Limitations on Measuring Alkaline Phosphatase (ALP)

By Pardeep Sharda MSc. PGdip, AIBMS, PRINCE 2 Practitioner

Greendale received an interesting sample which highlighted to me why one should be cautious when running the in-house machine. A vet found that on their in-house machine she was getting results which did not fit in, i.e. the ALT was coming out as 694 U/l and ALP was only reading 58 U/l. A previous sample had given a reading of over 10000 U/l.

The sample was then sent to Greendale for further investigation. We then diluted the sample serially and discovered that the ALP was actually measuring at 12611 U/l. The sample was therefore giving a result which was almost two hundred times lower then it should have been.

So why does this happen?

Measuring Alkaline Phosphatase

You may be surprised to note that this is a common occurrence when samples have such high values that they exceed the measuring capability of the analyser. To understand exactly why this occurs one must first understand the basic principles of the chemical reaction that takes place. The ALP in the blood sample reacts with specific chemicals in the slide. These chemicals then break down to produce a colour at a specific wavelength.

The more ALP there is in the sample, the more quickly and intense the chemical reaction would be. This change is then measured by a spectrophotometer and then interpreted into a concentration by using a calibration curve. If there is too much ALP in the blood then the chemicals in the slide are broken down too quickly, thus saturating the slide and resulting in a less intense colour change when measured by the machine. The machine then gives a false low result. Diluting the sample therefore reduces the amount of ALP in the blood and brings it down to a more acceptable level that the machine can measure.


The good thing is that if ALP is raised to such high levels, other chemistries will also be presented as abnormal. It should therefore be easy to spot any irregularities in the results you obtain. If this does occur send the sample to Greendale who can confirm whether this is a true result or not. It is important to note that other chemistries can also be affected by this way and if there are any uncertainties please do not hesitate to contact Greendale for some advice.