Step One: Does the pH Say Acidemia or Alkalemia?

Before you look at anything else on a gas, put the pH on one side of a line. Everything after that is a question about who pushed it there and who is pushing back, and neither question means much until you have committed to a side.

The published range

StatPearls gives a normal arterial pH between 7.35 and 7.45, with 7.40 as the average. Below 7.35 the blood is acidemic. Above 7.45 it is alkalemic.

The pH carries no units. It is a logarithmic scale rather than a concentration you can print in milligrams, which is why no unit is ever attached to it and you should not go hunting for one.

Reference ranges vary slightly between laboratories and across age groups, which the source states in its own words. That caveat travels with every value you learn, and it is part of why reading ABGs in three steps treats the method as more durable than the numbers.

Acidemia is not the same word as acidosis

The distinction is small, and it is the reason some students never get comfortable with gases. Acidemia and alkalemia describe the blood as measured, right now. Acidosis and alkalosis describe a process pushing in that direction.

A patient can have a process without the blood having crossed the line yet. A patient can also have two processes pulling opposite ways with a pH sitting innocently in the middle.

So step one answers a narrow question honestly. Not what is wrong with this patient, only which side of 7.40 this blood is sitting on. Narrow questions are the ones you can still answer under pressure.

Why the pH has to go first

Read the carbon dioxide before the pH and you have nothing to compare it against. A PaCO2 above its range is a driver in one patient and a compensation in another, and the only thing separating those two readings is which way the pH went.

Same for bicarbonate. A raised bicarbonate is either the cause of an alkalemia or the kidneys answering an acidemia, and the pH is the only witness.

That dependency is why this is a sequence and not a checklist. Step two: deciding respiratory versus metabolic only works once step one has handed it a direction.

What a normal pH does not rule out

Here is the part that catches people. A pH sitting between 7.35 and 7.45 does not mean the gas is normal.

A fully compensated disorder can put the pH back inside those two figures while the carbon dioxide and the bicarbonate both sit well outside their own ranges. The body has restored the pH without fixing what caused the problem.

So when step one gives you a normal pH, do not stop. Read the other two values anyway, and if either one is abnormal you are looking at compensation rather than a normal gas. Step three: spotting compensation in the numbers is where that gets worked out.

Then there is oxygenation, which lives on a different axis entirely. A patient can have a textbook pH and be dangerously hypoxic, which is exactly why the four step method assesses oxygenation on its own rather than folding it in here.

What step one hands to the rest of the reading

Step one produces two things and both get used later. It produces a direction, which step two needs before it can identify a driver. And it produces a distance, which tells you roughly how hard whatever is happening is pushing.

A pH of 7.33 and a pH of 7.10 are both acidemic and they are not the same clinical situation. The label is the same. The urgency is not.

So naming the side is a beginning rather than an answer. It sets up the next two questions and it quietly removes most of the wrong readings before you get to them.

The common ways step one goes wrong

Three mistakes account for most of them. Reading the carbon dioxide first and letting it colour the pH. Treating a pH inside its range as a finished gas. And using acidosis and acidemia interchangeably until the sentence stops meaning anything.

The first two are habits, and habits respond to sequence. Make yourself say the pH out loud before your eye reaches anything else on the panel.

The third is vocabulary, and vocabulary responds to repetition. Say acidemic when you mean the blood and acidosis when you mean the process, even in your own notes, until it stops requiring thought.

Where step one sits in the method

Step one is the smallest of the four steps and the one everything else leans on. The full order, with the published range for every value on the panel, is in arterial blood gases in a four step reading order.

If a stem gives you a pH and nothing else, it is almost certainly testing vocabulary rather than interpretation. Name the side. Then wait for the rest.

Saying it out loud

Practise step one as one sentence with two blanks. This blood is acidemic or alkalemic, and the pH is or is not inside its published range.

That sounds too simple to be worth rehearsing. It is not. Under pressure the common failure is not getting step three wrong, it is never properly committing to step one and then reasoning off a direction you never actually chose.

Pick a side first. Everything after it has somewhere to attach.