How the Monofilament Foot Test Works: A Patient's Step-by-Step Guide

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The monofilament foot test is one of the simplest checks in diabetes care, and one of the most useful. It takes a few minutes, involves no needles, and tells your doctor whether your feet can still feel the kind of pressure that warns you about an injury. This guide explains what happens during the test, how to prepare, and what the result means. Quick answer: A thin nylon filament is pressed against a few spots on the sole of each foot until it bends, while you keep your eyes closed and say when you feel it. If you can't feel it at some spots, you may have lost "protective sensation", which raises the risk of unnoticed foot injuries. The American Diabetes Association recommends this test at least once a year for everyone with diabetes. Key takeaways ✅ The test is painless, quick and needs no preparation beyond clean feet. ✅ It checks protective sensation, the feeling that alerts you to stones, blisters and cuts. ✅ Everyone with diabetes should have it at l...
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Urine Tests for Sugar, Ketones and Bile Salts: Benedict's, Rothera's and Hay's Tests

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✓ Medically reviewed by Dr. Ram Kumar, MBBS, MD

Before dipsticks and automated analysers, labs used simple chemical tests to check urine for sugar, ketone bodies and bile. Three of these are still taught in every medical and lab technology course and are used in many smaller labs in India: Benedict's test for sugar, Rothera's test for ketone bodies, and Hay's test for bile salts. This guide explains the principle, procedure and interpretation of each, and how they compare with modern tests.

Quick answer: Benedict's test detects reducing sugars (mainly glucose) in urine: the blue reagent turns green, yellow, orange or brick-red depending on how much sugar is present. Rothera's test detects acetone and acetoacetate: a purple-pink ring means ketones are present, as in diabetic ketoacidosis or starvation. Hay's test detects bile salts: sulphur powder sinks instead of floating, which suggests jaundice due to liver disease or blocked bile flow. A positive result is a clue, not a diagnosis, and should be confirmed with blood tests.

Key takeaways

  • ✅ Use a fresh, clean-catch urine sample for all three tests.
  • ✅ Benedict's test is semi-quantitative, giving a rough idea of how much sugar is present.
  • ✅ Rothera's test does not detect beta-hydroxybutyrate, the main ketone in severe DKA.
  • ✅ Urine sugar is not used to diagnose diabetes; blood glucose and HbA1c are.

Benedict's test for sugar in urine

Principle: Benedict's reagent contains copper sulphate, sodium carbonate and sodium citrate. When heated, reducing sugars such as glucose reduce blue cupric ions (Cu²⁺) to cuprous oxide (Cu⁺), which forms a coloured precipitate. The more sugar, the more precipitate and the more the colour shifts from blue towards red.

Procedure:

  1. Take 5 ml of Benedict's reagent in a clean test tube.
  2. Add 8 drops (about 0.5 ml) of urine and mix.
  3. Boil for 2 minutes over a flame, or place in a boiling water bath for 5 minutes.
  4. Let the tube cool and note the colour of the precipitate.
ColourReportApproximate sugar
Blue, no changeNil—
GreenTrace (+)about 0.1–0.5 g/dL
Yellow++about 0.5–1 g/dL
Orange+++about 1–2 g/dL
Brick red++++above 2 g/dL

Interpretation: glucose in urine (glycosuria) usually appears when blood sugar rises above the kidney threshold of about 180 mg/dL, as in uncontrolled diabetes. It can also occur in pregnancy, with a low renal threshold (renal glycosuria), and in people taking SGLT2 inhibitor diabetes medicines, which deliberately cause glucose loss in urine. Other reducing substances such as lactose (late pregnancy, breastfeeding), fructose, galactose and some medicines can give false positives, which is why the glucose-specific dipstick (glucose oxidase) is now preferred.

Rothera's test for ketone bodies

Principle: Acetoacetate and acetone react with sodium nitroprusside in an alkaline medium to form a purple-coloured complex.

Procedure:

  1. Take 5 ml of urine in a test tube and saturate it with ammonium sulphate powder.
  2. Add a few crystals of sodium nitroprusside (or 2 to 3 drops of freshly prepared solution) and mix.
  3. Gently layer 1 to 2 ml of strong liquor ammonia along the side of the tube.
  4. Look for a ring at the junction of the two layers within a few minutes.

Result: a purple or permanganate-coloured ring means ketone bodies are present. No ring means negative.

Causes of ketonuria: diabetic ketoacidosis, prolonged fasting or starvation, severe vomiting (including hyperemesis in pregnancy), very low-carbohydrate diets, prolonged exercise and alcoholic ketoacidosis. Because the test does not detect beta-hydroxybutyrate, it can underestimate ketosis in severe DKA; a blood ketone (beta-hydroxybutyrate) test is more reliable.

Hay's test for bile salts

Principle: Bile salts lower the surface tension of urine. Normally, fine sulphur powder floats on the surface of urine because of surface tension. When bile salts are present, the surface tension falls and the sulphur sinks.

Procedure:

  1. Take about 5 ml of fresh urine in a test tube (or beaker) and a similar amount of water in another as a control.
  2. Gently sprinkle a pinch of dry sulphur powder on the surface of each.
  3. Watch without shaking the tube.

Result: sulphur that sinks in urine (but floats in water) means bile salts are present. Sulphur that stays floating is negative.

Interpretation: bile salts appear in urine in obstructive (cholestatic) jaundice, such as with gallstones blocking the bile duct, and in some cases of hepatitis. They are usually absent in jaundice due to red cell breakdown (hemolytic jaundice). Bile pigments (bilirubin) are tested separately, for example with Fouchet's test or a dipstick.

Urine sample tips

  • Use a clean, dry container and a midstream, clean-catch sample.
  • Test within 1 to 2 hours. Ketones evaporate and bile products break down in light if the sample stands.
  • Record medicines, pregnancy and recent fasting, which can affect results.

Classic tests vs modern methods

Looking forClassic testModern alternative
SugarBenedict's testGlucose-oxidase dipstick; blood glucose and HbA1c for diagnosis
KetonesRothera's testNitroprusside dipstick; blood beta-hydroxybutyrate
Bile salts and pigmentsHay's test, Fouchet's testUrine bilirubin dipstick; blood liver function tests

Frequently asked questions

Can Benedict's test diagnose diabetes?

No. It can suggest high blood sugar, but diabetes is diagnosed with fasting glucose, HbA1c or a glucose tolerance test.

Is ketone in urine always dangerous?

Small amounts can appear after fasting or intense exercise. Ketones with high blood sugar, vomiting or abdominal pain in someone with diabetes need urgent medical attention.

Why is a control tube used in Hay's test?

Water shows how sulphur behaves with normal surface tension, making it easier to judge whether it sinks in the urine sample.

Related reading: Diabetes symptoms and the tests that confirm diabetes.

Last updated: September 2026. General health and laboratory education only, not a diagnosis. Laboratory procedures should follow your lab's standard operating procedures.

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