Kidneys and electrolytes
Fosfaat anorganisch
The phosphate in your blood; mainly relevant with kidney disease, an abnormal calcium or a problem with the parathyroid gland.
Everything about this testWhat it measures
Phosphate is a mineral that, together with calcium, makes bones and teeth strong and is needed for the energy supply of cells and the acidity of the blood. Most of it is in the bones and teeth; about 1 percent is in the blood. The kidneys, parathyroid hormone (PTH) and vitamin D keep the amount in the blood at the right level by absorbing more or less phosphate from food or excreting it in the urine.
- Too high a phosphate (hyperphosphatemia), especially with reduced kidney function
- Too low a phosphate (hypophosphatemia), for example with an overactive parathyroid gland, vitamin D deficiency, malnutrition or heavy alcohol use
- Additional information with an abnormal calcium
High
A raised phosphate mainly fits reduced kidney function, because the kidneys then excrete phosphate less well. Other causes are an underactive parathyroid gland, too much vitamin D or phosphate from supplements, and poorly controlled diabetes. In growing children and young people, phosphate is normally higher. A falsely raised value occurs when blood cells in the tube break down (hemolysis), for example with a difficult blood draw or a finger prick. A raised phosphate itself usually causes no symptoms.
Low
A low phosphate occurs with malnutrition and heavy alcohol use, with long-lasting diarrhea, with a vitamin D deficiency, with an overactive parathyroid gland (the calcium is then often raised) and with heavy use of antacids. Phosphate can also drop temporarily because it moves into the cells, for example under the influence of insulin or with rapid breathing. A slightly low value usually causes no symptoms; according to Het Acute Boekje of the Dutch internists, symptoms usually only appear at a very low value (below 0.3 mmol/l), such as muscle weakness, muscle pain and confusion.
Preparation
- Preferably do not eat for 12 hours before the sample is taken and drink only water. Phosphate varies with meals and over the day.
If your result is abnormal
With an abnormal phosphate, the GP usually repeats the measurement, if possible without hemolysis, together with calcium, albumin and kidney function. Depending on the outcome, PTH and vitamin D follow, or further assessment of kidney function.
Reliability
The measurement is accurate, but a single value says little about phosphate intake or the body's stores: according to the Health Council of the Netherlands, following EFSA, phosphate in the blood is not a reliable measure of these, because the value also depends on age, daily rhythm, season and vitamin D. An abnormal phosphate is assessed together with calcium, kidney function, PTH and vitamin D.
Limitations
- Hemolysis from a difficult blood draw, a finger prick or long transport gives a falsely raised value.
- The value varies with the time of day and with meals.
- Growing children and young people have higher values; reference values differ between laboratories.
- Very high triglycerides, bilirubin or proteins in the blood can interfere with the measurement.
How common is this
According to the Netherlands Nutrition Centre (Voedingscentrum), a shortage or excess of phosphorus does not occur in people who are healthy and eat a varied diet. A raised phosphate is mainly seen with clearly reduced kidney function; a low phosphate mainly with malnutrition, heavy alcohol use and in seriously ill people in hospital.
Who is this useful for
The result is most meaningful with reduced kidney function, with an abnormal calcium or a suspected parathyroid problem, with a vitamin D deficiency with bone symptoms, with malnutrition or heavy alcohol use, and when taking phosphate or vitamin D supplements. In healthy people who eat a varied diet, a shortage or excess of phosphate is rare. The result says nothing about whether you get enough phosphorus from your diet.
Downsides and risks
- A falsely raised phosphate due to hemolysis, for example with a finger prick, can lead to unnecessary worry without a repeat measurement.
Sources
- NVKC, allesovertesten.nl: Fosfaat
- Nederlandse Internisten Vereniging, Het Acute Boekje: Hypofosfatemie
- Nederlandse Internisten Vereniging, Het Acute Boekje: Hyperfosfatemie
- Voedingscentrum: Fosfor (fosfaat)
- Gezondheidsraad: Voedingsnormen voor vitamines en mineralen voor volwassenen (2018), met achtergronddocument
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results