Page 27 - Basic Monitoring in Canine and Feline Emergency Patients

plasma and RBCs, thus rendering the pre-programmed As previously discussed, when using colorimet-
calibration in units like the AlphaTRAK (Zoetis, ric reagent urine ketone test pads with either urine
VetBooks.ir Parsippany, New Jersey, United States) incorrect. or serum, they do not detect 3-HB. This can lead
to a decreased estimation of severity or even
Point-of-care units measure BG in whole blood
rather than plasma, so hemoconcentration will
with ketosis.
allow less non-RBC–bound glucose interaction potentially a false negative reading in a patient
with the sensors in the test strip, artifactually low-
ering BG. Anemia will have the opposite effect,
artifactually raising BG due to less RBCs to bind it. Lactate
Oxygen supplementation can interfere with glu- The most common situations affecting lactate
cose oxidase in the test strips, taking electrons levels are not related to the lactate meter itself,
from the enzyme and decreasing the ability of the but rather to pre-analytical variables. The first
enzyme to convert glucose to gluconic acid and obstacle is that there is no standardization
hydrogen peroxide. Therefore, high levels of oxy- between lactate values meaning that lactate
gen in the blood (as might be seen under general meters cannot be compared to a single standard
anesthesia) can falsely lower the BG reading. reference and as such cannot be compared to each
Certain drugs can also interfere with BG other. Also, delays in testing lactate will falsely
assessment when glucose is measured on point- increase the lactate level because red blood cell
of-care units. Ascorbic acid, acetaminophen, metabolism in the sample will continue to increase
dopamine, and mannitol can all interfere with lactate levels.
BG readings by affecting glucose oxidase’s con- The other large consideration with lactate testing
version of glucose or by being broken down by is how sample collection can impact the lactate
the glucose oxidase and therefore being mis- levels. Specifically, lactate levels can be falsely
interpreted as glucose. Thus, drug therapy can increased when obtaining the blood sample. This
falsely increase or decreased BG readings depend- occurs when a blood vessel is held off for long
ing on the situation. If patients are receiving periods, or, in the case with animals, there is signifi-
these drugs, it is better to verify any glucose cant restraint required to obtain the sample. In the
reading obtained from a point-of-care instru- former example, holding off blood flow from an
ment against a glucose result done on a standard area can decrease oxygen delivery to those local
laboratory analyzer. tissues, leading to anaerobic metabolism and lac-
tate production. Similarly, more significant restraint
can either lead to increased lactate production from
Ketones
a similar effect or possibly secondary to increased
When dealing with ketone strips, the same con- muscle activity related to the restraint. Sampling
cerns for manufacturing and storage as indicated from an ischemic limb (e.g. in a cat with an aortic
above for glucose strips are true. There is much thromboembolism affecting blood flow to its rear
less research into this area versus glucose strips, legs) can falsely increase lactate levels due to the
however. Hematocrit changes can also alter anaerobic metabolism naturally taking place in
ketone results. Low hematocrits can falsely ischemic tissue that lacks blood flow. This ‘artifact’
increase ketone levels; some meters have propri- is sometimes used diagnostically; if lactate in one
etary built-in corrections for hematocrit levels, limb is significantly higher than peripheral blood, it
but to the author’s knowledge these have not would raise concern for decreased regional perfu-
been validated in veterinary species. Ketone test- sion to that limb (e.g. possible thrombosis).
ing is similar to glucose testing in that substances Whole blood samples have a lower lactate level
such as vitamin C can interfere with the enzy- than plasma samples. This is because the RBCs
matic conversion of ketones to nitroprusside will sequester some of the circulating lactate, thus
because it alters the alkaline environment needed falsely lowering the lactate levels. Similarly, lac-
for that conversion. The degree of change of tate levels in venous blood will naturally be higher
ketones in response to vitamin C is still under than levels in arterial blood due to diffusion of
investigation in humans. Vitamin C is likely not lactate from metabolizing cells into the venous
important in most animal patients. blood.

Physical Examination and Point-of-care Testing 19