Current guidelines that state that blood from a clamped umbilical cord can be used for blood samples up to 1 hour after delivery should not be applied to tests for lactate as a marker for prenatal hypoxia. 

Scottish researchers analyzed 20 arterial samples and 18 venous samples from placentas of infants delivered by elective cesarean section. They paired samples of blood from clamped and unclamped cords at 0, 20, 40, and 60 minutes for lactate, base excess, pH, and PCO

In samples analyzed immediately, arterial and venous lactate levels were significantly higher than they were at 20 minutes in both clamped and unclamped vessels. In clamped vessels, pH did not change over the course of an hour, but in unclamped vessels, it changed significantly. There were also significant changes in base excess in both the clamped and unclamped samples. 

Armstrong L, Stenson B. Effect of delayed sampling on umbilical cord arterial and venous lactate and blood gases in clamped and unclamped vessels 

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Current guidelines that state that blood from a clamped umbilical cord can be used for blood samples up to 1 hour after delivery should not be applied to tests for lactate as a marker for prenatal hypoxia.