Urine tests, and the limits of a dipstick

The urine dipstick is one of the oldest and most widely used point-of-care tests in medicine. A strip with a row of coloured pads, dipped and read within a minute, produces a surprising amount of information.

It also produces a fair number of results that mean less than they appear to, and understanding what each pad actually detects explains why.

What the pads measure

Protein detection is weighted toward albumin and is comparatively insensitive to small amounts, which is why a specific albumin measurement is used where early kidney changes are the question.

Blood is detected by an enzyme-like activity of haemoglobin, which means the pad reacts to free haemoglobin and to muscle pigment as well as to intact red cells. A positive result does not by itself establish that whole blood cells are present.

Nitrite detects the conversion of dietary nitrate by certain bacteria. It is fairly specific when positive and misses a good deal, because not all relevant bacteria perform that conversion and because urine needs to sit in the bladder for some hours for enough nitrite to accumulate.

Leucocyte esterase detects an enzyme from white cells and is a more sensitive but less specific indicator of inflammation in the urinary tract.

Glucose, ketones, bilirubin, urobilinogen, pH and specific gravity make up the rest, each with its own limitations.

Concentration changes everything

A dipstick measures concentration, not total quantity. Very dilute urine after a large drink can turn a genuinely present substance into a negative result; concentrated first-morning urine can push a trace into a positive.

This is why specific gravity appears on the strip, and why laboratory measurements of protein are often expressed as a ratio against creatinine rather than as a raw concentration. The ratio corrects for how dilute the sample happens to be.

Collection matters more than people expect

Contamination from skin or genital flora is common and produces misleading results, particularly for white cells and bacteria. Instructions about mid-stream collection exist for a concrete reason.

Samples left standing at room temperature change. Bacteria multiply, altering counts and pH, and some components degrade. A sample that has sat for hours before being processed is a different sample from a fresh one.

Menstruation, recent vigorous exercise and some foods and medicines all affect dipstick results in ways that have nothing to do with the urinary tract.

Protein in urine and why it gets its own test

The kidneys normally allow very little protein through, so persistent protein in urine is one of the more informative findings the test can produce. The dipstick is a poor instrument for detecting the small amounts that matter earliest.

This is why a specific albumin measurement, expressed as a ratio against creatinine in the same sample, is used where early kidney changes are the question. Expressing it as a ratio removes the effect of how dilute the sample happens to be.

Transient protein appears after fever, after intense exercise, and in some people simply after standing upright for a period. A single positive is therefore commonly repeated on an early morning sample before it is treated as a finding.

What follows a positive dipstick

Microscopy examines the sample directly, identifying cells, crystals and casts that a dipstick cannot see. Casts in particular carry information about where in the kidney something is happening.

Culture grows any bacteria present and identifies them, along with which antibiotics they respond to. It takes a day or two and is what actually establishes an infection rather than suggesting one.

A dipstick is therefore best understood as a triage tool. It is designed to be fast and to over-refer rather than to miss, which means its positives frequently need confirming and its negatives are more informative than people assume.