Biochemistry Lab
Tan Zhao Rong. Tgo1.

Blood gas analyses are performed to evaluate the respiratory function of patients namely the acid-base status (pH), the oxygenation (pO2) and the ventilation status (pCO2). This blood test is specifically performed on blood from an artery and thus is somewhat more uncomfortable and difficult to perform compared to venipuncture.

The reference range for blood pH should range between 7.35 and 7.45. The pH status would indicate if the patient is acidemic or alkalimic.

The reference range for pO2 ranges between 75 – 100mmHg. A low o2 indicates that the patient is not respiring properly (hypoxemic). At a pO2 level of less than 26mmHg, the patient is at risk of death and must be oxygenated immediately as insufficient oxygen is transferred to the vital organs such as brain and heart.

The reference range for pCO2 should range between 35.0 – 45.0 mmHg. Any abnormal pH status would indicate a respiratory problem as the pCO2 in arterial blood is determined entirely by ventilation. A high pCO2 indicates under-ventilation, where respiratory acidosis occurs and acidity of blood rises. A low pCO2 would indicate hyperventilation where there is increased alveolar respiration.

ABL 825 flex blood gas module is the instrument responsible for carrying out blood gas analyses. The specimen type required would be 1 – 3 ml of arterial blood in heparinized syringes.

Procedure:
1. It is necessary to mix a blood sample before introducing it to the system/instrument so as to ensure homogeneity. RBC is to be evenly distributed and mixed within the syringe so as to prevent machine from aspirating a plasma phase or packed RBCs from the specimen. This may result in inaccuracy in results produced.
2. Check for any clots by injecting the first few parts of the specimen into a waste container.
3. Ensure that the analyzer is in READY mode and scan the barcode. (Barcodes would be labeled on the patient’s request form and specimen once it arrive.)
4. The inlet is a section of the instrument in which the analyzer aspirates specimens. Therefore, the next step would be to place the syringe tip firmly in the inlet and press START.
5. Press Aspirate to start the measurement.
6. Remove the sample when prompted by the analyzer.

When specimen volume is too low, it would be difficult for the machine to aspirate fully from the syringe as some parts may remain at the tip of the syringe. So as to fully utilize the whole specimen, the specimen can be transferred to a capillary tube before inserting into the inlet. A capillary tube can hold up to 200ul of blood specimen and the minimal requirement for a blood gas analysis would be to fill up to three quarter of the capillary tube (150ul).


Factors that may cause result to be inaccurate:

- Frothy samples or visible air bubbles would affect the pO2 and pCO2 level in the syringe causing inaccuracy in results.

- Sample not properly anti-coagulated are rejected as clots can cause the instrument to break down. Therefore, blood samples in normal unheparinized syringes are not accepted.

- Sample should not be left in room temperature for more than 30 minutes.

- Sample should be delivered in ice so as to slow the metabolic processes which causes inaccuracy. Delays in analysis (without chilling) may result in inaccurately low