Multisite Evaluation of a Hemoglobin-Based Control for pH and Blood-Gas,Electrolyte,and CO-Oximetry Instruments

Abstract

BACKGROUND: Aqueous and fluorocarbon emulsion controls do not have the same oxygen carrying capacity as human blood, and at Po₂ levels between 25-100 torr do not adequately assess either analyzer inaccuracy or imprecision. MATERIALS: EQUIL Plus Control (EPC) is a bovine hemoglobin (Hb) solution prepared at 3 levels of Hb and electrolyte concentration (P₅₀ range from 26-31 torr). The EQUILibrator Tonometer is a 3-channel bubble tonometer that uses a 6-mL syringe as a tonometry vessel. Tonometry of EPC with appropriate gas mixtures yields clinically relevant values for pH (7.2-7.6), P_O2 (40-100 torr), and P_CO2 (20-70 torr) and provides clinically useful values for Na+ (116-160 mmol/L), K+ (2.9-7.4 mmol/L), iCa++ (0.65-1.5 mmol/L), Cl- (76-120 mmol/L), tHb (90-175g/L), and Hb fractions. EVALUATION METHODS: Tonometered EPC was evaluated on 11 different blood-gas analyzers at 7 sites. Day-to-day imprecision was compared to imprecision obtained with aqueous and fluorocarbon controls. CO-oximetry and electrolyte imprecision were evaluated with EPC only. Post-tonometry stability of blood-gas values was assessed by analysis of 10 samples from a single syringe over 30-minute interval. RESULTS: From 40-100 torr, EPC monitors PO₂ inaccuracy within 5% or 3 torr of the theoretical (calculated) value for most analyzers tested. Day-to-day P_O2 standard deviations were 35-400% lower than the standard deviations found for aqueous controls in the same Po₂ range (on all instruments tested except one). From 20-70 torr, EPC also monitors the P_CO2 inaccuracy in most analyzers tested. Imprecision for PCO₂ and pH was similar for all 3 types of controls. Electrolyte was generally comparable to aqueous controls. CO-oximetry imprecision was also good for most analyzers tested. Blood-gas values sampled 10 times from a syringe and measured on an ABL 330 changed by <3% over 30 minutes. CONCLUSION: Tonometered EPC can monitor the P_O2 and P_CO2 inaccuracy of many blood-gas analyzers and improve the observed P()2 imprecision compared to aqueous controls. EPC electrolyte imprecision was comparable to aqueous controls for iCa++, and K+, and slightly greater for Nat. With standard gas mixtures, a single syringe of tonometered EPC can be used on multiple pH and blood-gas and electrolyte instruments for up to 30 minutes.