Laboratory values of central D.I. s. d.
Laboratory values of nephrogenic D.I. s. d.
- Laboratory values in primary polydipsia:
Increase in body water results in the following changes:
- Sodium drops (due to free water diuresis, the value usually stabilizes at 1 % - 2 % below the basal value).
- ADH secretion decreases
- Serum osmolality decreases
(Kasper 2015)
The ability to form a concentrated urine is usually between 500 - 600 mosmol / l (Krebs 2018), which is below the normal value of 800 mosmol / kg, because the osmotic gradient in the kidney is lost due to the high flow values (Kuhlmann 2015).
If an excretion volume of > 50 ml / kg bw (corresponds to > 3500 ml at 70 kg) and an osmolality of > 300 mosmol / l are present here, this confirms the diagnosis of D.i.
To identify the type of D.I., further diagnostic measures are required.
Further diagnostics regarding central or nephrogenic diabetes insipidus s. d.
For the diagnosis of primary polydipsia are available:
- 1. measurement of copeptin:
Measurement of copeptin is best for further differentiation and is now considered the gold standard in differentiating a D. I. (Christ- Crain 2020).
Copeptin can be used to mirror the vasopressin (ADH) concentration in the circulation. To diagnose primary polydipsia, the test requires stimulation. This can be done by hypertonic saline (osmotic stimulation) or by arginine (non-osmotic stimulation).
The copeptin test shows a sensitivity of 93% and a specificity of 100% in primary polydipsia (Crist- Crain 2020).
Test performance:
This involves first determining copeptin, then infusing a hypertonic 3% NaCl solution and measuring copeptin again.
Remains low in ZDI and NDI, increases in primary polydipsia.
rises in ZDI and NDI, remains normal in primary polydipsia.
remains low in ZDI, rises in RDI and primary polydipsia.
NaCl- test is more reliable than dehydration test at 95%.
(Herold 2021)
Copeptin - level is measured after 8 hours of dehydration in the morning fasting.
It lies in:
- renal diabetes insipidus > 20 pmol / l
- central diabetes insipidus < 2,6 pmol / l
If the determined value is between 2.6 - 20 pmol / l, a further fasting blood sample is required after 16 h of dehydration, in which serum sodium is determined in addition to copeptin. The copeptin index is calculated from these values. If this is < 20, it is a case of central diabetes insipidus partialis and if values are > 20, it is a case of primary polydipsia.
(Kasper 2015)
- 2. fluid deprivation in primary polydipsia:
The test begins in the morning at a well-defined time. The patient is allowed a light breakfast and fluid ad libitum.
The patient is then weighed and a urine sample is taken to determine urine osmolarity, a blood sample is taken to determine plasma osmolality, sodium concentration, ADH or copeptin concentration.
This is followed by the 12-hour thirst phase during which 1 x / h is measured:
- Urine volume
- urine osmolality
- Plasma osmolality
- Serum sodium
- body weight
- Pulse
- Blood pressure
After the end of the test, electrolytes and ADH / Copeptin are measured in addition to the above data.
Under fluid deprivation there is an increase in serum osmolality and sodium level. ADH release is adequate.
However, the concentration capacity of the urine is usually not immediately restored due to the persistent polyuria (Kasper 2015). The values are 500 - 600 mosmol / l (in healthy > 800 mosmol / l ).
(Krebs 2018)
However, the diagnostic reliability is poor in primary polydipsia, as shown in studies by Fenske in 2011 and 2018 (Christ- Crain 2020).