What is the unit of measurement for `quantity`?

[sebbacon]

The concept of quantity is central to most of our analyses. It is used to count levels of prescribing in some of our measures; it is used in our "price-per-quantity" tool; and it is used when comparing between products using ADQs.

The (archived) NHS Digital definition is:

The quantity of a drug dispensed is measured in units depending on the formulation of the product, which is given in the drug name...

Where the formulation is tablet, capsule, ampoule, vial etc. the quantity will be the number of tablets, capsules, ampoules, vials etc.

Where the formulation is a liquid, the quantity will be the number of millilitres

Where the formulation is a solid form (eg. Cream, gel, ointment), the quantity will be the number of grammes.

The monthly PCA data has a column "Standard Quantity Unit" (update: this column appears to have been removed without notice in December 2018) e which appears to indicate what we're looking for. The supplied glossary states:

Standard quantity unit (SQU): This code indicates the form of the drug and the units in which quantity is measured: Code 1 - a unit (e.g. one tablet, capsule, pack, aerosol etc) Code 3 - millilitres Code 6 - grammes Code 0 - individually formulated (unit varies)

Working with that definition

We have three sets of rules to formulate (developed using this SQL as a view):

• Discrete (Tablet, capsule, ampoule, vial etc): where the DT "form indicator" is Discrete. The unit of measure is then available in the "dose form unit of measure" field in dm+d. However, this discrete forms include some that could be measured as liquids or non-discrete solids: spoonful, sachet, cup and ml. Spoonful, in particular, is quite common, and appears to be measured in ml (rather than spoonfuls) in at least one example (see below). This category also includes "unit dose".
• Solid and continuous: where the DT "form indicator" is not Discrete and the form is in this list (or this temp table) (examples: Powder, Oral gel, Foam).
• Liquid and continous: Everything else where "form indicator" is not Discrete, per this list of form values (examples: Liquid, Ear drops)

How are spoonfuls denominated with respect to quantity?

Per our heuristic above, they are a discrete quantity, but in reality quantity appears to mean ml. For example, this Dicycloverine HCl solution is available in 120ml bottles. As can be seen on that page (from dm+d), its "unit dose" is a "5ml spoonful". The strength of its active ingredient is 2mg per ml.

Its ADQ is 30mg. So that's a average daily dose for adults of 15ml, or 3 spoonfuls: 24 days' worth in a bottle.

The question is: is a quantity of this 1ml, or 5ml? Based on all historic prescribing data, we can see spikes at 5, 10, 20, 30 and 50, which suggests GPs are prescribing in spoonfuls, but the data is recorded in ml. This is reminscent of the situation where we also know that in some cases quantity also represents packs.

Other strange corners

When applying the above heuristic, extra dm+d metadata leads us to question some of our categorisations. Some examples:

• Barium sulfate 94% powder for rectal suspension has a form of "Enema", and has "powder" and "suspension" in its name. Thus, based on its name only, it could be any of a discrete item, a continuous liquid, or a continuous cream
• Sprays would naturally be considered to occur in ml but when considering name only, some, like Betadine 2.5% dry powder spray could be intuitively either in mg or g
• Liothyronine 2.5microgram capsules should surely be Discrete but are listed as Continuous

We would normally expect the unit of measure of the denominator ingredient in dm+d to match what we consider quantity. For example, we would normally expect the denominator in a liquid to have a unit of measure of ml or l`. This gives us an opportunity to find items where the denominator unit of measure varies considerably within their class:

• Most sprays are treated as liquids, but some like Acetic acid 2% ear spray may be measured in grams.
• Shampoos occupy a strange zone: in dm+d, like most shampoos, Prioderm 1% cream shampoo would appear to be measured in grams (thus a solid); however, several, like Coal tar 4.3% shampoo and Dermax Therapeutic 0.5% shampoo would appear to be measured in ml
• Paint is similar to shampoo, though appears mostly to be in ml: Tioconazole 283mg/ml medicated nail lacquer is apparently in ml, Resorcinol 50% in Glycerol is apparently in mg
• Bath additives are similar again. Liquid paraffin light 85.09% bath oil is apparently in grams, but Liquid paraffin light 63.4% bath additive is in ml
• Per the official definition, gels are a solid form and should be measured in grams, but 6% of them have denominators measured in ml in dm+d, such as Lidocaine 1% gel and Nonoxinol-9 2% gel. This is also true of a small number of creams, e.g. Boots Skin Allergy Relief 2% cream
• Eye drops are mostly treated as a liquid, but sometimes they have a denominator in grams in dm+d - for example, Trisodium edetate 0.4% eye drops and Hypromellose 2% eye drops

Special containers

In the case of special containers, we can't rely on the quantity at all, as dispensers sometimes record a number of packs rather than tablets (for example), presumably to indicate entire packs were dispensed. How NHSBSA distinguishes between packs and tablets in such cases for the purpose of reimbursement is unclear.

From this NHSBSA glossary:

...where a product is packed in a 'special container' (see Drug Tariff Part II Clause 10) the pharmacist is not required to split the pack and may supply the number of containers or original packs which provide the quantity nearest to that prescribed. In some circumstances these items show quantity as the number of units supplied ie 1 or 2 even though a pack may contain 56 tablets.

Included in this category are preparations which are: sterile; effervescent or hygroscopic; liquid preparations for addition to bath water; coal tar preparations; viscous external preparations; preparations packed in a collapsible tube, pressurised aerosol, spray, puffer pack, squeeze pack, castor, shaker, roll-on bottle, drop bottle. container with an integral means of application, or any other container from which it is not practicable to dispense the exact quantity ordered; reconstituted from granules or powder, resulting in a liquid of limited stability.

This rule applies to Special Containers which can not have Broken Bulk applied. This data is available in dm+d at the AMPP level. However, as prescribing data is only at AMP/VMP level, we can only tell if an AMP/VMP may have this condition applied.

Questions:

• Is our heuristic right? Should we make an exclusion for spoonful and treat it as a liquid rather than a discrete dose?
• How does the value given for quantity find its way to the data? Is it from EPS systems and if so, what is their data source? As this data is the basis for paying dispensers, it presumably comes from them, even if the originating system was an EPS. What kinds of system do they use to record quantity? Is this, in fact, all reliant on the individual judgement of dispensers, hence the wide variation?
• Rather than rely on a guessed implementation of a loosely-worded definition, does NHSHSA have a programmatic way of defining quantity for us?
• As noted in price-per-dose/issues/12, BSA have told us there are "complexities regarding the authoring of the size of sachets in general, some have a container size of 1 and some have the weight of the sachet as the size. This is a historic issue. Work is under way to review this and agree a way forward". Is there any update on this?
• Is there anything we can do about the fact that special containers that cannot have broken bulk applied can be recorded as 1 (i.e. per-pack?) If not, should we drop such products from any quantity-based analysis?
• Does NHSBSA have a process for identifying and reporting when they find errors or inconsistencies?

[richiecroker]

@sebbacon can't find the issue at the moment, but there was also a problem with when quantity is always an integer, but pack sizes are 2.5ml

[sebbacon]

Another example of where quantity appears to mean different things - Fentanyl 400micrograms/dose nasal spray

Sometimes recorded by dose (which is what dm+d says it should be recorded by), sometimes by pack:

SELECT
  practice,
  items,
  net_cost/quantity AS ppq
FROM
  `ebmdatalab.hscic.normalised_prescribing_standard`
WHERE
  month = '2018-04-01'
  AND bnf_code = '0407020A0AABPBP'
ORDER BY
  ppq

[alexwalkerepi]

This doesn't appear to be related to EHR:

SELECT
  practice,
  items,
  net_cost/quantity AS ppq,
  Principal_Supplier,
  Principal_System
FROM
  `ebmdatalab.hscic.normalised_prescribing_standard`
LEFT JOIN
  ebmdatalab.hscic.vendors
ON
  practice = ODS AND Date = '2018-04-01'
WHERE
  month = '2018-04-01'
  AND bnf_code = '0407020A0AABPBP'
ORDER BY
  ppq```

[sebbacon]

From an NHS BSA newsletter:

Most inhalers, sprays and similar products areprescribed with the quantity measured in dosesrather than devices. In certain prescribing systemsa ‘pack conversion facility’ exists, allowingprescribers to enter the number of devices whichare converted to the appropriate equivalentquantity of doses. Unfortunately, where this facility exists, sometimes the prescriber enters the number of doses instead of the number of inhalers and excessive quantities are inadvertently prescribed. If an item is prescribed with an excessive quantity but you have dispensed less, you must populate the quantity that was actually dispensed.Example 1 – 3600 doses prescribed (this is equivalent to 60 x 60 dose devices), dispensed quantity 3600 doses. The item was returned to confirm the quantity dispensed and resubmitted with additional information, stating 60 x 60 dose devices were actually dispensed (£1380.00). Further contact confirmed only 1 x 60 dosedevice (£23.00) was supplied. This resulted in a delay to the contractor’s reimbursement:Endorsing the quantity dispensed in EPS and paper submissionsExample 2 – 9520 x 85g sachets prescribed (£24,656.00). The item was returned to confirm the quantity dispensed and was resubmitted, stating 9520 grammes (£290.08) was supplied. This resulted in a delay to the contractor’s reimbursement