Introduction

Quality Assurance in the clinical laboratory consists of an important chain of events that can be divided into three general phases: the pre-analytical phase, the analytical phase, and the post-analytical phase. These phases are further broken down into several links; the strength of each and every link is as important as the next regarding maintaining the quality of the chain as a whole and the ultimate impact on patient care.

The pre-analytical phase includes the multiple steps prior to the actual sample testing. Aspects relate to the order (right test and time), patient (preparation, identification) and sample (collection and handling). As the name implies, the analytical phase relates to the steps directly involved in generating a test result including the instrument performance, the method, manual technique, reagents, etc. The post-analytical phase involves all the steps occurring after the generation of the test result including reporting, data management, interpretation, diagnosis and treatment. Since the pre-analytical phase is by most accounts the most common source of errors related to laboratory testing, this session will focus on some of the more common causes and effects of these errors, and how to avoid them.

The pre-analytical phase involves many links in the quality assurance chain of events in laboratory testing. Insuring the appropriate test is requested at the appropriate time are links beyond the scope of this session. Patient identification is a huge source of error in the human testing world. Because of this, large hospitals go to great expense to automate this step with the use of bar-coded identification wristbands and this bar code follows the patient's sample throughout the testing process. This is not practical and is perhaps less of an issue in the smaller clinic. Needless to say, appropriate patient identification and accurate legible sample identification are important steps.

Patient preparation

Lipemia is one interferent that can sometimes be avoided with appropriate fasting of the patient prior to sample collection. Owners should be reminded to fast their pets prior to blood sampling whenever possible. However some metabolic conditions as hypothyroidism, diabetes mellitus and Cushing's disease may induce lipemic samples even in fasted states. Lipemia creates a turbid sample that may interfere with spectrophotometric methods as with hemoglobin concentration and many of the biochemical determinations. It also often induces hemolysis so the sample resembles tomato soup. Lipemia will falsely increase the hemoglobin concentration and since the calculation for MCHC uses this value, it will also be increased. One can easily check for lipemia by examination of the microhematocrit tube after spinning. Biochemical instruments can minimize these effects with sample blanking and lipemia is less of an interferent in systems using dry-slide technology. Lipemia and paraproteinemia can create pseudohyponatremia via volume exclusion in systems that dilute samples prior to measurements by ion-selectrode technology (ISE). Such systems are typically located in larger referral laboratories and are known as "indirect ISE" methods. The smaller systems that test undiluted samples with ISE technology, as i-STAT and IRMA, are not subject to this issue. For this reasons, comparisons between these two system types should be limited to non-lipemic samples and samples from myeloma patients should be excluded.