What Urine Colour Can And Cannot Indicate

Urine colour is the most widely used informal indicator of hydration. It reflects a real physiological variable, and it is also affected by enough unrelated factors to be easily misread.

The colour reflects concentration

The yellow pigment in urine is a breakdown product of haemoglobin and is produced at a fairly steady rate. Its concentration therefore depends mainly on how much water it is dissolved in.

When water is being conserved, less is excreted with the same amount of pigment, and the urine appears darker.

When intake exceeds requirement, the excess is excreted and the same pigment is diluted, producing a pale appearance.

Charts approximate a laboratory measure

Laboratories quantify this properly through specific gravity or osmolality, which measure the concentration of dissolved substances directly.

Colour charts approximate those measures and correlate with them reasonably well in controlled conditions, which is why they are used in sports and occupational settings.

The correlation is much weaker outside those conditions, since it depends on nothing else having altered the colour.

It also depends on lighting and on the container, since the same sample can look markedly different against white porcelain and against a clear vessel held to daylight.

Many things change the colour independently

Several B vitamins produce a distinctly bright yellow at doses that exceed what the body retains, and the colour then reflects excretion rather than concentration.

Certain foods, including beetroot and heavily coloured products, can tint urine, as can a number of common medications.

Anyone taking supplements or medicines is therefore reading a colour with an added component that the chart does not account for.

The added colour does not fade as fluid intake rises, which can lead someone to keep drinking in response to a shade that has nothing to do with concentration.

Timing shifts the reading

The first urine after waking is normally concentrated, because little fluid was taken during the night while losses continued.

Conversely, a sample shortly after a large drink is dilute regardless of overall status, since the excess is being cleared.

Assessing on a single sample without regard to timing produces results that reflect the last hour rather than any general state.

When colour is a medical sign

Red or brown discolouration, persistent very dark urine despite adequate drinking, or cloudy urine with pain are not hydration matters.

These can indicate blood, liver or muscle problems, or infection, and each warrants prompt medical assessment rather than more water.

Colour is a rough indicator of one variable in an otherwise well person, and any unexplained change belongs with a doctor rather than a chart.