Nursing Explained

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Why does a small drop in sats matter?

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The question

Mrs Smith's sats were 96 % this morning and are 91 % now: it's only five points and still in the 90s, so does it matter?

Understand

Mrs Smith's sats were 96% this morning and 91% now. Only five points, so does it matter? Oxygen saturation (sats, SpO2) is the share of haemoglobin carrying oxygen, and haemoglobin holds on to oxygen tightly. So the oxygen level in the blood (its pressure) can fall a long way while sats barely move: roughly, in a typical adult, 96% is about 80 mmHg and 91% about 60 mmHg, a fall of about a quarter. And at about 90% the curve turns steep, so a little less oxygen makes sats drop fast. And the clip can't show how fast someone is breathing. So recheck the reading, look at the person, count their breathing and report the drop from the last reading, not just the number. Some people have a different target set by their team. Charts and rules vary: follow your local policy. Mrs Smith and her numbers are fictional examples, and the buses and curve are simplified teaching drawings, not personal targets.

  1. Nearly all oxygen in blood rides on haemoglobin, up to 4 per molecule. Sats = the share of seats filled.
  2. Haemoglobin holds on tightly: on the flat top, the oxygen level can fall a lot while sats barely move.
  3. Roughly, 96% ≈ 80 and 91% ≈ 60 mmHg: the oxygen level is about a quarter lower.
  4. Around 90% the curve turns steep: sats then drop fast.

Key takeaways

  • Healthy adults usually sit at about 95% or above.
  • A fall matters even inside the 90s: compare with the last set and report the drop.
  • Some people have a different target set by their team (for example some people with COPD): follow it and your local policy.

In practice

  • Compare with the last reading: a fall matters, even in the 90s.
  • Recheck the reading, look at the person, count the breathing.
  • Reading and person don't match? Recheck and report. Follow your local policy.

In one line

A small drop in sats can hide a big fall in the oxygen level, close to the edge, so recheck the reading, look at the person and report the drop from the last reading, not just the number.

Check yourself

Sats go from 97% to 92% in two hours. Still in the 90s, so fine?

Show the answer

No. A small fall in sats can hide a much bigger fall in the oxygen level. Recheck, look at the person, count the breathing and report the drop.

Words to know

Haemoglobin
oxygen carrier in red blood cells (haemo- (blood) + globin (protein))
PaO₂
oxygen level in arterial blood (mmHg)
Hypoxaemia
low oxygen in the blood (hypo- (low) + ox- (oxygen) + -aemia (blood))

Transcript

Read the narration

Mrs Smith's oxygen sats were 96 percent this morning. At noon, they're 91. That's only five points, and still in the nineties.

Should Alex worry? Yes. Picture what the clip is reading. Nearly all your blood's oxygen rides on haemoglobin, inside red blood cells.

Picture it as buses with four seats each. Sats is the share of seats filled. Haemoglobin holds on to oxygen tightly. So the oxygen level around the buses can fall a long way before the seats empty, and the number hardly moves.

So when it does drop, 96 to 91, a lot has changed underneath. Roughly, the oxygen level around the buses has fallen by about a quarter. And at 91, she's near the edge of the flat top.

Past it, each empty seat makes the next let go, so seats empty fast. And she's breathing 24, above the usual 12 to 20. The clip can't show that.

A small drop in sats can hide a big fall in the oxygen level, close to the edge. So Alex rechecks the reading, looks at Mrs Smith, and reports the drop from 96, not just the 91.

Five points? Now you know why Alex makes the call.

Sources

  1. OpenStax Anatomy and Physiology 2e, 22.5 Transport of Gases: haemoglobin saturation is the percent of heme units bound to oxygen; one haemoglobin carries up to four oxygen molecules; saturation generally 95-99 % (read 13 October 2026)
  2. StatPearls - Physiology, Oxygen Transport: about 98 % of oxygen in blood is bound to haemoglobin, 2 % dissolved (read 13 October 2026)
  3. StatPearls - Oxyhemoglobin Dissociation Curve: sigmoid curve; plateaus at higher oxygen tension; steeper at lower tension (read 13 October 2026)
  4. Deranged Physiology (Australian ICU exam notes) - Oxyhaemoglobin dissociation curve: the flat upper plateau buffers large PaO2 changes; Severinghaus values about 97.5 % at 100, 95.8 % at 80 and 90.9 % at 60 mmHg (normal pH and temperature) (read 13 October 2026)
  5. Beasley R et al. Thoracic Society of Australia and New Zealand oxygen guidelines for acute oxygen use in adults. Respirology 2015;20:1182: hypoxaemia defined as SaO2 < 90 % or PaO2 < 60 mmHg; SpO2 is a vital sign to consider with respiratory rate; a reduction in SpO2 should lead to further assessment (read 13 October 2026)
  6. StatPearls - Pulse Oximetry (archived): cannot measure ventilation; monitor oxygenation trends; accuracy 2-4 % (read 13 October 2026)
  7. Healthify He Puna Waiora (NZ) - Pulse oximeter: normal saturation is 95-100 % for a resting healthy adult; often the change in oxygen level is as important as the level (read 13 October 2026)
  8. TGA - Limitations of pulse oximeters and the effect of skin pigmentation: a pulse oximeter estimates oxygen saturation; usually 95-100 % in healthy people (read 13 October 2026)
  9. Beasley R et al. Appropriate use of oxygen in acute medicine. Med J Aust 2015;203(3): most oximeters quote an accuracy of about 2 % (read 13 October 2026)
  10. Healthify He Puna Waiora (NZ) - Breathing rate: adults at rest usually 12-20 breaths per minute (read 10 October 2026)

Checked against these sources at the time of publishing. Sources change, so check the date above and your local policy. Spotted an error? Tell us and we'll correct it.

General education, not clinical advice. Check your local policy.