Measuring quantity in the Secondary Care Medicines Data

The Secondary Care Medicines Data (SCMD) reports the quantity of individual medicines and devices issued each month by NHS trusts in England. Before this can be used to analyse trends and variation in medicines usage, there are several complications to be aware of. This blog describes these complexities and how they can be addressed. In the following posts in this series, we will explain how the dictionary of medicines and devices (dm+d) can be used to get more information about individual medicines and how this can be used to calculate alternative measures of quantity.

Working with the quantity reported in the SCMD

Every product that is reported in the SCMD has an associated quantity. In many cases, this is all you need to get an understanding of how much of a medicine or device is used, but there are a few things to be aware of with the raw data:

1. Different products may be reported using the same type of units, but at different orders of magnitude. For example, some liquid preparations are reported in litres, others are reported in millilitres.
2. A small number of products are reported in varying types of units. For example, the same product can be reported in both grams and millilitres.
3. The structured data for individual products, such as the code which is used to identify them, or the associated name can change over time.

We’ll look at each of these issues and describe how we can handle them below.

Units of measure with different orders of magnitude

There are a lot of different units of measure reported in the SCMD. Most of these are distinct, but some represent the same type of units at different orders of magnitude. For example, liquids may be measured in microlitres, millilitres or litres; mass may be measured in micrograms, milligrams or grams. Below is an example from a previous blog where two similar products, which you may want to compare, are reported in litres and millilitres.

Unit of measure	Product name	Quantity
litre	Bupivacaine 1g/1litre (0.1%) / Fentanyl 2mg/1litre infusion bags	73
ml	Bupivacaine 250mg/250ml (0.1%) / Fentanyl 500micrograms/250ml infusion bags	6,789,870

Simlpy aggregating the quantity issued for these products would lead to an incorrect answer. To make it easier to compare quantities across these products, we can convert them to the same basis unit. As the most common unit for liquids in the raw data for liquids is ml, it makes sense to make this the basis unit and convert anything reported in litres or microlitres into ml. You can see all of the unit mapping we apply on OpenPrescribing Hospitals here. In total, the units for around 300 products are converted.

Products with changing units of measure over time

There are a small number of products in the SCMD which are reported using completely different units of measure at different points in time. Below you can see the products reported in the SCMD where this is the case, and the time periods where they report each unit (some of these are infrequently used, so don’t have any quantity issued, or associated unit of meausre in some months).

We’re not sure what is driving (let us know if you do!) this, but some of the changes align with changes to the product (VMP) name in the dm+d.

Changing between units of different types is more challenging than changing between units of the same type as it requires a deeper understanding of how the product is prepared. For example, to map Levothyroxine sodium 500microgram powder for solution for injection vials from micrograms to vials, we have to find the quantity of micrograms in each vial. For this product, you can understand this from its name, but it’s also evident from the other structured data within the dm+d. We then divide any quantity measured in micrograms by 500, to convert it into the quantity in vials (each containing 500 micrograms). You can see the conversions we use on OpenPrescribing Hospitals for the rest of the products above here.

Changes to product information in the dm+d

The SCMD is reported in dm+d format, which is is updated regularly. As a result, the structured information for individual products, such as the VMP code or the associated name can change (some multiple times!). For example, the identifier for Paracetamol 500mg tablets changed from 322236009 to 42109611000001109 in July 2023.

The SCMD contains a mix of provisional data for the most recent months and finalised data for more historic data. As the name suggests, finalised data is fixed, so when changes in the dm+d occur after publication of the finalised data, products like the one above can be represented using two different codes within the SCMD. This is shown in the figure below.

So that the correct trend for this single product over time can be analysed, data for outdated codes has to be aggregated with the data for the currently used codes. So the two separate traces in the chart above would appear as a single line, using the code 42109611000001109 .

As of May 2025, there were 14,394 unique product codes in the raw SCMD, 1,831 of which can be mapped to a more recent code.

Alternative measures of quantity

Once the three issue above have been handled, we end up with a quantity measure that can be reliably used to look at trends in the use of all of the individual products in the SCMD over time. We refer to this processed version of the raw quantity as the SCMD quantity.

In a previous post, we indicated that varying units of measurement make comparing quantity across different products is more challenging. But, the SCMD quantity can be used to calculate other measures of quantity which may be more appropriate when looking at trends across multiple different products. These include:

• Unit dose quantity - measured quantity of a medicine, packaged for administration to a patient in a single unit.
• Ingredient quantity - the quantity of each ingredient within a product.
• Defined Daily Doses - a unit of measure for medicines consumption that enables comparison of usage across groups of medicines. Defined Daily Doses are defined and mantained by the World Health Organisation, who define them as “the assumed average maintenance dose per day for a drug used for its main indication in adults”.

We’ll explain what each of these are and how they are calculated in more detail in the following posts in this series.