KidneyFish
Nephromaths
Cover
Disclaimer
Acknowledgements
I Introduction
1
Introduction
1.1
Work in progress
1.2
What this book is - and what it is not
1.3
How to use this book
2
Basic Concepts
2.1
Validity & assumptions
2.2
Balance
2.3
Testing during stress & “normal ranges”
2.4
Renal clearance & fractional excretion
2.4.1
Derivation of FE:
2.4.2
FEs: caveats & practicalities
2.5
Tmax
2.6
Osmolarity, osmolality & tonicity
2.7
Gaps
3
General approach to a patient with electrolyte disturbance
3.1
Vomiting & diarrhoea
3.1.1
Electrolyte content of body fluids
3.1.2
Vomiting
3.1.3
Quantitative assessment after vomiting
3.1.4
Diarrhoea
3.2
Drugs
3.3
IV fluids & electrolyte supplements
3.4
Diet
3.5
Basic set of investigations
II Acid-base
4
Acid-base & chloride
4.1
Acid-base homeostasis
4.2
pH
4.3
Carbonic acid buffer system
4.4
Renal control of acid-base homeostasis
4.4.1
Normal renal response to chronic acidosis
4.4.2
Citrate metabolism in acidosis
4.4.3
Renal response to alkalosis
5
Metabolic acidosis
5.1
Serum anion & osmolar gaps
5.2
Investigations in HCMA
5.2.1
Urine pH
5.2.2
Urine anion gap, osmolar gap, ammonium
5.3
Causes of a metabolic acidosis
5.3.1
Approach to metabolic acidosis
6
Metabolic alkalosis (& chloride homeostasis)
6.1
Causes of a metabolic alkalosis
6.1.1
Specific syndromes and their mechanisms
6.2
Chloride
6.2.1
Chloride-depletion alkalosis
7
Respiratory acid-base disorders
7.1
Classification of respiratory acid-base disorders
7.2
A-a gradient
7.2.1
Alveolar gas equation
7.2.2
Explanation
7.2.3
Interpretation
8
Blood gas interpretation
8.1
General approach
8.2
VBG vs. ABG
8.3
Measures of acid-base status
8.3.1
What is on the blood gas report?
8.3.2
What about TCO2?
8.3.3
Unit conversions
8.3.4
Principles
8.3.5
“Boston Rules”
8.3.6
Severinghaus rules for SBE:
8.3.7
Error bars
8.4
Further testing in metabolic disorders
III Water and sodium
9
Water and sodium
9.1
Total body water
9.2
Plasma sodium
9.3
Renal water excretion
9.4
Urine osmolality
10
Hyponatraemia
10.1
Drug causes of hyponatraemia
10.2
Correction for hyperglycaemia
10.3
Urine sodium
10.3.1
FE
Na
10.3.2
FE
urate
as an alternative index of volume status
10.4
Free water clearance
10.4.1
Calculating free water clearance
10.4.2
Clinical utility of free water clearance
10.4.3
Urine flow rate in hyponatraemia
10.5
SIAD
10.5.1
Diagnosis
10.5.2
Subtypes
10.5.3
Treatment
11
Hypernatraemia
11.1
Causes of hypernatraemia
11.2
Diagnosis in hypernatraemia
11.3
Treatment of hypernatraemia
12
Polyuria
12.1
Causes
12.2
Investigation of polyuria
12.3
Uses for copeptin testing
12.3.1
Assay details
12.3.2
Polyuria-polydipsia (hypotonic polyuria)
12.3.3
Hypernatraemia
12.3.4
Hyponatraemia
IV Potassium
13
Potassium
13.1
Quantification
13.1.1
Interpretation
13.2
TTKG
Assumptions & pre-requisites
Interpretation
14
Hypokalaemia
14.1
Causes of hypokalaemia
14.2
Causes of K loss
14.3
Diagnostic approach
14.4
Vomiting
14.5
Non-reasbsorbable anions
14.6
Cola-induced hypokalaemia
14.7
Hypokalaemic periodic paralysis
15
Hyperkalaemia
15.1
Causes of hyperkalaemia
15.2
Causes of spurious hyper- and hypokalaemia
V Phosphate, Calcium, Magnesium
16
Phosphate
16.1
Urinary phosphate
16.1.1
General approach
16.1.2
Measuring urinary phosphate and calculating TmP/GFR
16.1.3
Reference ranges
16.2
Hypophosphataemia
16.2.1
Consequences
16.2.2
Causes of hypophosphataemia
16.2.3
Notes on specific causes
17
Calcium
17.1
Ca2+ homeostasis
17.1.1
Drug causes of hypercalcaemia:
17.1.2
Polyuria in hyperCa
17.2
Investigations in hypercalcaemia:
17.3
Urine calcium
17.4
Interpreting the results
17.4.1
Unit conversions
17.4.2
Normal calcium excretion:
17.4.3
Hypercalciuria (e.g. in hyperPTH, other causes of hypercalcaemia, RTA):
17.4.4
Hypocalciuria (e.g. FHH, Gitelman):
18
Magnesium
18.1
Mg homeostasis
18.2
Hypomagnesaemia
18.2.1
Consequences
18.2.2
Differential diagnosis of hypomagesaemia
18.2.3
Notes on specific causes
18.3
Investigations
18.3.1
Management
VI Tubulopathies
19
Tubulopathies
19.1
Inherited
19.1.1
Notes on some of the inherited tubulopathies
19.2
Acquired
19.3
Tubulopathy syndromes
19.3.1
Interpretation of these electrolyte syndromes
19.4
Diagnostic approach in suspected tubulopathy
19.4.1
Fractional excretions
19.4.2
Specialised testing
19.5
Sjogren syndrome
19.6
Lithium-associated tubulopathy
19.6.1
Pathogenesis
19.6.2
Polyuria
19.6.3
TIN
19.6.4
Podocytopathy
19.7
Salt-wasting tubulopathies
20
Renal tubular acidosis
20.1
Classification of RTA
20.1.1
Pathogenesis of hypokalaemic RTA
20.1.2
Pathogenesis of hyperkalaemic RTA
20.1.3
Associations
20.1.4
Investigation of suspected RTA
20.1.5
Causes
20.2
Fanconi syndrome
20.3
Management of adult dRTA
Appendices
A
Appendix 1 - notation & abbreviations
A.1
Notation
A.2
Glossary of abbreviations
B
Appendix 2 - rules of thumb
B.1
pH and [H
+
]
B.2
Urine specific gravity and UOsm
C
Appendix 3 - what’s the mechanism?
C.1
Mechanisms in dRTA (see ESPN clinical practice points, NDT 2021):
C.2
Mechanisms in salt-wasting tubulopathy (See Dutta & Layton, AJP Renal 2024 – a modelling study)
D
Appendix 4 - references
Published with bookdown
Nephromaths
Nephromaths
Robert W Hunter
2026-05-14
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