Clinical Q&A: Urine glucose and black stools

Our experts answer questions posted on our website by GPs

Is urine dipstick testing in a routine health check of the under-40s cost effective?

The short answer is ‘no’.  Routine urinalysis for glycosuria involves the use of reagent strips capable of detecting 0.1 per cent glucose concentrations. This may then seem like a reasonable way of screening for diabetes.

Unfortunately, this has not proven to be the case and all protocols for diabetes screening recommend testing of blood (usually in the fasting state).

There are a number of issues. There is only a moderate correlation between blood glucose levels and semi-quantative measurements of urinary glucose. So, in a study assessing the urine of 400 diabetic patients, urine testing strips failed to detect glycosuria in 9–16 per cent of samples from patients with plasma glucose levels greater than 11mmol/l.

In the primary care setting in Sweden, urinalysis had a sensitivity of only 23 per cent for detecting new cases of type-2 diabetes.

As the diagnostic thresholds for diabetes fall, these figures will worsen.

Furthermore, the renal threshold for diabetes varies considerably between individuals, ranging from 8.0 to 11.0mmol/l.

Note that screening for type-1 diabetes is not feasible since the asymptomatic, hyperglycaemic prodrome is short.

Despite the widely publicised ‘epidemic’ of type-2 diabetes in young people, type-1 remains more prevalent in the under 40s and should be the default diagnosis, leading to urinary ketone testing.

Professor Stephen Bain, professor of medicine, University of Wales, Swansea

A female patient presented with jet black, tarry and smelly stools.  She was on aspirin, lansoprazole and iron tablets. I could not decide whether it was melaena or black stool due to the iron tablets. How one can tell the difference?

Telling the difference between black stools associated with iron therapy and an upper gastrointestinal bleed, on history alone, can be difficult.

Characteristically, melaena is tarry and has a distinct odour.

This patient has a history of aspirin ingestion, which can be a cause of upper GI bleeding, but is also on lansoprazole,  which should counter any ulcerogenic effects.

However, it is not clear why she is on lansoprazole. She may have a history of peptic ulceration which would make an upper GI bleed more likely.

We also need to consider why she is on iron therapy. Does she have an iron deficiency anaemia and if so has the cause of it been identified?

In a woman in her reproductive years menorrhagia is a possible cause of iron deficiency anaemia, but in post-menopausal patients it should be assumed that they have a malignancy in their gastrointestinal tract.

Caecal carcinoma is associated with occult blood loss and presents with anaemia. However, occasionally it may lead to melaena.

Gastric cancer presents with the triad of anorexia, weight loss and anaemia. While a peptic ulcer can produce pain, anaemia and bleeding it may be silent and only identified on gastroscopy.

On the basis of what I know I would recommend the patient be referred urgently to a medical or surgical gastroenterologist for further evaluation and investigation.

If the patient has signs suggesting that she is bleeding she should be referred up to hospital as an emergency.

Mr Dugal Heath, consultant gastroenterologist and laparoscopic surgeon,
The London Clinic

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