A blood gas can look less alarming after compensation without the underlying disorder having disappeared. It can also contain two primary disturbances pulling the pH in different directions.
This acid-base quiz asks you to connect the numbers to the mechanism. There are seven original exercises, including a paired case in which changing carbon dioxide changes the interpretation.
All values are fictional teaching data. Gas values are arterial, and accompanying chemistry is assumed to have been sampled at the same time and to agree with the supplied bicarbonate. Real discrepancies require investigation rather than forcing results into a tidy pattern.
The cases are interpretation exercises, not instructions for giving bicarbonate, saline, potassium or ventilatory treatment. An unwell patient needs assessment while the results are being interpreted.
Units and conventions for this quiz
PaCO2 is supplied in both kPa and mmHg. Use the mmHg value in the stated compensation equations; approximately 7.5 mmHg equals 1 kPa. Bicarbonate, sodium and chloride are in mmol/L.
For the exercises, use an illustrative arterial pH interval of 7.35-7.45, PaCO2 of 35-45 mmHg and bicarbonate of 22-26 mmol/L. Use an anion-gap comparison value of 12 mmol/L, calculated without potassium. Actual laboratory intervals vary.
The distinction between a process and the resulting pH matters. Acidosis and alkalosis describe processes; acidaemia and alkalaemia describe the net pH. The MSD acid-base regulation overview explains the relationship between bicarbonate, carbon dioxide and buffering.
Attempt the cases before opening the worked answers.
The seven exercises
1. A low bicarbonate with a lower carbon dioxide
A patient with diabetes and markedly raised blood ketones is undergoing urgent assessment.
| Variable | Result |
|---|---|
| pH | 7.29 |
| PaCO2 | 3.47 kPa / 26 mmHg |
| Bicarbonate | 12 mmol/L |
| Sodium | 140 mmol/L |
| Chloride | 104 mmol/L |
| Albumin | 40 g/L |
Tasks: Identify the primary disturbance, calculate the anion gap and decide whether the respiratory response is consistent with expected compensation.
2. Change the respiratory response
Now consider a separate teaching variation of case 1. Keep bicarbonate, sodium and chloride unchanged, but change PaCO2 to 5.33 kPa / 40 mmHg. The corresponding pH is approximately 7.10, not 7.29.
Tasks: Is a PaCO2 within an ordinary reference interval reassuring here? What additional disturbance does the comparison suggest?
3. A different route to a low bicarbonate
An adult with profuse diarrhoea is being assessed. There is no raised lactate or ketonaemia in the supplied case, and renal assessment has not identified a primary renal explanation.
| Variable | Result |
|---|---|
| pH | 7.32 |
| PaCO2 | 4.27 kPa / 32 mmHg |
| Bicarbonate | 16 mmol/L |
| Sodium | 140 mmol/L |
| Chloride | 112 mmol/L |
| Albumin | 40 g/L |
Tasks: Calculate the gap, identify the pattern and connect it to the gastrointestinal history. Is the measured PaCO2 consistent with the expected response?
4. Gastric losses and a raised bicarbonate
An adult has prolonged vomiting and clinical volume depletion. There has been no recent diuretic exposure, and renal function is sufficiently preserved for the urine result to be interpretable.
| Variable | Result |
|---|---|
| pH | 7.50 |
| PaCO2 | 6.40 kPa / 48 mmHg |
| Bicarbonate | 36 mmol/L |
| Potassium | 3.0 mmol/L |
| Urine chloride | 8 mmol/L |
Tasks: Name the primary disturbance. Explain why the gastric history and urine chloride support the proposed mechanism, rather than considering the bicarbonate in isolation.
5. The duration changes the expected adaptation
A patient with established chronic alveolar hypoventilation has the following stable pattern, similar to previous results:
| Variable | Result |
|---|---|
| pH | 7.35 |
| PaCO2 | 8.00 kPa / 60 mmHg |
| Bicarbonate | 32 mmol/L |
Tasks: Explain why the raised bicarbonate can represent adaptation to a respiratory disturbance. Would the same bicarbonate response be expected immediately after a sudden rise in PaCO2 from 40 to 60 mmHg?
6. Alkalaemia does not exclude a metabolic acidosis
An acutely ill patient has an independently established cause of hyperventilation and a raised lactate. The cause and treatment are being assessed; the quiz asks only about the acid-base pattern.
| Variable | Result |
|---|---|
| pH | 7.53 |
| PaCO2 | 2.67 kPa / 20 mmHg |
| Bicarbonate | 16 mmol/L |
| Sodium | 140 mmol/L |
| Chloride | 100 mmol/L |
| Albumin | 40 g/L |
Tasks: Calculate the gap. Is the low PaCO2 simply the expected compensation for the bicarbonate value, or is a second primary disturbance present?
7. The apparently ordinary gap with low albumin
A separate chemistry panel gives sodium 138 mmol/L, chloride 108 mmol/L, bicarbonate 18 mmol/L and albumin 20 g/L.
Tasks: Calculate the unadjusted gap. Using an albumin reference value of 40 g/L, estimate the albumin-adjusted gap. Does the unadjusted result alone justify concluding that no increased-gap component exists?
No blood gas is supplied, so do not invent a pH or respiratory compensation assessment for this final case.
Worked answers
1. Increased-anion-gap metabolic acidosis with an expected respiratory response
The low pH and low bicarbonate identify the metabolic acidifying process. The fictional ketone result supports a mechanism but is not inferred solely from the gap.
Anion gap = sodium - chloride - bicarbonate = 140 - 104 - 12 = 24 mmol/L.
For metabolic acidosis, Winter's estimate is:
Expected PaCO2 = 1.5 x bicarbonate + 8, with an approximate range of +/-2 mmHg.
Here, 1.5 x 12 + 8 = 26 mmHg, giving approximately 24-28 mmHg. The measured 26 fits that estimate.
The MSD metabolic-acidosis guidance describes the increased-gap pattern and assessment of compensation. The arithmetic above uses the original case values.
An expected respiratory response does not make the metabolic problem resolved. It describes how the measured carbon dioxide relates to it.
The clinical connections game offers a separate opportunity to connect physiological concepts, without requiring today's puzzle to contain acid-base calculations.
2. The carbon dioxide is inappropriately high for the bicarbonate
The expected value remains approximately 26 mmHg, because the bicarbonate remains 12 mmol/L. A measured 40 mmHg is substantially above that expectation.
The interpretation is metabolic acidosis with an additional respiratory acidosis, not a reassuring normal carbon dioxide result.
The accompanying fall in pH follows the changed relationship between bicarbonate and dissolved carbon dioxide. The stated pH was recalculated rather than incorrectly retained from case 1.
This is an application of the compensation comparison, not proof of a particular cause of inadequate ventilation. Fatigue, medicines, neurological disease and other possibilities would require clinical assessment; none has been supplied as the established diagnosis here.
3. Normal-gap, hyperchloraemic metabolic acidosis
The anion gap is 140 - 112 - 16 = 12 mmol/L. In the stated context, gastrointestinal bicarbonate loss provides a plausible explanation for the normal-gap metabolic acidosis.
Winter's estimate gives 1.5 x 16 + 8 = 32 mmHg, approximately 30-34 mmHg. The measured 32 is consistent with it.
The distinction between gastrointestinal or renal bicarbonate loss and other acidifying processes is described in OpenStax's acid-base chapter.
The history makes gastrointestinal loss the intended explanation, but a normal gap is not uniquely diagnostic of diarrhoea. The question explicitly supplies supporting context rather than expecting the chloride value to identify the source alone.
4. Metabolic alkalosis associated with gastric acid and chloride loss
The pH is high and bicarbonate is raised. The vomiting history, low potassium and low urine chloride support a gastric-loss and chloride-depletion explanation.
The MSD metabolic-alkalosis guidance discusses gastric losses, factors maintaining alkalosis and the usefulness and limitations of urine chloride.
The increased PaCO2 is in the compensatory direction. Using a rough rise of 0.6-0.75 mmHg for each 1 mmol/L bicarbonate increase above 24 gives an expected value around 47-49 mmHg, compatible with 48.
These relationships guide interpretation; they do not supply a universal fluid prescription. Recent diuretic exposure, impaired renal function or another simultaneous disorder could alter the assessment.
5. Chronic respiratory acidosis with renal adaptation
The persistently high PaCO2 is the respiratory disturbance. Over time, renal adaptation can raise bicarbonate substantially more than occurs immediately after an acute rise.
The MSD respiratory-acidosis account describes that time-dependent response. The approximate chronic rule of a 3-4 mmol/L bicarbonate rise per 10 mmHg PaCO2 increase gives 30-32 mmol/L from a starting bicarbonate of 24 when PaCO2 rises by 20.
The measured 32 fits the chronic teaching model. An acute response would be smaller; using 26 mmol/L as an illustrative acute value with PaCO2 60 gives a pH around 7.26.
The near-reference pH does not mean the underlying ventilatory problem has disappeared. New symptoms or a changed trend would require reassessment rather than assuming this remains the patient's stable state.
6. Respiratory alkalosis and increased-gap metabolic acidosis coexist
The gap is 140 - 100 - 16 = 24 mmol/L. The metabolic component is therefore important despite the net alkalaemia.
Expected PaCO2 for bicarbonate 16 would be approximately 32 mmHg, not 20. The lower measured value indicates an additional respiratory alkalosis rather than compensation alone.
The MSD respiratory-alkalosis discussion explains the primary carbon dioxide reduction and the need to interpret it in clinical context.
The pH gives the net result of opposing processes. In this constructed example, the respiratory alkalinising effect predominates, while the elevated gap and low bicarbonate still demand an explanation.
7. Low albumin can conceal an increased gap
The unadjusted gap is 138 - 108 - 18 = 12 mmol/L.
A commonly used approximation adds 2.5 mmol/L for each 10 g/L by which albumin is below 40 g/L:
Adjusted gap = 12 + 0.25 x (40 - 20) = 17 mmol/L.
The MSD acid-base-disorders overview describes albumin's influence on the gap and the approximate adjustment.
Relative to the comparison value used here, the adjusted estimate raises concern for an increased-gap component. It does not establish its cause, and the absence of a gas prevents a complete respiratory analysis.
An adjustment is a model, not a substitute for measuring relevant acids or evaluating the patient.
Check the consistency before naming the disorder
The numerical relationships provide a useful error check for the exercise itself.
Using the standard approximation pH = 6.1 + log10[bicarbonate / (0.03 x PaCO2 in mmHg)], changing PaCO2 without changing bicarbonate changes pH. A question that leaves all the other values untouched may accidentally create inconsistent teaching data.
In real reports, gas bicarbonate is often calculated while chemistry bicarbonate or total carbon dioxide is measured through a different process. Sample timing and method therefore matter when results disagree. The MSD evaluation guidance discusses that distinction.
For this quiz, the numbers have been constructed to agree after ordinary rounding. The exercise asks you to explain them, not rescue an inconsistent panel.
What would you review after a missed answer?
Choose the precise step that caused difficulty: identifying the primary process, calculating the gap, using the correct units or comparing measured with expected compensation.
Those are different learning needs. Repeating every formula may be less useful than explaining why a normal-looking PaCO2 is inappropriate in case 2.
A later case can change one variable while preserving the rest of the setting. That tests the relationship rather than memory for the original answer.
Frequently asked questions
Can a patient have metabolic acidosis when the pH is high?
Yes. Another simultaneous process can raise the net pH. Case 6 illustrates respiratory alkalosis coexisting with an increased-gap metabolic acidosis.
Can I put a kPa value directly into Winter's formula as written here?
No. This version returns PaCO2 in mmHg. Use the matching units or convert correctly before comparing the result.
Are compensation equations exact physiological laws for every patient?
No. They are clinical approximations used with the history, timing, laboratory method and other findings. Departures prompt further interpretation rather than an automatic diagnosis of the cause.