I Have the Blood Results Back - Now What Do I Do?

Take Home Message—Interpretation of clinical pathology values can be difficult if consideration of the vagaries of testing, the reason for sampling, and organ or tissue specificity are not considered. Initial testing with comprehensive panels interpreted with consideration of clinical exam findings will enable more targeted laboratory testing to monitor progress of condition.

I. INTRODUCTION

This presentation is not intended to be a comprehensive review of the entire spectrum of clinical pathological investigation of the horse. Rather, it seeks to highlight considerations in the interpretation of common and uncommon clinical pathology results, and review common and interesting clinical scenarios in equine practice.

II. CONSIDERATIONS WHEN INTERPRETING CLINICAL PATHOLOGY VALUES

Why Did You Take the Blood in the First Place?

Client pressure to exhaustively test the horse and a veterinary desire to not miss anything creates an environment where over-testing is possible. Maximum information for the financial expenditure is the goal. Consider the likelihood that the test in question will yield useful information. The predictive value of the test is increased in situations where the likelihood of abnormality or disease is higher.

Chances of an Abnormality are High with Multiple Analytes

The reference interval is set by convention to encompass the central 95% of the reference values obtained from a healthy population of comparable individuals to that for which the test is designed to provide meaningful information. Statistical methods are used for the selection of reference limits, being based on the number and distribution of the reference values. The 2.5th and 97.5th percentiles serve as the lower and upper reference limits, that is, the bottom 2.5% and upper 2.5% are set outside the normal range.^1,2 This means a total of 5% of values will occur outside the reference interval for a ‘normal’ population of tested animals even though the animal may be judged ‘healthy’.

The chance of getting a value within the reference range for any single analyte is therefore 0.95. Routine biochemical panels measure up to 20 separate clinical variables. Using this as an example, the chance of getting all 20 variables within the normal range in a healthy horse is (0.95)²⁰, or 0.36. The probability of getting at least one value outside the normal range in these circumstances is therefore equal to 1-(0.95)²⁰, or 0.64 i.e. 64% of the time at least one (or more) result will lie outside the reference range in a healthy horse. When 10 analytes are measured, this chance is equal to 40%.

Duration and Method of Venipuncture

The hemogram is altered by venipuncture for longer than 30 seconds due to splenic contraction resulting from the actions of the sympathetic-adrenal and hypothalamic-pituitary activity. Regarding method, the use of vacuum tubes was associated with cell damage in one study, and higher-gauge needles are considered preferable by some practitioners. Significant differences in hematological parameters between venipuncture and intravenous catheter blood draws has not been shown.³

Order of Sample Drawing

The order of blood tube filling can be important. EDTA tube prior to a serum or heparin tube for biochemical testing can allow contamination with the potassium EDTA anticoagulant on the needle of the collection syringe, yielding a spurious high potassium concentration.

Proper Tube Mixing

Shaking can lead to hemolysis. Invert all tubes with additives to mix the additive evenly with the blood (8-10 times) to prevent clotting.

Correct Specimen Volume

Ensure the proper amount of blood is added to the tube for the amount of additive. This is very important when assessing clotting times (citrate) and using small volume tubes. [...]