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Objectives The aim was to evaluate the stability of serum bicarbonate at room temperature, depending on time to centrifugation and air exposure. Methods Stability study was conducted in the laboratory of Clinical Hospital Centre Rijeka, Croatia in January-February 2022. Nine samples from 10 volunteers were collected in clot activator gel tubes (Greiner Bio-One). Bicarbonate was measured on Beckman Coulter AU480 (Beckman Coulter, Brea, USA). Three tubes were left at room temperature for 30 min, three tubes for 2 h, three tubes for 4 h until centrifugation. First tube from first group (baseline) was measured immediately after centrifugation. Other measurements were expressed as percentage deviation (PD%) from baseline. First tube was remeasured after 1 and 2 h (OT_0h_1h; OT_0h_2h). Second and third tubes were opened 1 and 2 h after centrifugation (C_0h_1h; C_0h_2h). Second group of tubes was processed the same way with 2-hour centrifugation delay (WB_2h; OT_2h_1h; OT_2h_2h; C_2h_1h; C_2h_2h), and third group with 4-hour delay (WB_4h; OT_4h_1h; OT_4h_2h; C_4h_1h; C_4h_2h). PD% was compared to Maximum Permissible Difference (MPD=5.69%). MedCalc statistical software was used (MedCalc, Ostend, Belgium). Results Bicarbonate baseline mean value (range) was 27.3 (23.4–29.6) mmol/L. Obtained PD% (95%CI) were: C_0h_1h 0.46 (−1.21, 2.12); C_0h_2h 0.18 (−2.22, 2.57); OT_0h_1h −6.46 (−7.57, −5.36); OT_0h_2h −10.67 (−12.13, −9.21); WB_2h −0.15 (−2.04, 1.74); C_2h_1h 0.01 (−1.52, 1.54); C_2h_2h −0.40 (−2.65, 1.85); OT_2h_1h −5.43 (−7.30, −3.55); OT_2h_2h −11.32 (−13.57, −9.07); WB_4h −0.85 (−3.28, 1.58); C_4h_1h −2.52 (−4.93, 0.11); C_4h_2h −3.02 (−5.62, 0.43); OT_4h_1h −7.34 (−9.64, −5.05); OT_4h_2h −11.85 (−14.38, −9.33). Conclusions Serum bicarbonate is stable for 4 h in closed uncentrifuged tubes, another 2 h in closed tubes after centrifugation, and is unstable within 1 h in opened tube.

Introduction: Vitamin D testing is excessively used in clinical practice, despite of the clinical guidelines statements against population screening for vitamin D deficiency. This study aimed to assess an annual number of performed 25-hydroxy vitamin D (25(OH)D) tests that were unsupported by the national guidelines for prevention, detection and therapy of vitamin D deficiency in adults and to calculate associated financial burden for the publicly funded healthcare. Materials and methods: A representative sample of requested 25(OH)D tests in 2018 (N = 474) was formed after selection and randomisation of data set (N = 5298) collected from the laboratory information system database of the Clinical Department for Laboratory Diagnostics, the Clinical Hospital Centre Rijeka. Records were classified in two groups depending on associated medical condition(s) according to the national guidelines. An annual cost of the total and group specific vitamin D testing was calculated on the base of a single test price reimbursed by the Croatian Healthcare Insurance Fund (CHIF). Results: Medical conditions with high-risk for vitamin D deficiency were detected in 43% (206/474) of vitamin D requests (group 1). Conditions not associated with vitamin D deficiency were detected in 57% (268/474) requests (group 2). A total cost of 25(OH)D testing for the CHIF was 58,729.50 EUR (25,523.79 EUR in the group 1 and 33,205.71 EUR in the group 2). Conclusions: More than half of all 25(OH)D tests performed in the clinical laboratory represent avoidable cost for the public healthcare. Prevention of population screening by vitamin D testing is needed.