability of lactate in heparinized whole blood samples.⁵ The investigators found lactate concentrations in whole blood to be stable (defined as less than a 20 percent increase in lactate concentrations over baseline values) for 16 minutes in samples stored at room temperature and up to one hour for samples kept at 4°C. However, in samples with leukocytosis, storage of samples at room temperature was limited to 10 minutes; lactate concentrations in samples with leukocytosis kept at 4°C remained unchanged. Samples collected into tubes containing F/Ox are significantly more stable, even when leukocytosis is present. The authors of another study reported that lactate concentrations in samples with leukocytosis collected in tubes containing F/Ox were stable for at least eight hours at room temperature.⁶ Lactate has been reported to be stable in plasma at –70°C for up to 24 months.⁷

Lactate is not measured in serum because of the significant time delay that occurs when the specimen is allowed to clot and the additional time then required to centrifuge the sample to obtain serum. We do not recommend using serum for measuring lactate in patients with sepsis due to the significant leukocytosis often observed in these individuals.

References

1. James JH, Luchette FA, McCarter FD, et al. Lactate is an unreliable indicator of tissue hypoxia in injury or sepsis. Lancet. 1999;354:505–508.
2. Bakker J. Lactate: may I have your votes please? Intensive Care Med. 2001;27:6–11.
3. Sacks DB. Carbohydrates. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, Pa.: W.B. Saunders Co.; 1999:787–789.
4. Luft D, Deichsel G, Schmulling RM, et al. Definition of clinically relevant lactic acidosis in patients with internal diseases. Am J Clin Pathol. 1983;80:484–489.
5. Calatayud O, Tenias JM. Effects of time, temperature, and blood cell counts on levels of lactate in heparinized whole blood gas samples. Scand J Clin Lab Invest. 2003;63:311–314.
6. Astles R, Williams CP, Sedor F. Stability of plasma lactate in vitro in the presence of antiglycolytic agents. Clin Chem. 1994;40:1327–1330.
7. Dube MP, Kitch DW, Parker RA, et al. The effect of long-term storage on measured plasma lactate concentrations and prospective lactate results from a multicenter trial of antiretroviral therapy. Clin Chem Lab Med. 2005;43:947–952.

Steven Kazmierczak, PhD, DABCC
Professor, Department of Pathology
Oregon Health and Science University
Portland

Liaison, CAP Chemistry
Resource Committee

Q: It has been our laboratory policy to call every critical value we get. Some of our physicians and special care areas do not want all these test results. How do we comply?

A. Whether a lab result requires rapid clinical attention depends on the clinical setting—that is, inpatient versus outpatient versus nursing home patient versus renal dialysis patient, or initial versus repeat value. It is acceptable for the lab director, in consultation with clinicians, to set critical results that vary with clinical settings. The laboratory should have a procedure in place and review these values as necessary.

This issue is addressed in the CAP laboratory general checklist. Checklist question GEN.41320 reads:

Does the laboratory have procedures for immediate notification of a physician (or other clinical personnel responsible for patient care) when results of certain tests fall within established “alert” or “critical” ranges?

Note: This includes results received on specimens sent to reference laboratories for testing. Alert or critical values are those results that may require rapid clinical attention to avert significant patient morbidity or mortality. These values should be defined by the laboratory director, in consultation with the clinicians served.

Nancy Yeransian, MT(ASCP)SC
Manager, Accreditation Services
Laboratory Accreditation Program
College of American Pathologists
Northfield, Ill.