The aim of this prospective, observational cohort study is to assess the impact of iron deficiency anaemia on the incidence of perioperative complications and the quality of recovery after surgery in patients undergoing colorectal cancer surgery. The main questions the study aims to answer are:

• whether the presence of preoperative iron deficiency anaemia leads to a poorer quality of postoperative recovery in patients undergoing colorectal cancer surgery
• whether different combinations of complete blood count parameters (red blood cell indices) could be suitable diagnostic tools for the detection of iron deficiency in the latent stage (without laboratory-confirmed anaemia) in colorectal cancer patients.

Blood samples for laboratory analyses will be collected from each study patient admitted to the surgical ward one day prior to elective surgery and on the first postoperative day during the stay in the intensive care unit. The pre-operative laboratory analyses include a complete blood count and serum iron status parameters (iron concentration, ferritin concentration, TIBC, UIBC and TSAT). Laboratory parameters analysed on the first postoperative day include complete blood count, serum concentration of electrolytes (Na, K, Ca, Cl, Mg), serum concentration of urea and creatinine, parameters of haemostasis (aPTT, PT, INR), serum concentration of C-reactive protein and procalcitonin.

Data about overall morbidity, intraoperative complications, quality of postoperative recovery, red blood cell transfusion rate, all-cause infection rate, antibiotic usage, as well as length of hospital stay will be collected.

The researchers will compare the group of patients with iron deficiency anaemia, the group of patients with iron deficiency in the latent stage and the control group to determine whether patients with iron deficiency have a higher incidence of perioperative complications and impaired recovery after surgery. The researchers will investigate whether iron deficiency can be detected at an early stage, when anaemia is not yet present, by calculating various red blood cell indices.

The following erythrocyte indices based on the preoperative complete blood count results will be calculated for each patient one day prior to surgical treatment:

• Mentzer index: MCV / RBC
• Green and King index: MCV2 x RDW / (100 x HGB)
• RDW index: MCV x RDW / RBC
• Shine and Lal index: MCV2 x MCH / 100
• England and Fraser index: MCV - RBC - (5 x HGB) - 3.4
• Srivastava index: MCH / RBC
• Ricerca index: RDW / RBC
• Ehsani index: MCV - (10 x RBC)
• Sirdah index: MCV - RBC - (3 x HGB)
• Sehgal index: MCV2 / RBC

The Ganzoni equation for calculating total iron deficit will be calculated for each patient one day prior to surgical treatment, using the following formula:

total iron deficit [mg] = body weight [kg] x (target hemoglobin [g/L] - actual hemoglobin [g/L]) x 2.4 + iron depot [mg]

Inclusion Criteria:

• Adult patients (˃ 18 years of age)
• ASA III clinical status
• Patients undergoing radical surgical treatment of colorectal cancer
• Signed written informed consent

Exclusion Criteria:

• Patients undergoing palliative surgical treatment of colorectal cancer
• Anaemic patients without iron deficiency, defined as: normal serum iron concentration, TSAT, TIBC, UIBC, and decreased HGB, HCT and RBC
• Presence of other type of anaemia than iron deficiency anaemia (e.g. alpha- or beta-thalassemia, sickle-cell anaemia, etc.)
• History of red blood cell transfusion in the period of 120 days prior to hospital-admission
• Stage III, IV, or V of chronic kidney disease (creatinine clearance < 60 mL/min)
• Significant intraoperative bleeding, which requires transfusion of red blood cell products, calculated using the Gross-formula:

allowable blood loss [mL] = (estimated blood volume [mL] x (initial HGB [g/L] - HGB level when transfusion is required [g/L])) / average of initial HGB and HGB level when transfusion is required [g/L] The cut-off value for HGB level when transfusion is required is set to 80 g/L.

Consecutive patients undergoing elective surgical treatment of colorectal cancer.