Laboratory testing plays an integral part in medical decision making. However, laboratory results can sometimes vary significantly, leading to anomalous outcomes that are not consistent with the clinical picture. These anomalies can occur even in the best of laboratories simply because the total testing process includes elements that are not totally under the laboratory’s control. For example, variations in patient preparation and sample collection procedures, as can happen at physician offices or patients receiving intravenous fluids, are major contributors to these anomalies.