Before considering the test, polyuria must be established with an accurate 24 hr urine output
measurement. Urine output >4 mL/kg/hr in infants and children is suggestive of polyuria.
The overnight test is reserved for situations where the diagnosis cannot be easily made by stopping oral fluid intake for a few hours and obtaining sodium and osmolality measurements.
Children with massive polyuria (>4L/24 hr) should start the test in the morning when medical staff are present as the test will usually last 2–4 hrs.
Thyroid and adrenal function must be normal or adequately replaced
The patient must be kept under close surveillance throughout the test to avoid surreptitious water drinking and in order to be monitored for any signs of dehydration.
Protocol
1. The night before the test (at 2200h), take blood for plasma osmolality, urea, electrolytes, glucose and Co-peptin. The test can only be carried out if the plasma osmolality is <295 mmol/kg. Plasma osmolality can be calculated from the urea, electrolyte and glucose results using the formula: 2 x Na + K + Glucose +Urea The osmolality sample will be analysed by the lab first thing in the morning before the test commences.
If the test is to proceed, weigh the patient undressed, record the weight and insert a reliable i.v. cannula.
3. Assess the patient: If there is a low level of suspicion of DI and the patient is >2 years of age, stop all fluid intake at midnight. If there is a high index of suspicion of DI (i.e. patients are polyuric or borderline hyperosmolar)
4. At 0900h weigh the patient undressed and record the weight. Calculate and record 5% of the weight. Collect blood and urine samples for osmolality, urea, electrolytes and Co-peptin The samples should be sent immediately to the Biochemistry laboratory. If the osmolality is >295 mmol/kg the water deprivation test must not be undertaken
Copeptin (also known as CT-proAVP) is a 39-amino acid-long peptide derived from a pre-pro-hormone consisting of vasopressin, neurophysin II and copeptin. Arginine vasopressin (AVP), also known as the antidiuretic hormone (ADH), is involved in multiple cardiovascular and renal pathways and functions
5. Continue to weigh the child hourly and simultaneously collect blood and urine samples for osmolality measurements. Ensure that the child is undressed on each occasion. The weight should be recorded and the test terminated, with DDAVP given, if 5% of the initial body weight is lost (see step 7).
6. The test is normally continued until 3 consecutive urines have shown a total rise in urine osmolality of <30 mmol/kg (normally about 12 midday) or until either :- The urine osmolality exceeds 750 mmol/kg (or 500 mmol/kg in infants) 5% of initial weight is lost or thirst is unbearable Plasma osmolality exceeds 300 mmol/kg N.B. It may be necessary to prolong the test in compulsive water drinking, especially if the child has been drinking excessively immediately prior to the start.
7. At 12 midday, or when the test is terminated, take blood samples for urea, electrolytes, osmolality and Co-peptin, along with a urine sample for osmolality. N.B. If 5% weight loss or extreme distress occurs give DDAVP (5 micrograms intra-nasally or 0.3 microgram i.m.) and free fluids immediately after test is terminated
8. If the child shows no evidence of urinary concentration, proceed with the DDAVP test to allow differentiation between central and nephrogenic DI.
DDAVP Test 1. Allow the patient to drink but not excessively or a dilutional hyponatraemia may occur. N.B. Fluid intake should be no more than twice the volume of urine passed during fluid restriction. Fluid intake should be monitored closely. 2. Give DDAVP as follows: Children aged 12 – 18 years should receive 2 micrograms i.m. or 20 micrograms intranasally Children aged 2 to 12 years 0.5 – 1 microgram i.m. or 10 – 20 micrograms intra-nasally 3. Collect blood and urine samples for osmolality hourly (if possible) for the next 4 hours. Stop if the urine osmolality reaches >750 mmol/kg.
Samples Na, K, Urea & Plasma Osmolality 1 mL lithium heparin blood (orange top) Glucose 1 mL venous blood in a fluoride oxalate tube (yellow top) Urine osmolality 1-2 mL urine in a plain bottle Co-peptin 1 ml lithium heparin (orange top). This sample will only be sent for analysis if the urine and plasma osmolality results are indicative of Diabetes Insipidus.
Interpretation Normal and CWD: Plasma osmolality does not exceed 295 mmol/kg and the urine osmolality rises three-fold to >750 mmol/kg. Central DI: Plasma osmolality >295 mmol/kg with inappropriately dilute urine (<300 mmol/kg). DDAVP produces normally concentrated urine. Nephrogenic DI: As for Central DI, but DDAVP produces no response. Partial DI: Patients have moderate elevation of plasma osmolality and urine osmolality typically between 300-750 mmol/kg. Copeptin: There are currently no reference ranges for Co-peptin in children. The following ranges are derived from limited studies in adult populations: Baseline Co-peptin levels (without prior thirsting): ≥21.4 pmol/L – Suggests nephrogenic DI <21.4 pmol/L – Suggests other polyuria-polydipsia syndromes (including cranial DI) <2.6 pmol/L – Suggests cranial DI Stimulated Co-peptin levels (plasma osmolality >300 mmol/kg): <4.9 pmol/L – Suggests cranial DI
The overnight test is reserved for situations where the diagnosis cannot be easily made by stopping oral fluid intake for a few hours and obtaining sodium and osmolality measurements.
Children with massive polyuria (>4L/24 hr) should start the test in the morning when medical staff are present as the test will usually last 2–4 hrs.
Thyroid and adrenal function must be normal or adequately replaced
The patient must be kept under close surveillance throughout the test to avoid surreptitious water drinking and in order to be monitored for any signs of dehydration.
Protocol
1. The night before the test (at 2200h), take blood for plasma osmolality, urea, electrolytes, glucose and Co-peptin. The test can only be carried out if the plasma osmolality is <295 mmol/kg. Plasma osmolality can be calculated from the urea, electrolyte and glucose results using the formula: 2 x Na + K + Glucose +Urea The osmolality sample will be analysed by the lab first thing in the morning before the test commences.
If the test is to proceed, weigh the patient undressed, record the weight and insert a reliable i.v. cannula.
3. Assess the patient: If there is a low level of suspicion of DI and the patient is >2 years of age, stop all fluid intake at midnight. If there is a high index of suspicion of DI (i.e. patients are polyuric or borderline hyperosmolar)
4. At 0900h weigh the patient undressed and record the weight. Calculate and record 5% of the weight. Collect blood and urine samples for osmolality, urea, electrolytes and Co-peptin The samples should be sent immediately to the Biochemistry laboratory. If the osmolality is >295 mmol/kg the water deprivation test must not be undertaken
Copeptin (also known as CT-proAVP) is a 39-amino acid-long peptide derived from a pre-pro-hormone consisting of vasopressin, neurophysin II and copeptin. Arginine vasopressin (AVP), also known as the antidiuretic hormone (ADH), is involved in multiple cardiovascular and renal pathways and functions
5. Continue to weigh the child hourly and simultaneously collect blood and urine samples for osmolality measurements. Ensure that the child is undressed on each occasion. The weight should be recorded and the test terminated, with DDAVP given, if 5% of the initial body weight is lost (see step 7).
6. The test is normally continued until 3 consecutive urines have shown a total rise in urine osmolality of <30 mmol/kg (normally about 12 midday) or until either :- The urine osmolality exceeds 750 mmol/kg (or 500 mmol/kg in infants) 5% of initial weight is lost or thirst is unbearable Plasma osmolality exceeds 300 mmol/kg N.B. It may be necessary to prolong the test in compulsive water drinking, especially if the child has been drinking excessively immediately prior to the start.
7. At 12 midday, or when the test is terminated, take blood samples for urea, electrolytes, osmolality and Co-peptin, along with a urine sample for osmolality. N.B. If 5% weight loss or extreme distress occurs give DDAVP (5 micrograms intra-nasally or 0.3 microgram i.m.) and free fluids immediately after test is terminated
8. If the child shows no evidence of urinary concentration, proceed with the DDAVP test to allow differentiation between central and nephrogenic DI.
DDAVP Test 1. Allow the patient to drink but not excessively or a dilutional hyponatraemia may occur. N.B. Fluid intake should be no more than twice the volume of urine passed during fluid restriction. Fluid intake should be monitored closely. 2. Give DDAVP as follows: Children aged 12 – 18 years should receive 2 micrograms i.m. or 20 micrograms intranasally Children aged 2 to 12 years 0.5 – 1 microgram i.m. or 10 – 20 micrograms intra-nasally 3. Collect blood and urine samples for osmolality hourly (if possible) for the next 4 hours. Stop if the urine osmolality reaches >750 mmol/kg.
Samples Na, K, Urea & Plasma Osmolality 1 mL lithium heparin blood (orange top) Glucose 1 mL venous blood in a fluoride oxalate tube (yellow top) Urine osmolality 1-2 mL urine in a plain bottle Co-peptin 1 ml lithium heparin (orange top). This sample will only be sent for analysis if the urine and plasma osmolality results are indicative of Diabetes Insipidus.
Interpretation Normal and CWD: Plasma osmolality does not exceed 295 mmol/kg and the urine osmolality rises three-fold to >750 mmol/kg. Central DI: Plasma osmolality >295 mmol/kg with inappropriately dilute urine (<300 mmol/kg). DDAVP produces normally concentrated urine. Nephrogenic DI: As for Central DI, but DDAVP produces no response. Partial DI: Patients have moderate elevation of plasma osmolality and urine osmolality typically between 300-750 mmol/kg. Copeptin: There are currently no reference ranges for Co-peptin in children. The following ranges are derived from limited studies in adult populations: Baseline Co-peptin levels (without prior thirsting): ≥21.4 pmol/L – Suggests nephrogenic DI <21.4 pmol/L – Suggests other polyuria-polydipsia syndromes (including cranial DI) <2.6 pmol/L – Suggests cranial DI Stimulated Co-peptin levels (plasma osmolality >300 mmol/kg): <4.9 pmol/L – Suggests cranial DI
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