Diagnostic laboratory tests are an integral part of managing hospitalized patients. In particular, patients in the intensive care units (ICUs) can experience a concerning amount of blood loss due to diagnostic testing, which can increase the risk developing iatrogenic anemia. Several interventions exist to curtail avoidable blood loss, for example computerized decision support, smaller phlebotomy tubes, and other blood conservation devices. Nevertheless, use of these interventions is not standardized. Therefore, the objective of our study was to quantify the daily phlebotomy volume taken from patients who had undergone major cardiac or vascular surgery.We estimated the number of blood analyses and volumes of drawn blood of 400 consecutive patients (≥ 18 yr) undergoing major cardiac or vascular surgery. The amount of blood saved using small-volume tubes and in combination with blood conservation device rather than standard-volume tubes was estimated for serum chemistry (serum), ethylenediaminetetraacetic acid (EDTA) tubes, sodium citrate coagulation (SCC) tubes, and arterial blood gas (ABG) analysis.The mean total blood loss due to phlebotomy drawing using standard-volume tubes during hospitalization was 167.9 mL (95% confidence interval [CI], 158.0 to 177.8), 255.6 mL (95% CI, 226.5 to 284.6), and 695.3 mL (95% CI, 544.1 to 846.4) for patients undergoing cardiac surgery with a hospital length of stay (LOS) of 0-10, 11-20, and ≥ 21 days, respectively. The mean total blood loss due to phlebotomy during hospitalization was 80.5 mL (95% CI, 70.5 to 90.6), 225.0 mL (95% CI, 135.1 to 314.8 mL) and 470.3 mL (95% CI, 333.5 to 607.1) for vascular surgery patients with LOS 0-10, 11-20, and ≥ 21 days, respectively. Patients with at least a two-day stay at the ICU had a mean blood loss of 146.6 mL (95% CI, 134.6 to 158.6 mL) and those with ≥ 11 days incurred a loss of 1,428 mL (95% CI, 1,117.8 to 1,739.2). The use of closed blood collection device and small-volume tubes (serum, EDTA, SCC, and ABG) reduced blood loss by 82.8 mL for patients with an ICU stay of 2 days and up to 824.0 mL for patients with a ICU stay of ≥ 11 days.Diagnostic laboratory tests are associated with significant patient blood loss, but are a modifiable risk factor. The use of small-volume tubes and closed blood collection devices decreases the volume of patient blood drawn for analysis and prevents blood waste.