About this product

St Mark’s solution (also known as St Mark’s Electrolyte mix or E-mix) is a glucose based oral rehydration solution and is not commercially available.

It is prepared by mixing specific quantities of glucose, sodium bicarbonate (or sodium citrate) and salt with potable (tap) water.

Clinical use and mechanism of action

St Mark’s solution is used in the management of high output stoma, enterocutaneous fistula or intestinal failure, also known as short bowel syndrome. As a result, the small intestine’s ability to maintain fluid and electrolyte balance is reduced.

Oral rehydration solutions like St Mark’s solution are isotonic fluids with a similar solute concentration as blood. They promote passive absorption of salt and water to maintain adequate fluid balance. At least 90mmol/L of sodium is necessary to maximise water and sodium absorption.

Short bowel syndrome

Most patients with short bowel syndrome will require fluid, electrolyte, and nutrient supplementation to avoid or reduce diarrhoea and maintain health and growth. Oral rehydration solutions like St Mark’s solution can help, but some people may still require parental nutrition or intravenous fluids.

Intestinal failure

In patients with intestinal failure, drinking hypotonic fluids (which have a lower solute concentration than blood) can lead to diarrhoea as sodium and fluid move into the intestine. Hypotonic fluids include water, tea and coffee. Intake of these, as well as other fluids should therefore be restricted.

Differences from other oral rehydration solutions

Oral rehydration solutions have four main constituents: electrolytes, a bicarbonate source to correct or prevent metabolic acidosis, water to replace fluid losses and a carbohydrate source to maximise fluid and electrolyte absorption.

St Mark’s solution was originally produced by St Mark’s Hospital in London for their patients due to the lack of suitable alternatives available with the same properties. Other products available include:

• The World Health Organisation (WHO) original standard oral rehydration solution contains the right amount of sodium, but it also contains potassium which can lead to high potassium levels in some people.
• Commercially available oral rehydration products generally contain potassium as well as having a lower concentration of sodium.
• Sports drinks are not designed for clinical use as oral rehydration products, and they often have a high sugar and a lower sodium content.

There are variations of the basic St Mark’s formula that may have different electrolyte content. Locally prepared formulas should be checked.

Preparation

The solution must be prepared freshly every day using the following recipe. Measuring spoons should be used to measure out the ingredients:

• 20g (six level 5mL spoonfuls) of glucose powder OR glucose powder with vitamin C*
• 2.5g (one heaped 2.5mL spoonful) of sodium bicarbonate powder (baking soda)**
• 3.5g (one level 5mL spoonful) of sodium chloride (table salt)
• 1 Litre of cold tap water to dissolve the powders

The solution is prepared by allowing each of the powders to dissolve in the water.

*If glucose powder is not available to buy, then the same quantity of glucose powder with vitamin C may be substituted instead (see adverse effects).

**If the sodium bicarbonate is not tolerated due to a taste (bitter or salty) or unavailable, then the same quantity of sodium citrate can be substituted instead.

Composition

Once prepared, this mixture provides 90mmol/L of sodium.

Obtaining ingredients

At St Mark’s hospital, patients are provided with instructions on how to prepare St Mark’s solution. They are generally advised to purchase ingredients from pharmacies or supermarkets. Other hospitals may follow different practices.

Patients may be asked by hospital teams to continue using the solution long term.

Prescribing

Purchasing the individual powders from community pharmacies and/or supermarkets is often cheaper than a single prescription charge.

If prescribed on an NHS prescription, the prescriber can prescribe an original pack of glucose powder (or glucose with vitamin C powder) and ask the patient to purchase sodium bicarbonate powder (baking soda) and sodium chloride powder (table salt) from any supermarket. This is preferable to prescribing an unlicensed ‘special’ product for which costs can be high.

Patient counselling

The following points should be covered when counselling patients on use of St Mark’s solution.

Administration

The prescribed volume of prepared solution should be sipped slowly throughout the day for maximum absorption, and not all at once.

Approximately one litre per day may be necessary to maintain hydration but this can vary. Tolerance to their prescribed volume will be built slowly.

Storage

The prepared solution can be stored at room temperature or in the fridge. It should be discarded 24 hours after it is first prepared. A fresh solution should be prepared each day.

Improving taste

St Mark’s solution can taste bitter due to the sodium bicarbonate component. It can be made more palatable by:

• storing in the refrigerator and drinking it chilled
• freezing into ice cubes and drinking as a slush
• sipping through a straw which can also help minimise the bitter taste

If the taste continues to be a problem, sodium bicarbonate can be replaced by the same quantity of sodium citrate, which can be prescribed.

Alternatively, the taste may be masked by adding a very small amount of fruit juice or squash. This should be done when making up the whole solution rather than adding this to each glass of solution. The aim is to add flavour but not to dilute the solution so that the correct concentration of sodium per litre of water is maintained.

Avoid plain water

Patients should avoid drinking plain water when they are thirsty. This should be substituted with St Mark’s solution instead.

Limit other fluids

Patients should restrict their intake of drinks (such as tea, coffee, soft drinks, fruit juice) to one litre per day but specific advice given to individuals may vary.

Measure accurately

Patients should ensure that they accurately measurement out the ingredients. This is because inaccurate measurement of ingredients or adding too much water or ice can alter the sodium to carbohydrate ratio. This results in the solution being less effective.

Separate fluids with food

In patients who experience a bowel movement immediately after eating, separating eating and drinking are often beneficial. Patients can be advised to withhold fluids for 30 minutes before and after food.

Adverse effects

Adverse effects are not usually reported with St Mark’s solution.

If glucose with vitamin C is used, patients should be advised that large amounts of vitamin C can cause stomach pain, diarrhoea, and flatulence. A large dose of vitamin C is considered more than 1g (or 1000mg) per day. The amount of vitamin C in St Mark’s solution that a patient would normally have each day would be well below this.

However, if adverse effects do occur, they will resolve once the vitamin C is stopped. In such cases, plain glucose powder without vitamin C should be used to make up St Mark’s solution.

Acknowledgements

With thanks to the pharmacy team at St Mark’s hospital, part of the London North West University Healthcare NHS Trust.

Bibliography

Full referencing is available on request.