Quality control in blood gas and critical care testing

Routine daily quality control on blood gas instruments

Most current blood gas/electrolyte/metabolite analyzers have onboard internal quality control (QC) systems that automatically monitor the stability and performance of reagents, electrodes, and electronics of the analyzer. These control systems make these “hybrid” analyzers well-suited to use in both laboratory and Point-of-Care Testing (POCT) areas. Two such analyzers use easily replaced multiuse packs that contain controls, calibrants, rinse solution, and waste. Some also have the flow system and analyte sensors within the pack. They are described in the following.

One model of a hybrid blood gas analyzer measures pH, p CO ₂ , p O ₂ , Na ⁺ , K ⁺ , Cl ⁻ , Ca ²⁺ , glucose, lactate, total hemoglobin (Hb), and several forms of Hb including oxy-Hb, carboxy-Hb, and met-Hb. Part of its QC system utilizes four process control solutions (PCS) all contained within a multiuse test pack to monitor the stability of the analytical responses. PCS A and B are run at least every 4 h and after every sample. PCS C is run once per 24 h, and PCS D is run every 12 h. Once the test pack is validated, an active QC program monitors the analytical process before, during, and after sample measurement, with automatic error detection. During the analysis, the sensor response is continually evaluated for abnormal responses that might indicate microclots, microbubbles, or some chemical interferences. If an error is detected in the analysis, the system automatically tries to correct the system error and will either document any corrective actions or disable that test channel if the problem cannot be fixed.

Another brand of hybrid blood gas analyzer measures pH, p CO ₂ , p O ₂ , Na ⁺ , K ⁺ , Ca ²⁺ , Cl ⁻ , glucose, lactate, bilirubin, and Hb with cooximetry. This analyzer utilizes a replaceable cassette of solid-state amperometric and potentiometric ion-selective sensors, optical p O ₂ detection, and spectrophotometric measurements for oximetry. QC solutions are contained within a multiuse pack containing three calibrant solution pouches (with one also used for rinse), one gas mixture pouch, three QC solution pouches, a flow selector, pump tubing, and a waste pouch. To compensate for drift, a calibrant solution is run to bracket every test measurement. Every 8 h, one level of each of three levels of liquid internal QC solutions is automatically run. All controls enter through the sample probe and flow through the entire system. Clots are usually trapped by a clot catcher near the sample inlet, plus pressure sensors detect any blockage of flow along the analysis path. If abnormal pressure is detected, the analyzer initiates a clot removal process to rid the offending clot from the flow path. To ensure that accurate results are reported, a question mark appears next to any invalid result that may be caused by an inhomogeneous sample, insufficient sample, or some problem with the calibration.

Hand-held portable blood gas analyzers utilize both a liquid calibrant contained within a single-use test card, and an external multiuse electronic simulator card that checks the acceptability of the electronic sensors within the instrument. This simulator is often referred to as providing equivalent quality control (EQC) in place of external control solutions. By itself, it verifies only the electronic function of the analyzer, but does not check the reagent or sensor stabilities of the test card. Every 24 h when cartridges are being tested, the external electronic simulator is used to verify any internal electronic failures. Twice per year, this simulator performs the required thermal probe check. The electronic simulator sends signals that are below and above the measurement ranges of the tests to simulate signals of a test cartridge during an actual analysis of a sample and these simulator results are stored in the instrument's memory. However, these electronic checks typically have almost no variation and do not represent the more realistic precision results obtained with either liquid controls or patient samples.

Another single-use disposable cartridge contains microsensors, a calibrant solution, flow system, and a waste chamber. Sensors for analysis of total CO ₂ , sodium, potassium, chloride, ionized calcium, glucose, creatinine, urea as blood urea nitrogen (BUN) and hematocrit are available in a variety of panel configurations. Approximately two–three drops (100–150 μL) of anticoagulated blood are dispensed into the cartridge sample well, which is sealed before inserting the cartridge into the analyzer. Inserting a test card brings the card's sensors into contact with the instruments electrical contacts. This contact activates a heater assembly to bring the measurement region to 37°C. This also opens the fluidic valve in the card to deliver calibrant fluid from the reservoir to the measurement region.

After calibration, and upon a prompt by an LED and an audio beep, the user introduces a blood sample for measurement through the blood sample port to the card's measurement region. When blood contacts the analyte sensors, an electrical signal is generated that is proportional to analyte concentrations in the blood sample.