Blood Gases: 4-5-6, 7,8,9 Rule

Technically speaking, so long as you have an accurate pulse oximetry reading, you shouldn't have to draw an ABG to determine how well a person is oxygenating.  This is because the oxyhemoglobin dissociation curve allows you to estimate one by knowing the other.  

For instance, all you have to do use the following formula:

• Estimated PO2 = SpO2 minus 30

This can also be drawn out as the  4-5-6-7-8-9 Rule.  It goes like this.

• 4...................7
• 5...................8
• 6...................9

or...

• 40 PO2 = 70 SpO2
• 50 PO2 = 80 SpO2
• 60 PO2 = 90 SpO2

Essentially, SpO2 can be used to estimate PO2 and level of hypoxemia. 

90% SpO2	60 PO2	Mild Hypoxemia	No oxygen needed
80% SpO2	50 PO2	Moderate Hypoxemia	Oxygen needed
70% SpO2	40 PO2	severe Hypoxemia	Oxygen needed

Further reading:

1. ABG Interpretation Made Easy: Acid Base Balance
2. ABG Interpretation Made EAsy: Oxygenation
3. Oxyhemoglobin Dissociation Curve