Lab Test


PO2 (0.5), PO2 at half saturation, Oxygen Dissociation

Test Codes





P50 specimens should ONLY be collected at the Grosse Pointe & Royal Oak Hospital Campus.

Specimen Collection Criteria

Collect: Arterial, venous, or capillary, heparinized whole blood.

  • A 1.0 mL or 3.0 mL plastic syringe containing dry heparin is preferred for specimen collection.
  • At least 1.0-2.0 mL (0.3 mL for pediatric patients) of blood is needed to ensure proper assay.
  • Arterial specimens are collected by a physician, nurse, or respiratory therapist.

Collection Requirements:

  • For specimens collected by syringe, any air bubbles present in the syringe must be immediately expelled, the needle removed, and the syringe tightly capped. The syringe is rolled between the palms of the hands to ensure mixing.
    • Note: The Radiometer safePico Self-fillTM syringe has a metal ball to ensure homogeneous mixing when the syringe is rolled between the palms of the hands.
  • Characterization of oxygen enrichment, at the time of arterial puncture, is required. This may be expressed as FiO2 fractional inspired oxygen concentration provided (e.g. 0.50), FiO2% (e.g. 50%), room air, or nasal cannula flow rate in liters per minute. The FiO2 must be noted on the requisition or entered as part of the electronic order for any arterial blood gas specimens. It is not possible to fully evaluate arterial blood gas results without knowledge of the oxygen concentration or nasal flow rate being delivered to a patient at the time of blood draw.
  • The specimen must be properly labeled.
  • Place on wet ice for transport. Ice and Specimen must be kept separated.
    • Suggested: Place specimen in front pocket of the biohazard bag. Place ice in zip lock section of biohazard bag.
  • The specimen must be transported immediately to the Laboratory.    

Rejection Criteria

  • Clotted specimens.
  • Specimens not collected and processed indicated.
  • Specimens received past 1 hour from the time of collection.

Inpatient Specimen Preparation

The specimen must be transported STAT, on wet ice, to the Laboratory. 

In-Lab Processing

Immediately analyze the whole blood, heparinized, iced specimen upon receipt.


Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): Unacceptable
Refrigerated (2-8°C or 36-46°F): 1 hour
Frozen (-20°C/-4°F or below): Unacceptable

Specimen Storage in Department Prior to Disposal:

Specimens are disposed within 8 hours of testing.


Royal Oak STAT Laboratory
Grosse Pointe Chemistry Laboratory


Sunday – Saturday, 24 hours a day.
Results available within 1 hour of receipt in the laboratory.

Reference Range

25-29 mm Hg.
Newborn infants: 18-24 mm Hg.

Test Methodology

Amperometry and Absorption Spectrophotometry measurements followed by calculated results.


  • A reduced P50 indicates an increase in hemoglobin-oxygen affinity that may occur with decreased levels of 2,3 diphosphoglycerate, significantly increased levels of fetal hemoglobin, and presence of a hemoglobin variant with increased oxygen affinity (e.g. Hgb Ranier).
  • An increased P50 indicates a decrease in hemoglobin oxygen affinity that may occur with increased levels of 2,3 diphosphoglycerate and in the presence of a hemoglobin variant with decreased oxygen affinity (e.g. Hgb Kansas).

Clinical Utility

The P50 is defined as the oxygen tension at which 50% of the hemoglobin is saturated when the blood is at 37°C, has a PCO2 value of 40 mm Hg and has a pH of 7.40. The concept was designed to evaluate factors, other than temperature, pH, and PCO2 that change hemoglobin-oxygen affinity.

CPT Codes

LOINC:  19214-6


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