A ten-pad urine strip is the most-used point-of-care test in general practice and the most casually read. Almost all of the error is put in before anyone looks at the colours.
What a strip can and cannot do. It is a good rule-out test
for urinary infection and a poor rule-in one: in the manufacturer's own multicentre data,
93% of samples negative for both leukocytes and nitrite were confirmed negative on
culture. A positive strip is much weaker, because the things that turn those pads positive are
not only infection. And in over-65s and catheterised patients it should not be used to diagnose
infection at all — see urinary tract infection.
1. Getting the sample right
- Midstream, into a clean container. The first part of the stream carries urethral and perineal cells and organisms, which is where most false-positive leukocytes come from.
- Fresh. Test within about 2 hours at room temperature. Refrigerated, it holds for around 24 hours — but bring it back to room temperature before dipping, because strip reactions are temperature-dependent and cold urine under-reads. The optimum is 22–26 °C.
- Standing urine changes its own result. Bacteria multiply, glucose is consumed, ketones and bilirubin degrade in light, red cells lyse, and pH rises as urea breaks down to ammonia — which then produces a false-positive protein.
- In a woman, note where she is in her cycle. Menstrual blood makes the blood pad meaningless.
- Do not dip from a catheter bag. Sample from the port, or not at all.
2. Running it, and the two mistakes everyone makes
Dip briefly, then drag and blot — do not stand it upright
Immerse all pads for no more than a second, then drag the edge along the rim of the container and blot the edge on paper. Mistake one: leaving excess urine on the strip lets reagent run between pads. It has a name — runover — and the classic version is acid buffer from the protein pad running onto the pH pad and falsely lowering the pH. Never hold the strip vertically with urine on it.
Read each pad at its own time, against the bottle
Not "about a minute". The pads develop on different schedules and reading them together is the second mistake. Times in section 3. Read in good light, holding the strip right against the chart on the bottle.
Never read the leukocyte pad late
It keeps developing. A colour change appearing after two minutes is disregarded — it is not a weak positive, it is nothing. This single habit accounts for a large share of false-positive strips and therefore of unnecessary antibiotics.
Keep the bottle shut, and check the date
The pads are moisture-sensitive; cap immediately and tightly, and do not decant into another container or add a desiccant of your own. Do not touch the pads.
3. The ten pads, in the order they are read
| Read at | Pad | What it detects |
|---|---|---|
| 30 s | Glucose (GLU) | Glucose specifically — not lactose, galactose or fructose |
| 30 s | Bilirubin (BIL) | Conjugated bilirubin |
| 40 s | Ketones (KET) | Acetoacetate, and only weakly acetone. Not beta-hydroxybutyrate |
| 45 s | Specific gravity (SG) | Ionic concentration, as a proxy for how concentrated the urine is |
| 60 s | Blood (BLO) | Peroxidase activity — so haemoglobin, free or in cells, and myoglobin |
| 60 s | pH | 4.5 to 9. Can be read immediately |
| 60 s | Protein (PRO) | Mainly albumin. Insensitive to light chains |
| 60 s | Urobilinogen (URO) | Normal range 0.2–1.0 mg/dL; 2.0 is the transition to abnormal |
| 60 s | Nitrite (NIT) | Nitrite, produced by bacteria that reduce dietary nitrate |
| 120 s | Leukocytes (LEU) | Esterase from neutrophils, eosinophils, basophils and monocytes — not lymphocytes. Detects lysed cells too, so it can be positive when microscopy shows few intact white cells |
4. What each pad means when it is positive
| Pad | Positive means | Worth knowing |
|---|---|---|
| Nitrite | Nitrate-reducing bacteria — E. coli, Klebsiella, Proteus and relatives. Reasonably specific for infection | A negative nitrite excludes far less than people think. It misses organisms that do not reduce nitrate — Enterococcus, Staphylococcus saprophyticus, Pseudomonas, and most that cause infection in children — and it needs the urine to have sat in the bladder about four hours to accumulate. Someone passing urine every twenty minutes because it hurts is the person least likely to produce a positive nitrite |
| Leukocytes | White cells, present or lysed, anywhere in the urinary tract | Sensitive and not specific. Positive in infection, but also with vaginal or perineal contamination, urethritis and sexually transmitted infection, stones, catheters, recent instrumentation, interstitial nephritis, tumour, and vigorous exercise |
| Blood | Haemoglobin or myoglobin | Cannot distinguish red cells from free haemoglobin from muscle breakdown. So it is positive in crush injury and rhabdomyolysis with no bleeding at all. Also menstruation, catheter trauma, exercise, and infection itself — bacterial peroxidases can turn this pad positive |
| Protein | Albumin above roughly 0.15 g/L | Blind to Bence Jones light chains, so a negative pad does not exclude myeloma. Trace protein in very concentrated urine is often nothing. Persistent protein needs quantifying properly with an albumin:creatinine ratio, not repeat dipping |
| Glucose | Blood glucose above the renal threshold, roughly 10 mmol/L | Glucose in the urine with normal blood glucose is not diabetes. The commonest cause now is an SGLT2 inhibitor, which works by causing glycosuria. Otherwise pregnancy or a renal tubular disorder |
| Ketones | Acetoacetate | The most misleading pad on the strip in a sick patient. Section 5 |
| Bilirubin | Conjugated bilirubin reaching the urine | Any positive, including trace, is abnormal — there is none in normal urine. Suggests hepatocellular disease or biliary obstruction, and often precedes visible jaundice |
| Urobilinogen | Above 1.0 mg/dL: haemolysis, or hepatocellular disease | Its absence is the more interesting finding — with a positive bilirubin it suggests complete biliary obstruction, because no bilirubin is reaching the gut to be converted |
| pH | — | Persistently above 7 with a positive nitrite suggests a urea-splitting organism, classically Proteus, which alkalinises urine and forms struvite stones. Persistently below 5.5 goes with uric acid stones and with acidosis |
| Specific gravity | — | High with low urine output suggests dehydration or a pre-renal state. Fixed near 1.010 regardless of intake means the kidney has lost its ability to concentrate or dilute. Very low with high volume raises diabetes insipidus or excess intake |
5. The ketone pad, and why it misleads in diabetic ketoacidosis
It measures the wrong ketone
- The dominant ketone in ketoacidosis is beta-hydroxybutyrate. The strip does not measure it. It measures acetoacetate, which is a minority species early on.
- So the strip understates severity at presentation, when the ratio is pushed hard toward beta-hydroxybutyrate.
- And during successful treatment it can appear to get worse. As the patient improves, beta-hydroxybutyrate is converted to acetoacetate — the thing the strip measures — so urine ketones can rise while the person is recovering. Treating that as deterioration is a known and avoidable error.
- Blood beta-hydroxybutyrate is the measurement that matters in suspected ketoacidosis, and urine ketones should not be used to monitor treatment.
- Ketones without glucose is a different and often benign picture: fasting, vomiting, a very low-carbohydrate diet, alcohol, or pregnancy. Ketones with glucose in an unwell person is a same-day emergency, and normal blood glucose does not exclude it — SGLT2 inhibitors cause euglycaemic ketoacidosis.
6. Combinations — what patterns actually mean
The pads are not independent. Reading them together is where a strip becomes useful.
The urinary infection question
| Nitrite | Leukocytes | Reading |
|---|---|---|
| Positive | Positive | Infection likely. Treat on symptoms |
| Positive | Negative | Infection likely. Nitrite is the more specific of the two — treat if there are symptoms |
| Negative | Positive | Possible. Culture, and think about the other causes of white cells — contamination, sexually transmitted infection, stones, a non-nitrate-reducing organism |
| Negative | Negative | Unlikely. The strip's strongest result. Look for another explanation for the symptoms |
Other patterns worth recognising
| Pattern | Suggests |
|---|---|
| Blood + protein, with nitrite and leukocytes negative | Glomerular disease, not infection. This is the pattern that most needs recognising, because it gets antibiotics and needs a nephrologist. Ask about recent sore throat, rash, joint pain and swelling, and check blood pressure and kidney function |
| Blood alone, repeatedly, no infection | Needs a haematuria pathway — stones, and urinary tract cancer. Age over 45 with visible blood, or over 60 with non-visible blood plus other features, meets urgent referral criteria |
| Blood positive with no red cells on microscopy | Myoglobin — muscle breakdown from crush, prolonged immobility, extreme exertion, seizures, statins or an electrical injury. Or free haemoglobin from haemolysis |
| Protein alone | Could be nothing (concentrated urine, fever, upright posture, exercise) or early kidney disease. Quantify with an albumin:creatinine ratio on a fresh early-morning sample |
| Glucose + ketones | Ketoacidosis until excluded. Section 5 |
| Glucose alone, well patient | Hyperglycaemia, an SGLT2 inhibitor, pregnancy, or renal glycosuria |
| Bilirubin + raised urobilinogen | Hepatocellular disease — hepatitis, drug injury, alcohol |
| Bilirubin positive, urobilinogen absent | Biliary obstruction — stone or tumour. Pale stools and dark urine fit |
| Raised urobilinogen, bilirubin negative | Haemolysis. Check haemoglobin and reticulocytes |
| pH above 7 + nitrite positive | Urea-splitting organism; consider Proteus and struvite stones. Also just an alkaline sample that has been standing |
| Leukocytes + blood, nitrite negative, dysuria | Consider chlamydia and gonorrhoea in a sexually active person — sterile pyuria with symptoms is a classic presentation and is missed by treating for cystitis |
| High specific gravity with trace protein and trace everything | Likely just concentrated. Repeat on a better sample rather than acting on it |
| Everything negative in a symptomatic young woman | Infection is unlikely but not excluded — a frequently-voided dilute sample can be negative. Culture, and consider urethritis, vulvovaginal causes, interstitial cystitis and sexually transmitted infection |
7. Interference — the results that are about the sample, not the patient
| Cause | Effect |
|---|---|
| Vitamin C — ascorbic acid, above roughly 25 mg/dL | False-negative nitrite, blood and glucose, and reduced leukocytes. Common, invisible, and the reason a strip can be clean in a genuinely infected person taking supplements or drinking a lot of fortified juice |
| High specific gravity — concentrated urine | False-positive protein; falsely low glucose, blood, nitrite and leukocytes |
| Alkaline urine, including a stale sample | False-positive protein; reduced nitrite sensitivity |
| Very high glucose, above about 3 g/dL | Falsely low leukocytes |
| Cefalexin, cephalothin, tetracycline, gentamicin | Falsely low leukocytes |
| Nitrofurantoin, phenazopyridine, rifampicin, beetroot, senna, azo dyes | Colour the urine and mask the pads. Not a false result so much as an unreadable one — if the urine is bright yellow, orange or red, say so rather than reporting a value |
| Strong oxidising agents, including some cleaning residue in the container | False-positive leukocytes and blood |
| Chlorhexidine or antiseptic on the skin or in the pot | False-positive protein |
| Formaldehyde as a preservative | False-negative urobilinogen and leukocytes |
| Captopril, levodopa, valproate, and some sulfhydryl drugs | False-positive or masked ketones |
| Para-aminosalicylic acid, sulfonamides | False-positive urobilinogen |
| Reading late | False-positive leukocytes above all, and drift on most other pads |
| Reading cold | Everything under-reads. Reactivity rises with temperature |
The habit worth building. Before you interpret a strip, ask three questions: was the sample fresh and midstream, was each pad read at its own time, and is the person on vitamin C, an SGLT2 inhibitor, nitrofurantoin or a drug that colours urine? Those three account for most of the strips that mislead, and none of them is visible in the result.
Try the combinations. The
dipstick UTI interpreter lets you set all ten pads
and shows what the pattern suggests, which pads are pulling the interpretation, and what would change it.
It reads patterns; it does not diagnose, and it will say so more than once.