Despite taking higher doses of warfarin, my patient’s INR won’t budge. What am I missing?

Poor compliance is probably the most common and least “exciting” explanation for low INRs despite seemingly adequate or high warfarin doses.  Otherwise, consider the following: 

Increased vitamin K intake: Since warfarin acts by inhibiting vitamin K recycling by VKORC1 (Vitamin K epOxide Reductase Complex), find out if your patient takes multivitamins or loves foods or products rich in vitamin K, ranging from leafy green vegetables to nutritional supplements( eg, Ensure) and even chewing tobacco!1 

Drug interactions: Warfarin is notorious for interacting with many drugs, although its effect is more often enhanced than counteracted. Run the patient’s med list and look for “counteractors” of warfarin,  including carbamazepine, phenobarbital, phenytoin, rifampin, and dexamethasone.2 

Hypothyroidism: Thyroid hormone seems to be necessary for efficient clearance of the vitamin K-dependent clotting factors (II, VII, IX, and X). Thus, larger doses of warfarin may be needed when patients are hypothyroid.3 

Hyperlipidemia and obesity: High lipid levels may allow for high vitamin K levels (since it’s lipid-soluble and carried in VLDL), especially at the start of therapy.4,5 

What if the INR is falsely low? This is usually not the problem although one major trial took a lot of heat for using a point of care INR device that gave low readings in anemic patients.6  When in doubt, check a chromogenic factor Xa test to confirm; 20-30% activity correlates with a true INR of 2-3.7

If none of these explanations fit the bill, consider genetic testing for warfarin resistance.8,9

Bonus Pearl: Did you know that use of warfarin (introduced in 1948 as a rodenticide) has already led to some selective pressure for VKORC1 mutations in exposed rat populations.10


  1. Kuykendall JR, et al. Possible warfarin failure due to interaction with smokeless tobacco. Ann Pharmacother. 2004 Apr;38(4):595-7.
  2. Zhou SF, et al. Substrates, inducers, inhibitors and structure-activity relationships of human Cytochrome P450 2C9 and implications in drug development. Curr Med Chem. 2009;16(27):3480-675.
  3. Bucerius J, et al. Impact of short-term hypothyroidism on systemic anticoagulation in patients with thyroid cancer and coumarin therapy. Thyroid. 2006 Apr;16(4):369-74.
  4. Robinson A, et al. Lipids and warfarin requirements. Thromb Haemost. 1990;63:148–149.
  5. Wallace JL, et al. Comparison of initial warfarin response in obese patients versus non-obese patients. J Thromb Thrombolysis. 2013 Jul;36(1):96-101.
  6. Cohen D. Rivaroxaban: can we trust the evidence? BMJ 2016;352:i575.
  7. Sanfelippo MJ, et al. Use of Chromogenic Assay of Factor X to Accept or Reject INR Results in Warfarin Treated Patients. Clin Med Res. 2009 Sep; 7(3): 103–105.
  8. Rost S, et al. Mutations in VKORC1 cause warfarin resistance and multiple coagulation factor deficiency type 2. Nature. 2004;427:537–41.
  9. Schwarz UI, et al. Genetic determinants of response to warfarin during initial anticoagulation. N Engl J Med. 2008 Mar 6;358(10):999-1008.
  10. Rost S, et al. Novel mutations in the VKORC1 gene of wild rats and mice–a response to 50 years of selection pressure by warfarin? BMC Genet. 2009 Feb 6;10:4.

Contributed by Nicholas B Bodnar, Harvard Medical School student, Boston, MA.

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Despite taking higher doses of warfarin, my patient’s INR won’t budge. What am I missing?

Does the time of day matter when performing cosyntropin stimulation test on my patient with suspected adrenal insufficiency?


No, it doesn’t! Although there is a diurnal variation in serum cortisol level, time of the day does not have an appreciable impact on the synthetic ACTH, also known as cosyntropin (Cortrosyn), stimulation test results.

A 2018 retrospective cohort study found that outcomes from cosyntropin stimulation (CS) testing was not affected by time of the day (0800-1000 h vs 1001-1200 h vs after 1200 h).1

An experimental study involving healthy volunteers with normal adrenal function also found that the time of day of CS testing (250  mcg IV) did not influence the peak or the delta of cortisol levels when measured by immunoassay.2 Similarly, an experiment involving normal volunteers concluded that while compared to testing at 0800 h the afternoon (1600) cortisol response to CS was more pronounced at 5 and 15 min, there was no significant difference in cortisol levels at 30 min.3  Parenthetically, peak cortisol level is usually obtained at 1 h after IV cosynstropin administration.

So if you think your patient should undergo CS testing, there is no reason to wait until the next morning!

Bonus Pearl: Did you know that while the half-life of cortisol is between 70-120 min, the half-life of cosyntropin is only 15 min? 4

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  1. Munro V, Elnenaei M, Doucette S, et al. The effect of time of day testing and utility of 30 and 60 min cortisol values in the 250 mcg ACTH stimulation test. Clin Biochem 2018;54:37-41.
  2. Jonklaas J, Holst JP, Verbalis JG, et al. Changes in steroid concentration with the timing of corticotropin stimulation testing in participants with adrenal insufficiency. Endocr Pract 2012;18:66-75.
  3. Dickstein G, Shechner C, Nicholson WE, et al. Adrenocorticotropin stimulation test: effect of basal cortisol level, time of day, and suggested new sensitive low dose test. J Clin Endocrinol Metab 72:773-78.
  4. Hamilton DD, Cotton BA. Cosyntropin as a diagnostic agent in the screening of patients for adrenocortical insufficiency. Clinical Pharmacology Advances and Applications 2010;2:77-82.

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Massachusetts General Hospital, Harvard Catalyst, Harvard University, its affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

Does the time of day matter when performing cosyntropin stimulation test on my patient with suspected adrenal insufficiency?

Should my patient with COPD and recurrent exacerbations undergo evaluation for antibody deficiency?

Although there are no consensus guidelines on when to evaluate patients with COPD for antibody deficiency, we should at least consider this possibility in patients with recurrent exacerbations despite maximal inhaled therapy (long-acting beta-2 agonist-LABA, long-acting muscarinic antagonist-LAMA and inhaled corticosteroids).1

Couple of retrospective studies of common variable immunodeficiency (CVID) in patients with COPD have reported a prevalence ranging from 2.4% to 4.5%. 1 In another study involving 42 patients thought to have had 2 or more moderate to severe COPD exacerbations per year—often despite maximal inhaled therapy— 29 were diagnosed  with antibody deficiency syndrome, including 20 with specific antibody deficiency (SAD), 8 with CVID and 1 with selective IgA deficiency.2  Although systemic corticosteroids may lower IgG and IgA levels, the majority of the patients in this study were not taking any corticosteroids at the time of their evaluation.

In another study involving patients undergoing lung transplantation, pre-transplant mild hypogammaglobulinemia was more prevalent among those with COPD (15%) compared to other lung conditions (eg, cystic fibrosis), independent of corticosteroid use.3  A favorable impact of immunoglobulin therapy or chronic suppressive antibiotics on reducing recurrent episodes of COPD exacerbation in patients with antibody deficiency has also been reported, supporting the clinical relevance of hypogammaglobulinemia in these patients. 2,4 

Remember that even normal quantitative serum immunoglobulin levels (IgG, IgA, and IgM) do not necessarily rule out antibody deficiency. Measurement of IgG subclasses, as well as more specific antibodies, such as those against pneumococcal polysaccharides may be required for further evaluation.

See a related pearl at

Contributed in part by Sydney Montesi, MD, Mass General Hospital, Boston, MA.


  1. Berger M, Geng B, Cameron DW, et al. Primary immune deficiency diseases as unrecognized causes of chronic respiratory disease. Resp Med 2017;132:181-188.
  2. McCullagh BN, Comelias AP, Ballas ZK, et al. Antibody deficiency in patients with frequent exacerbations of chronic obstructive pulmonary disease (COPD). PLoS ONE 2017; 12: e0172437.
  3. Yip NH, Lederer DJ, Kawut SM, et al. Immunoglobulin G levels before and after lung transplantation 2006;173:917-21.
  4. Cowan J, Gaudet L, Mulpuru S, et al. A retrospective longitudinal within-subject risk interval analysis of immunoglobulin treatment for recurrent acute exacerbation of chronic obstructive pulmonary disease. PLoS ONE 2015;10:e0142205.

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Should my patient with COPD and recurrent exacerbations undergo evaluation for antibody deficiency?

Should I choose a bactericidal over bacteriostatic antibiotic in the treatment of my patient with pneumonia complicated by bacteremia?

You don’t have too!  Although “bacteriostatic” antibiotics have traditionally been regarded as inferior to “bactericidal” antibiotics in the treatment of serious infections, a 2018 “myth busting” systemic literature review1 concluded that bacteriostatic antibiotics are just as effective against a variety of infections, including pneumonia, non-endocarditis bacteremia, skin and soft tissue infections and genital infections; no conclusion can be made in regards to endocarditis or bacterial meningitis, however, due insufficient clinical evidence.1-3

Interestingly, most of the studies included in the same systemic review showed that bacteriostatic antibiotics were more effective compared to bactericidal antibiotics.1 So, for most infections in hospitalized patients, including those with non-endocarditis bacteremia, the choice of antibiotic among those that demonstrate in vitro susceptibility should not be based on their “cidal” vs “static” label.

Such conclusion should not be too surprising since the definition of bacteriostatic vs bactericidal is based on arbitrary in vitro constructs and not validated by any available in vivo data. In addition, static antibiotics may kill bacteria as rapidly as cidal antibiotics in vitro at higher antibiotic concentrations.3

Another supportive evidence is a 2019 study finding similar efficacy of sequential intravenous-to-oral outpatient antibiotic therapy for MRSA bacteremia compared to continued IV antibiotic therapy despite frequent use of bacteriostatic oral antibiotics (eg, linezolid, clindamycin and doxycycline). 4



  1. Wald-Dickler N, Holtom P, Spellberg B. Busting the myth of “static vs cidal”: as systemic literature review. Clin Infect Dis 2018;66:1470-4.
  2. Steigbigel RT, Steigbigel NH. Static vs cidal antibiotics. Clin Infect Dis 2019;68:351-2.
  3. Wald-Dickler N, Holtom P, Spellberg B. Static vs cidal antibiotics; reply to Steigbigel and Steigbigel. Clin Infect Dis 2019;68:352-3.
  4. Jorgensen SCJ, Lagnf AH, Bhatia S, et al. Sequential intravenous-to-oral outpatient antbiotic therapy for MRSA bacteraemia: one step closer.  J Antimicrob Chemother 2019;74:489-98.


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Should I choose a bactericidal over bacteriostatic antibiotic in the treatment of my patient with pneumonia complicated by bacteremia?

When should I consider prophylaxis for Pneumocystis pneumonia (PCP) in my patient on prednisone?

It is generally recommended that patients on ≥20 mg of daily prednisone (or its equivalent) for ≥1 month be considered for PCP prophylaxis. 1

Couple of studies in 1990s helped define the dose and duration of corticosteroids (CS) that should prompt PCP prophylaxis. A Mayo Clinic study of patients without AIDS found that a median daily CS dose of 30 mg of prednisone or equivalent—with 25% of patients receiving as little as 16 mg of prednisone daily— was associated with PCP.The median duration of CS therapy before PCP was 12 weeks. A similar study found a mean CS dose of 33 mg of prednisone or equivalent with mean duration of 7 months (range 1-154 months) among patients with PCP without AIDS. 3

A 2018 retrospective study4  of patients with rheumatic diseases receiving prolonged high-dose CS therapy (≥30 mg prednisone for ≥4 weeks) found that PCP prophylaxis with trimethoprim/sulfamethoxazole (TMP/STX) resulted in 93% reduction in the incidence of PCP with an overall number needed to treat (NNT) of 52. It was suggested that PCP prophylaxis could be discontinued in patients receiving < 15 mg of prednisone daily.

Bonus Pearl: Did you know that TMP/STX may be given either as double-strength 3x/week or single-strength daily? 5,6

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1. Limper AH, Knox KS, Sarosi SA, et al. An official American Thoracic Society statement: Treatment of fungal infections in adult pulmonary and critical care patients. Am J Respir Crit Care Med 2011;183:96-128.

2. Yale SH, Limper AH. Pneumocystis carinii pneumonia in patients without acquired immunodeficiency syndrome: associated illness and prior corticosteroid therapy. Mayo Clin Proc 1996;71:5-13.

3. Arend SM, Kroon FP, van’t Wout JW. Pneumocystis carinii pneumonia in patients without AIDS, 1980 through 1993: An analysis of 78 cases. Arch Intern Med 1995;155:2436-2441.

4. Park JW, Curtis JR, Moon J, et al. Prophylactic effect of trimethoprim-sulfamethoxazole for Pneumocystis pneumonia in patients with rheumatic diseases exposed to prolonged high-dose glucocorticoieds. Ann Rheum Dis 2018;77:664-9.

5. Anevlavis S, Kaltsas K, Bouros D. Prophylaxis for pneumocystis pneumonia (PCP) in non-HIV infected patients. PNEUMON 2012;25, October-December.

6. Stern A, Green H, Paul M, Leibovici L. Prophylaxis for pneumocystis pneumonia (PCP) in non-HIV immunocompromised patients (Review). Cochrane data of Systematic Reviews 2014, issue 10. DOI: 10.1002/14651858.CD005590.pub3.


When should I consider prophylaxis for Pneumocystis pneumonia (PCP) in my patient on prednisone?