Differentials
Common
Current diuretic therapy
History
prescribed or illicit use of loop and thiazide diuretics
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
based on history of diuretic use
Other investigations
- urinary drug screen:
positive for diuretics
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Gastric secretion loss
History
vomiting, gastric drainage
Exam
dry mouth, decreased skin turgor
1st investigation
- urinary chloride:
low (<10 mmol/L or 10 mEq/L)
Other investigations
Post-diuretic therapy
History
discontinuation of diuretic use (loop and thiazide diuretics)
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
no specific tests recommended
Other investigations
Post-hypercapnia
History
when chronic hypercapnia is rapidly corrected with mechanical ventilation, the patient continues to have high plasma HCO₃ levels for some time as renal correction takes a few hours
Exam
no specific examination findings
1st investigation
- ABG:
raised PaCO₂
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Other investigations
Bicarbonate administration
History
ingestion of alkali antacids or milk for gastro-oesophageal reflux disease or overtreatment of metabolic acidosis
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
no specific tests recommended
Other investigations
Milk-alkali syndrome
History
excessive intake of absorbable antacids, milk, or calcium supplements
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
no specific tests recommended
Other investigations
Uncommon
Primary hyperaldosteronism
History
muscular weakness, paraesthesia, headache, polyuria, polydipsia
Exam
raised BP
Other investigations
- 24-hour urine aldosterone:
>55 nanomol/24 hours (>20 micrograms/24 hours)
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Secondary hyperaldosteronism
Renal artery stenosis
History
uncontrolled hypertension, presents in young women (<40 years of age) or patients >50 years of age with risk factors for cardiovascular disease
Exam
raised BP
Other investigations
- duplex renal ultrasound:
high flow through stenotic lesion
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Cushing's syndrome
History
weakness, obesity, diabetes mellitus, hypertension, backache, menstrual irregularities
Exam
central obesity, facial plethora, proximal myopathy, hirsutism, abdominal striae, acne, bruises
1st investigation
Other investigations
Liquorice ingestion
History
liquorice ingestion, may be included in complementary medical therapies such as antacids
Exam
no specific examination findings
1st investigation
- serum potassium:
low
- 24-hour urinary free cortisol to cortisone ratio:
high
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Other investigations
Tobacco chewing
History
tobacco chewing
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
no investigation required
Other investigations
Apparent mineralocorticoid excess
Liddle's syndrome
Bartter's syndrome
History
positive family history (autosomal recessive), usually presents in children, polyuria, dehydration, mimics loop diuretic use
Exam
normal to low BP, rarely associated with sensorineural defects
Other investigations
- serum potassium:
low
Gitelman's syndrome
Profound potassium depletion
History
diarrhoea or diuretic use (other medications that cause hypokalaemia include beta-agonists, theophylline, or insulin), muscular weakness, fatigue, muscle cramps
Exam
no specific examination findings
1st investigation
- serum potassium:
low
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Other investigations
Hypercalcaemia of non-hyperparathyroid aetiology
History
constipation, polyuria
Exam
severe hypercalcaemia can cause stupor
1st investigation
- serum calcium:
high
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Other investigations
- PTH:
normal or low
Post-starvation refeeding syndrome
History
history of anorexia nervosa, alcohol misuse, post-operative patients, or patients who are fed through nasogastric or percutaneous endoscopic gastrostomy tubes are also at risk
Exam
no specific examination findings
1st investigation
- serum phosphate:
low
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Other investigations
Transfusion of blood products (sodium citrate)
History
transfusion of >10 units of blood (sodium citrate is metabolised to bicarbonate [HCO₃])
Exam
no specific examination findings
1st investigation
- clinical diagnosis:
no specific tests recommended
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Other investigations
Villous adenoma
History
severe diarrhoea (secretory diarrhoea syndrome)
Exam
dry mouth, decreased skin turgor
1st investigation
- urinary chloride:
low (<10 mmol/L or 10 mEq/L)
Other investigations
- faecal chloride:
high
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Congenital chloride diarrhoea
History
may be positive family history (autosomal recessive); history of polyhydramnios, history of premature birth; voluminous watery stools from a few weeks of age
Exam
dry mouth, decreased skin turgor; faltering growth
1st investigation
- urinary chloride:
low (<10 mmol/L or 10 mEq/L)
- faecal chloride:
high
Other investigations
Cystic fibrosis
History
typically known diagnosis; variable phenotype including faltering growth, recurrent pneumonia; chronic obstructive lung disease, diarrhoea, dehydration in hot weather, and infertility; rarely presents as metabolic alkalosis
Exam
depends on phenotype; includes poor weight gain, wheeze, thick sputum production, clubbing, hepatomegaly
1st investigation
- sweat test:
sweat chloride value >60 mmol/L (60 mEq/L)
Other investigations
- genetic testing:
CFTR gene mutation
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