Lesson
Move From Lipid A Binding to Membrane Failure
Polymyxins are cyclic cationic lipopeptides. Their charged ring binds lipid A, displaces calcium and magnesium, destabilizes the gram-negative outer membrane, and permits lethal cellular leakage.
- Cyclic peptide ring
- Fatty acyl tail
- Lipid A
- Calcium and magnesium displacement
- Bactericidal membrane injury
The charged peptide approaches the lipopolysaccharide surface.
Stabilizing ionic bridges leave the outer membrane.
The amphipathic scaffold disrupts the barrier.
Permeability failure produces bacterial death.
Structure creates selective attraction
Positively charged diaminobutyric-acid residues attract the negatively charged lipid A region of lipopolysaccharide. The fatty acyl segment supports insertion into the membrane environment.
Divalent cations normally stabilize the surface
Calcium and magnesium bridge neighboring lipopolysaccharide molecules. Polymyxin binding displaces these ions and weakens the organized outer-membrane barrier.
Leakage produces rapid bacterial killing
Permeability rises, intracellular contents escape, and the envelope loses function. The result is concentration-dependent bactericidal activity when adequate exposure reaches a susceptible organism.
The mechanism also predicts toxicity
A membrane-active compound cannot be treated as perfectly bacteria selective. Renal tubular injury, neurotoxicity, and neuromuscular blockade belong beside the mechanism, not in an isolated adverse-effect list.
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Lesson
Keep Two Agents and Three Unit Systems Separate
Colistimethate sodium is an inactive prodrug that forms colistin. Polymyxin B is given as active drug. Their names, strengths, unit conventions, distribution, renal handling, and preparation cannot be exchanged.
- Colistimethate sodium
- Active colistin
- Polymyxin B
- Colistin base activity
- Units and milligrams
An inactive prodrug that forms active colistin.
The current United States vial displays colistin base activity.
Active drug supplied as 500,000 units per vial.
This mass conversion belongs only to polymyxin B.
Colistimethate is not the active moiety
The injectable prodrug converts variably to active colistin after administration. Conversion and renal elimination create more variable plasma exposure than direct administration of polymyxin B.
The United States vial displays colistin base activity
Each current colistimethate vial contains the equivalent of 150 mg colistin base activity. The order and preparation record should preserve the CBA convention so that the number never becomes an unspecified mass.
Polymyxin B uses a different convention
The current vial contains 500,000 polymyxin B units. One milligram of pure polymyxin B base equals 10,000 units, so one vial corresponds to 50 mg of base by that convention.
Ambiguity is a stop signal
Never convert from a remembered ratio when the product or strength convention is unclear. Verify the vial, label, order, protocol, calculation, and independent check before compounding or administration.
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Lesson
Read the Organism Before the Last-Line Label
Polymyxins can act against selected susceptible aerobic gram-negative bacilli, but intrinsic gaps and acquired lipid A modification can eliminate activity before dose selection begins.
- Pseudomonas
- Acinetobacter
- Selected Enterobacterales
- Intrinsic resistance
- mcr and lipid A modification
Use current organism and MIC data within a justified site.
Current guidance still places preferred agents ahead of polymyxins.
Intrinsic resistance defeats a class-level shortcut.
Chromosomal pathways and mcr genes can reduce binding.
The spectrum is narrow despite rescue use
Potential activity includes selected Pseudomonas aeruginosa, Acinetobacter species, Escherichia coli, Klebsiella, and other Enterobacterales. Gram-positive bacteria, fungi, and gram-negative cocci are not covered.
Species identity can reveal an immediate gap
Proteus, Providencia, Morganella, Serratia, and several other organisms may have intrinsic or unreliable polymyxin activity. A class-level gram-negative label is not enough.
Resistance changes the membrane target
Chromosomal regulatory pathways and mobile mcr genes can modify lipid A, reduce negative surface charge, and weaken polymyxin binding. Membrane activity does not make resistance impossible.
Source control reduces toxic exposure
Drain an abscess, remove an infected device, debride devitalized tissue, and obtain useful cultures when possible. Prolonged polymyxin exposure cannot compensate safely for an uncontrolled source.
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Lesson
Use the MIC Without Calling It Susceptible
FDA now recognizes unconventional intermediate and resistant polymyxin criteria for Enterobacterales, Pseudomonas, and Acinetobacter. There is no susceptible category, and MIC testing has important technical and outcome limitations.
- Intermediate at or below 2 mg/L
- Resistant at or above 4 mg/L
- No susceptible category
- Testing limitations
- Alternative analysis
An MIC at or below 2 mg/L uses this category.
An MIC at or above 4 mg/L is resistant.
Available evidence does not support this category.
Technical limitations make one number insufficient.
Two milligrams per liter is intermediate
For the specified organism groups, an MIC at or below 2 mg/L is intermediate. Do not rename the category susceptible in a note, presentation, or decision aid.
Four milligrams per liter is resistant
An MIC at or above 4 mg/L is resistant. Increasing dose beyond a narrow toxicity threshold is not a defensible response to resistance.
The recognition is explicitly unconventional
FDA concluded that available pharmacokinetic, pharmacodynamic, and clinical-outcome data do not support a susceptible category. MIC may help a difficult benefit-risk decision but does not predict success by itself.
Testing quality matters
Polymyxins diffuse poorly and can adhere to testing materials. Review the laboratory method, quality controls, organism identity, repeated findings, site, exposure, and clinical response before acting on a small MIC difference.
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Lesson
Match the Agent to the Site and the Current Era
Current resistance guidance places safer active agents ahead of polymyxins in most settings. When a polymyxin is unavoidable, urinary conversion favors colistin in selected urinary rescue use, while active polymyxin B offers more predictable systemic exposure outside the urinary tract.
- Urinary colistin formation
- Minimal polymyxin B urine
- Systemic exposure
- Poor lung exposure
- CRAB rescue
Prodrug conversion can support a limited urinary role.
Predominantly nonrenal clearance limits urinary exposure.
Intravenous epithelial lining-fluid activity is suboptimal.
Use only when safer active agents cannot be used.
Urinary pharmacokinetics separate the agents
Colistimethate can convert to active colistin in the urinary tract. Current IDSA guidance retains a limited alternative role for uncomplicated resistant cystitis, while polymyxin B should not be used for UTI because little active drug reaches urine.
Systemic rescue favors predictable active exposure
Polymyxin B is administered as active drug and usually provides more predictable plasma exposure than colistimethate. That pharmacokinetic advantage matters only after safer active systemic options are excluded.
The lung remains a difficult compartment
Intravenous polymyxins have suboptimal epithelial lining-fluid exposure. Current guidance prefers active newer agents for difficult-to-treat Pseudomonas pneumonia when available, rather than relying on polymyxin potency in vitro.
CRAB use is a constrained alternative
For invasive carbapenem-resistant Acinetobacter, current IDSA guidance reserves polymyxin B as an alternative component when resistance precludes sulbactam-durlobactam or as interim therapy while access is arranged. This is not routine first-line monotherapy.
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Lesson
Calculate Colistimethate in Colistin Base Activity
The current United States label uses 2.5 to 5 mg/kg/day CBA in two to four divided doses for normal renal function, bases dosing on ideal body weight in obesity, and lowers dose or frequency with renal impairment.
- 2.5 to 5 mg/kg/day CBA
- Ideal body weight in obesity
- Renal adjustment table
- 75 mg/mL after reconstitution
- Fresh final infusion
Use CBA and divide the total into two to four doses.
Apply the exact current table or approved protocol.
The 150 mg CBA vial becomes 75 mg/mL.
Fresh preparation has a shorter stability window.
The labeled amount is a daily dose
For normal renal function, calculate 2.5 to 5 mg/kg/day of CBA and divide it into two to four doses. The maximum daily amount is 5 mg/kg/day, not 5 mg/kg per dose.
Renal function changes the schedule
The label suggests 2.5 to 3.8 mg/kg/day divided twice daily for creatinine clearance 50 to 79 mL/min, 2.5 mg/kg/day once daily or divided twice daily for 30 to 49 mL/min, and 1.5 mg/kg every 36 hours for 10 to 29 mL/min. Use a current institutional protocol for dialysis and rapidly changing kidney function.
Reconstitution establishes 75 mg per milliliter
Add 2 mL sterile water to the 150 mg CBA vial and swirl gently to avoid frothing. A prescribed 75 mg CBA dose requires 1 mL of the correctly reconstituted solution before route-specific final preparation.
Final infusion stability is shorter
Compatible continuous-infusion solutions are listed in the product label, but any final IV infusion containing colistimethate should be freshly prepared and used for no longer than 24 hours. Label preparation and discard times clearly.
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Lesson
Show the Label and Consensus on the Same Page
The polymyxin B product label and endorsed international consensus differ in dose framing and renal adjustment. This conflict must be documented and managed through infectious-disease, pharmacy, and institutional-protocol oversight.
- 500,000 units per vial
- 10,000 units per milligram
- Label daily dose
- Consensus loading and maintenance
- Renal conflict
The label states a total daily range and ceiling.
The endorsed consensus uses total body weight for severe infection.
Consensus maintenance is infused over one hour.
Label reduction and consensus nonadjustment require oversight.
The current label gives a total daily unit range
For normal kidney function, the label gives 15,000 to 25,000 units/kg/day IV and caps the total at 25,000 units/kg/day. It describes infusion every 12 hours and instructs reduction in renal impairment.
The consensus uses a pharmacokinetic regimen
For severe infection, the endorsed consensus recommends a 2 to 2.5 mg/kg total-body-weight loading dose, followed by 1.25 to 1.5 mg/kg every 12 hours infused over one hour. One milligram equals 10,000 units.
Renal guidance conflicts directly
The label recommends reduction when kidney function is impaired. The consensus recommends no loading or maintenance adjustment because polymyxin B clearance is largely nonrenal and lower exposure may fail. Do not hide this disagreement.
Preparation begins with an exact vial
Each vial contains 500,000 units, equivalent to 50 mg of pure polymyxin B base by the label conversion. The IV label dissolves one vial into 300 to 500 mL of D5W for continuous drip, while specialist protocols may differ and must be followed exactly.
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Lesson
Monitor the Tubule, Not Creatinine Alone
Polymyxin exposure can cause acute tubular injury, falling urine output, azotemia, and renal electrolyte wasting. Pseudo-Bartter physiology may present with hypokalemic metabolic alkalosis and low magnesium despite a normal creatinine.
- Acute kidney injury
- Urine output
- Potassium and bicarbonate
- Magnesium and calcium
- Nephrotoxin burden
Falling output or rising azotemia changes the exposure plan.
Hypokalemic alkalosis can reveal tubulopathy.
Renal wasting can occur with normal creatinine.
Concurrent and recent exposures compound risk.
Renal function is a trajectory
Diminishing urine output, rising BUN, rising creatinine, and declining clearance require immediate exposure review. Colistimethate can accumulate as kidney function falls, increasing both renal and neurologic risk.
Tubular wasting may arrive first
Postmarketing pseudo-Bartter cases include hypokalemia, metabolic alkalosis, hypomagnesemia, hypocalcemia, urinary potassium loss, normal blood pressure, and sometimes normal serum creatinine. Electrolyte surveillance cannot wait for azotemia.
The medication list is part of the kidney exam
Map aminoglycosides, vancomycin, contrast, NSAIDs, calcineurin inhibitors, diuretics, hemodynamic insults, and recent sequential nephrotoxins. Remove avoidable exposures and assign monitoring to every necessary overlap.
Response depends on agent and evidence source
For colistimethate, renal adjustment is explicit. For polymyxin B, the label and consensus conflict. In either case, new kidney injury prompts diagnostic confirmation, alternative analysis, source review, toxicity management, and a documented dose or stop decision.
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Lesson
Treat Weakness as a Respiratory Warning
Paresthesia, dizziness, ataxia, drowsiness, weakness, visual symptoms, and slurred speech can progress to neuromuscular blockade, apnea, respiratory arrest, and death, especially with renal accumulation, anesthesia, or muscle relaxants.
- Perioral paresthesia
- Ataxia and weakness
- Neuromuscular blockade
- Anesthesia and paralytics
- Pregnancy and lactation evidence
Treat paresthesia as an exposure signal.
Check progression, renal function, and interactions.
Neuromuscular effects can combine dangerously.
Hypoventilation or apnea is an emergency.
Early symptoms deserve active assessment
Circumoral tingling, extremity numbness, vertigo, dizziness, visual blurring, speech change, drowsiness, and ataxia are exposure signals. Check dose history, renal function, interacting medicines, and progression.
Respiratory paralysis is an emergency
New generalized weakness, shallow breathing, reduced ventilation, or apnea requires immediate drug cessation, assisted respiration as needed, and evaluation for polymyxin neuromuscular blockade.
Anesthesia and paralytics amplify the risk
Curariform muscle relaxants, succinylcholine, and other agents that impair neuromuscular transmission can precipitate respiratory depression. Recent procedural medications belong in the interaction review.
Use current reproductive risk language
Human pregnancy and lactation evidence remains limited. Replace retired letters with a narrative that includes infection severity, agent and route, placental or milk evidence, active alternatives, timing, and maternal and infant monitoring.
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Lesson
Close the Loop Before the First Unit Is Ordered
A defensible polymyxin plan makes the resistant infection, current evidence, exact product, unit convention, site, source, dose source, preparation, toxicity ownership, response, and exit criteria visible to every person in the care chain.
- Diagnosis and organism
- MIC and category
- Alternative analysis
- Exact product and units
- Monitoring and exit
Confirm organism, site, burden, and source action.
Current preferred agents come before polymyxins.
Document dose source, vial, volume, infusion, and timing.
Write the conditions to narrow, replace, or stop.
Start with evidence, not last-line reputation
Name the syndrome, organism, resistance mechanism, MIC method and category, burden, source, and severity. State why current preferred and alternative agents are inactive, unsafe, unavailable, or unsuitable.
Make the order reconstructable
Write colistimethate or polymyxin B, the strength convention, patient weight and weight type, loading and maintenance source, dose, route, interval, vial count, concentration, final solution, infusion, and discard time.
Assign safety ownership
Specify renal and urine-output frequency, electrolytes, neurologic and respiratory checks, interacting nephrotoxins and paralytics, response measures, laboratory owners, and thresholds for escalation.
Write the exit before exposure accumulates
Define culture review, source intervention, clinical response, toxicity limits, availability of a safer active agent, duration endpoint, and the conditions that will narrow, replace, or stop the polymyxin.
Quick check
Module test
Check the connections.
Each attempt draws 10 questions from the complete 148 question bank.
Each attempt draws a fresh set and rearranges the answer choices.