Acid base teaching – Case 2

An 18 year old girl consulted her GP because of tiredness, weight loss, polydipsia and polyuria increasingly present over a few weeks. 

What investigations would you like the GP to undertake?

This is an quick and easy test which can indicate a cause for this girl’s presentation. It can show glucose or ketones in the urine. In females of childbearing age a pregnancy test (with consent) may also be appropriate.

Whilst important in a young female patient with tiredness to rule out anaemia, the added features here make this an unlikely sole diagnosis. Therefore this is not the most applicable investigation.

A capillary blood glucose would be a a good indicator of high blood glucose levels however a venous sample would be required. A GP practice may not be the best setting in which this should be done as this would take time to be sent to hospital labs, which could potentially take time and delay further treatment.

Whilst important to determine if there is a renal cause of her thirst and increased urine output, however this alone may not deliver the answers to her current presentation.

You may want to reconsider your answer as some of the above requires some further investigation.

The GP conducts the investigations that you have suggested

The GP found she had glycosuria and arranged for her to be admitted to hospital for further tests. Later that day she became faint and vomited and acute admission was arranged.

You are the receiving doctor in the Emergency Department, which of these investigations would be most helpful to provide a diagnoses?

From the history provided there is no indication to undertake a chest x-ray for this girl. However is any abnormalities of the respiratory system was found on examination this should be considered.

Whilst an important test, this would not be likely to provide a diagnosis. It does form a part of initial investigations however it may not provide the diagnosis in this case.

Whilst this is an important to determine if there is any organ dysfunction the rest of the U+Es do not form the most important diagnostic test.

Whilst useful to give an indication of blood glucose, a venous sample would be more appropriate to get an accurate level. NB BMs of around 30 will just read ‘high’ on a glucometer.

Blood gases can give results quickly and lots of them. This includes acid-base results. But the results also give some electrolyte levels. It does also show blood glucose however you will need to send an official blood glucose to the labs.


An ABG was performed

Her results were:
[H+] = 89 nmol/L   pH = 7.05                     Blood Glucose 34.4
PaCO2 = 2.0 kPa
[HCO3] = 5 mmol/L

How would you describe the above result?

Look again, metabolic acidosis presented with elevated bicarbonate levels, is this picture seen above?

Whilst partially correct, there is a better answer available. Above shows an acidotic picture with raised hydrogen ion concentration and low bicarbonate.

This is correct. There is acidaemia with raised hydrogen ions. Bicarbonate is low indicating a metabolic cause. Carbon dioxide is also low, this indicates there is respiratory compensation.

Typical respiratory acidaemia presents with increased PaCO2, usually in the context of respiratory distress. There may be a compensatory bicarbonate rise.

Look again, hydrogen ions are high. Respiratory alkalaemia usually occurs due to hyperventilation of the lungs and results in significant low PaCO2 and a fall in hydrogen ions.

Further compensation requires renal tubular mechanisms for regenerating bicarbonate. The function of these may be limited by the likely dehydration caused by an osmotic diuresis due to the presence of glucose and ketones in the urine.

For further information on interpreting blood gas pictures click here.

What is the patient’s diagnosis?

(Choose one of these and then click to see if you are correct)

Polydipsia alone should not present with any acid base disturbance.

There is no suggestion of anorexia which usually presents with normal acid base picture or metabolic alkalaemia due to vomiting or use of diuretics. (hydrochloric acid loss in vomit)

In view of her age, symptoms and laboratory results it is the most likely diagnosis.

Diarrhoea usually presents with a metabolic acidaemia due to bicarbonate and electrolyte loss via the stool.

Severe anaemia should not change the acid base picture

The glycosuria suggests diabetes mellitus, the acidaemia being due to “ketone body” production. Diabetic ketoacidosis would be confirmed by the detection of ketones in the urine on dip-stick analysis. DKA would be expected to produce a significant anion gap.


Would you be worried about this patient? Explain your reasoning

Take a minute to write down your answers to this question

DKA is a life threatening emergency and should be treated promptly. Patients become really dehydrated as our bodies try and restore balance – hence why patients get polyuria and polydipsia.

One complication is Cerebral Oedema so if they have reduced consciousness you should escalate to a senior (also at risk of aspiration pneumonia)

They get disturbances in electrolytes which can lead to cardiac arrhythmias amongst other things. They’re at risk of: Hypokalaemia, Hypomagnesiumia and Hypophosphataemia

DKA is a pro-thrombotic state so patients are at increased risks of thromboembolism

Can you think of other causes of metabolic acidosis? (think about your answer before clicking on the reveal)

Metabolic acidosis can occur as a result of either:

Increased acid production or acid ingestion

  • DKA,
  • lactic acidosis (seen in sepsis)
  • Aspirin overdose

Decreased acid excretion or increased loss of gastrointestinal and renal HCO3

  • Addison’s disease
  • Renal Tubular Atrophy (retain H+)
  • GI losses of bicarb (eg diarrhea, ileostomy)

Your patient was seen by the diabetes team as an inpatient and started on an insulin regime with appropriate education. She recovered well and was fit for discharge 3 days after admission.

Well done you have completed this case.