Calculation of Oral Medications

Calculation of Oral Medications

After reviewing this chapter, you should be able to:

1. Identify the forms of oral medication

2. Identify the terms on the medication label to be used in calculation of dosages

3. Calculate dosages for oral and liquid medications using ratio and proportion, the formula method, or dimensional analysis

4. Apply principles learned concerning tablet and liquid preparations to obtain a rational answer

The easiest, most economical, and most commonly used method of medication administration is by mouth (p.o.). Medications for oral administration are available in solid forms such as tablets and capsules or as liquid preparations. To calculate dosages appropriately the nurse needs to understand the principles that apply to administration of medications by this route.

FORMS OF SOLID MEDICATIONS

Tablets

Tablets are preparations of powdered medications that have been molded into various sizes and shapes. Tablets come in a variety of dosages that can be expressed in metric or apothecary units—for example, milligrams and grains. There are different types of tablets and shapes (Figure 17-1, A).

Figure 17-1 A, Various shapes of tablets. B, Tablets scored in halves and fourths. (From Kee JL, Marshall SM: Clinical calculations: with applications to general and specialty areas, ed. 6, St. Louis, 2009, Saunders.)

CAPLETS

A caplet is a tablet that has an elongated shape like a capsule and is coated for ease of swallowing. Tylenol is available in caplet form.

SCORED TABLETS

Scored tablets are designed to administer a dosage that is less than what is available in a single tablet. In other words, scored tablets have indentations or markings that allow you to break the tablet into halves or quarters. Only scored tablets should be broken because there is no way to determine the dosage being administered when a nonscored tablet is broken. Breaking a tablet that is not scored could lead to the administration of an inaccurate dosage if the tablet is not divided equally. The purpose of the groove or indentation is to provide a guide for breaking a whole tablet into a fractional part (Figure 17-1, B). Figure 17-2 shows an example of a scored tablet.

Figure 17-2 Clonazepam tablet scored. (From Mosby’s drug consult 2007, St. Louis, 2007, Mosby.)

Breaking an unscored tablet is risky and dangerous and can lead to the administration of an unintended dosage.

A pill or tablet cutter is readily available in most pharmacies that can be used to evenly cut tablets appropriately. Figure 17-3 shows a pill/tablet cutter. An appropriate medication reference should always be consulted before cutting a tablet. Many tablets come in a form that allows slow and steady release of the active drug. These forms cannot be cut, crushed, or chewed. Capsules and enteric-coated, timed-release, sustained-release, and controlled-release tablets cannot be cut. Nurses must use caution when instructing clients to cut tablets.

Figure 17-3 Pill/tablet cutter. (From Kee JL, Marshall SM: Clinical calculations: with applications to general and specialty areas, ed. 6, St. Louis, 2009, Saunders.)

Tips for Clinical Practice

If a tablet is not scored, it should not be divided. If the calculation of a medication dosage requires that a tablet be cut in half, divide the tablet along the scoring created by the manufacturer. If possible use a pill or tablet cutter to divide a tablet in half to help ensure acccuracy of a dose.

ENTERIC-COATED TABLETS

Enteric-coated tablets have a special coating that protects them from the effects of gastric secretions and prevents them from dissolving in the stomach. They are dissolved and absorbed in the intestines.

The enteric coating also prevents the medication from becoming a source of irritation to the gastric mucosa, thereby preventing gastrointestinal upset. Examples include enteric-coated aspirin and iron tablets, such as ferrous gluconate. Enteric-coated tablets should never be crushed, because crushing them destroys the special coating and defeats its purpose. Always consult an appropriate medication reference or the pharmacist when in doubt about the safety of crushing a tablet or opening a capsule.

SUBLINGUAL TABLETS

Sublingual tablets are designed to be placed under the tongue, where they dissolve in saliva and the medication is absorbed. Sublingual tablets should never be swallowed because this will prevent them from achieving their desired effect. Nitroglycerin, which is used for the relief of acute chest pain, is usually administered sublingually.

LAYERED TABLETS

Some tablets contain different layers or have cores that separate different medications that may be incompatible with one another; thus incompatible ingredients may be separated and released at different times as the tablet passes through the gastrointestinal tract (Figure 17-4).

Figure 17-4 Layered tablet. (From Clayton BD, Stock YN, Harroun RD: Basic pharmacology for nurses, ed. 14, St. Louis, 2007, Mosby.)

Medications in a layered form have become available in which one or more medications can be released immediately from the coating, whereas the same or other medications can be released on a sustained basis from the tablet core. An example of this is Ambien CR. Ambien CR is formulated in a two-layer tablet. The first layer of the tablet dissolves quickly to help in falling asleep, and the second layer dissolves slowly over the night to help the person stay asleep.

FILM TAB

A film tab is a tablet sealed with a film. The special coating helps to protect the stomach. Some medications that come as film tabs include Biaxin (clarithromycin) and E.E.S. 400 (erythromycin).

TIMED-RELEASE AND EXTENDED-RELEASE TABLETS

Look for abbreviations such as SA, LA, or XL. Medication from these types of tablets is not released immediately but released over a period of time at specific intervals. These types of preparations should not be crushed, chewed, or broken. These types of preparations should be swallowed whole. If a timed-release or extended-release tablet is crushed, chewed, or broken, all of the medication will be administered at one time and absorbed rapidly. Examples include Procardia, Calan, and theophylline.

Not all medications can be crushed. Medications such as timed-release or extended-release tablets and capsules have special coatings to prevent the medicaion from being absorbed too quickly. Always refer to a medication manual or some other reference book before crushing a medication to ensure that a medication can be safely crushed to avoid harm to the client.

Capsules

A capsule is a form of medication that contains a powder, liquid, or oil enclosed in a hard or soft gelatin. Capsules come in a variety of colors, sizes, and dosages. Some capsules have special shapes and colorings to identify which company produced them. Capsules are also available as timed release, sustained release, and spansules and work over a period of time.

Capsules should always be administered whole to achieve the desired result (e.g., sustained release). Sustained-release and timed-release capsules cannot be divided or crushed (Figure 17-5). Always consult an appropriate reference or pharmacist when in doubt as to whether to open a capsule.

Figure 17-5 Timed-release capsule. (From Clayton BD, Stock YN, Harroun RD: Basic pharmacology for nurses, ed. 14, St. Louis, 2007, Mosby.)

Examples of medications that come in capsule form are ampicillin, tetracycline, Colace, and Lanoxicaps. Lanoxicaps are an example of a capsule that has liquid medication contained in a gelatin capsule (Figure 17-6, A). On some labels, in addition to capsules, may be seen the term kapseals (e.g., kapseals is seen on a label for Dilantin extended capsules).

Figure 17-6 Various types of capsules. A, Lanoxicap. B, Different types of capsules. (A from Mosby’s drug consult 2003, St. Louis, 2003, Mosby. B courtesy Amanda Politte, St. Louis.)

Although there are other forms of solid preparations for oral administration—such as lozenges and troches—tablets, capsules, and pulvules (proprietary capsules containing a dosage of a medication in powdered form) are the most common forms of solids requiring calculation encountered by the nurse. Figure 17-6 shows forms of solid oral medications, including capsules. Figures 17-6 through 17-8 show various types of capsules and an enteric-coated tablet.

Figure 17-7 Forms of solid oral medications. Top row, Uniquely shaped tablet, capsule, scored tablet; bottom row, gelatin-coated liquid capsule, extended-release capsule, enteric-coated tablet. (From Potter PA, Perry AG: Fundamentals of nursing, ed. 7, St. Louis, 2009, Mosby.)

CALCULATING DOSAGES INVOLVING TABLETS AND CAPSULES

When administering medications, you will have to calculate the number of tablets or capsules needed to administer the dosage ordered. To help determine if your calculated dosage is sensible, accurate, and safe, remember the following points:

The rule of 10% variation is often applied with adults, but not necessarily in the pediatric setting.

Points to Remember

• Converting medication measures from one system to another and one unit to another to determine the dosage to be administered can result in discrepancies, depending on the conversion factor used.

• Example: Aspirin may indicate on the label 5 grains (325 mg). This is based on the equivalent 65 mg = gr 1. On the other hand, another label on aspirin may indicate 5 grains (300 mg). Here the equivalent 60 mg = gr 1 was used. Both of the equivalents are correct. Remember, equivalents are not exact. Use the common equivalents when making conversions—for example, 60 mg = gr 1.

• When the precise number of tablets or capsules is determined and you find that administering the amount calculated is unrealistic or impossible, always use the following rule to avoid an error in administration: No more than 10% variation should exist between the dosage ordered and the dosage administered. For example, you may determine that a client should receive 0.9 tablet or 0.9 capsule. Administration of such an amount accurately would be impossible. Following the stated rule, if you determined that 0.9 tablet or 0.9 capsule should be given, you could safely administer 1 tab or 1 cap. This variation should only occur when conversions are made between apothecary and metric measurements because approximate equivalents are used.

• Capsules are not scored and cannot be divided. They are administered in whole amounts only. If a client has difficulty swallowing a capsule, check to see if a liquid preparation of the same drug is available. Never crush or open a timed-release capsule or empty its contents into a liquid or food; this may cause release of all the medication at once. There are, however, some instances in which a soft gelatin capsule filled with liquid may be pierced with a small sterile needle and the medication squeezed out for sublingual use. For example, Procardia (nifedipine) has been used in this way for severe hypertension. This drug is not approved by the Food and Drug Administration (FDA) for use in this manner. When used in this manner, the action of the medication is erratic and short term. When administered in this manner, it can cause a hypotensive effect that is not easy to control. Precipitous drops in blood pressure can spell disaster for some clients. Over the years there have been reports of stroke and other complications resulting from lowering the blood pressure too much. It is important to note that administration of this medicine in this manner is not a common practice.

• Pulvules are proprietary capsules containing a dosage of a medication in powder form. For example, the popular antidepressant Prozac comes in pulvule form (proprietary capsules owned by a corporation under a trademark or patent).

• Tablets and capsules may be available in different strengths for administration, and you may have a choice when giving a dosage. For example, 75 mg of a medication may be ordered. When you check what is available, it may be in tablet or capsule form as 10, 25, or 50 mg. In deciding the best combination of tablets or capsules to give, the nurse should always choose the strength that would allow the least number of tablets or capsules to be administered without breaking a tablet, if possible, because breaking is found to result in variations in dosage. In the example given, the best combination for administering 75 mg is one 50 mg tablet or capsule and one 25 mg tablet or capsule.

• Only scored tablets are intended to be divided. It is safest and most accurate not to divide tablets, and give the fewest number of whole, undivided tablets possible.

• The maximum number of tablets or capsules given to a client to achieve a single dosage is usually three. Recheck your calculation if a single dose requires more. It is important to note that although the maximum number of tablets or capsules given to a client to administer a single dosage is usually three, for some medications the client may have to take more than three to achieve the desired dosage. This is true with some of the solid forms of HIV medications (e.g., tablets, capsules). Examples: Viracept 1,250 mg p.o. b.i.d. (available 250 mg per tab), ritonavir 400 mg p.o. q12h (available 100 mg per tab). Although many HIV medications come in liquid form, many clients prefer to take tablets or capsules. Remember: Except for special medications, any more than three capsules or tablets to achieve a single dosage is unusual and may indicate an error in interpretation of the order, transcription, or calculation. Think! Always question any order that exceeds this amount.

• When using ratio and proportion, the formula method, or dimensional analysis to calculate tablet and capsule problems, remember that each tablet or capsule contains a certain weight of the medication. The weight indicated on a label is per tablet or per capsule. This is particularly important when you are reading a medication label on a bottle or single unit-dose package.

• In calculating oral dosages you may encounter measures other than apothecary or metric measures. For example, electrolytes such as potassium will indicate the number of milliequivalents (mEq) per tablet. Units is another measure you may see for oral antibiotics or vitamins. For example, a vitamin E capsule will indicate 400 units per capsule. Measurements of units and milliequivalents are specific to the drug they are being used for. There is no conversion between these and apothecary or metric measures. (These are discussed in Chapter 18.)

• Always consult a medication reference or pharmacist when in doubt as to whether a capsule may be opened or pierced or whether a tablet can be crushed.

Remembering the points mentioned will be helpful before starting to calculate dosages. Any of the methods presented in Chapters 14, 15, and 16 can be used to determine the dosage to be administered.

To compute dosages accurately it is necessary to review a few reminders that were presented in previous chapters.

Reminders

1. Read the problem carefully and

a. Identify known factors

b. Identify unknown factors

c. Eliminate unnecessary information that is not relevant

2. Make sure that what is ordered and what is available are in the same system of measurement and units, or a conversion will be necessary. When a conversion is necessary, it is usual to convert what is ordered into what you have available or what is indicated on the medication label. You can, however, convert the measure in which the medication is available into the same units and system of measure as the dosage ordered. The choice is usually based on whichever is easier to calculate. Use any of the methods presented in Chapters 8 and 16 to make conversions consistent to avoid confusion. If necessary, go back and review these methods.

3. Consider what would be a reasonable answer based on what is ordered.

4. Set up the problem using ratio and proportion, the formula method, or dimensional analysis. Label each component in the setup, including x.

5. Label the final answer (tablet, capsule).

6. For administration purposes, for oral dosages that are given in fractional dosages (e.g., scored tablets), state answers to problems in fractions. Example: ½ tab or 1½ tabs, instead of 0.5 tabs or 1.5 tabs.

Question a dosage that seems unreasonable or requires administering a medication by a route other than what the form indicates. Regardless of the source of error, if you administer the wrong dosage or give a medication by a route other than that which is intended, you have committed a medication error and are legally responsible for the error.

Here are at some sample problems calculating the number of tablets or capsules to administer.

Example 1:

Order: Digoxin 0.375 mg p.o. daily

Available: Digoxin (scored tablets) labeled 0.25 mg

1. No conversion is necessary; the units are in the same system of measure.

Order: 0.375 mg

Available: 0.25 mg

2. Think critically: Tablets are scored; 0.375 mg is larger than 0.25 mg; therefore you will need more than 1 tab to administer the correct dosage.

3. Solve using ratio and proportion, the formula method, or dimensional analysis.

Solution Using Ratio and Proportion

0.25 mg:1 tab=0.375 mg:x tab(known)(unknown)(what is available)(what is ordered)0.25x0.25=0.3750.25x=0.3750.25

Therefore x = 1.5 tabs or 112 tabs. (It is best to state it as 112 tabs for administration purposes.)

Solution Using the Formula Method

(D)0.375 mg(H)0.25 mg×(Q)1 tab=x tabx=0.3750.25x=112 tabs

You can administer 1½ tabs because the tablets are scored. The ratio and proportion method could have been written in fraction format as well. (If necessary, review Chapter 4 on ratio and proportion.)

Solution Using Dimensional Analysis

Example 2:

Order: Ampicillin 0.5 g p.o. q6h

Available: Ampicillin capsules labeled 250 mg per capsule

1. Order: 0.5 g

Available: 250 mg capsules

2. After making the necessary conversion, think about what is a reasonable amount to administer.

3. Calculate the dosage to be administered using ratio and proportion, the formula method, or dimensional analysis.

4. Label your final answer (tablets, capsules).

A conversion is necessary. The ordered dosage and the available dosage are in the same system of measurement (metric), but the units are different (g and mg). Before calculating the dosage to be administered, you must have the ordered dosage and the available dosage in the same units.

1. Convert grams to milligrams. Equivalent: 1,000 mg = 1 g

1,000 mg:1 g=x mg:0.5 gx=1,000×0.5x=500 mg

Therefore 0.5 g is equal to 500 mg. Converting the grams to milligrams eliminated a decimal, which is often the source of calculation errors. Converting milligrams to grams would necessitate a decimal. Whenever possible, conversions that result in a decimal should be avoided to decrease the chance of error in calculating. Remember, a ratio and proportion could also be stated as a fraction. If necessary, review Chapter 4 on ratio and proportion. Because the measures are metric in this problem (grams, milligrams), the other method that can be used is to move the decimal the desired number of places

2. After making the conversion, you are now ready to calculate the dosage to be given, using ratio and proportion, the formula method, or dimensional analysis. In this problem we will use the answer obtained from converting what was ordered to what is available (0.5 g = 500 mg). Remember that if dimensional analysis is used, you need only one equation; even if conversion is required, you can choose to do conversion first and then set the problem up in dimensional analysis.

Solution Using Ratio and Proportion

250 mg:1 cap=500 mg:x cap250x250=500250x=500250x=2 caps

Two caps is a logical answer. Capsules are administered in whole amounts; they are not divisible. Using the value obtained from converting milligrams to grams in this problem would also net a final answer of 2 caps.

Solution Using the Formula Method

(D)500 mg(H)250 mg×(Q)1 cap=x cap

Solution Using Dimensional Analysis

Set up if conversion done first, then set up in dimensional analysis to calculate dosage:

Set up in dimensional analysis after conversion made:

Note: It is easier to set up the problem by using one equation that will allow you to convert and calculate the dosage required.

Example 3:

Order: Nitroglycerin gr 1/150 sublingual p.r.n. for chest pain

Available: Sublingual nitroglycerin tablets labeled 0.4 mg

In this problem it was easier to change grains to milligrams; gr 1/150 is equal to 0.4 mg. When possible, convert apothecary measures to their metric equivalents. The metric system is the predominant system of measurement used to calculate dosages.

1. Conversion is required.

Order: gr 1/150

Available: 0.4 mg

Equivalent: 60 mg = gr 1½

Convert what is ordered to the same system and units as what is available (gr is apothecary, mg is metric).

60 mg:gr 1=x mg:gr1150

x=60×1150x=60150x=0.4 mg

2. Think critically—it is obvious after making the conversion that you will give 1 tab.

3. Solve to obtain the desired dosage.

Solution Using Ratio and Proportion

0.4 mg:1 tab=0.4 mg:x tab0.4x0.4=0.40.4x=0.40.4x=1 tab

Solution Using the Formula Method

(D)0.4 mg(H)0.4 mg×Q(1 tab)=x tabx=0.40.4x=1 tab

Solution Using Dimensional Analysis

Example 4:

Order: Nembutal gr 1½ p.o. at bedtime p.r.n.

Available: Nembutal capsules labeled 100 mg; what should the nurse administer?

1. Convert gr 1½ to mg. Equivalent: 60 mg = gr 1

60 mg:gr 1=x mg:gr 11260 mg:gr 1=x mg:gr 32x=60×32=1802x=90 mg

2. Solve for the dosage to be administered.

Solution Using Ratio and Proportion

100 mg:1 cap=90 mg:x capsule100x100=90100x=90100x=0.9 cap

Administer 1 capsule.

Tags: Calculate with Confidence

Feb 11, 2017 | Posted by admin in PHARMACY | Comments Off

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