Why would my patient with Covid-19 infection test negative by PCR?

There are several potential reasons why someone who is infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the agent of Covid-19, may test negative by PCR. These including the threshold for detection of virus (which can vary among different manufacturers from as low as 100 viral copies/ml to >6,000 copies/ml),1 timing of the sample collection with respect to infection stage (lowest false-negative rate [~20%] on day 3 of symptoms or 8 days post-infection),specimen storage and transport and, particularly in the case of nasopharyngeal specimens, the adequacy of the sample obtained. 3

Suboptimal specimen collection from nasopharynx has long been suspected as an explanation for false-negative PCR tests in patients who subsequently have a positive test or are highly suspected of having Covid-19, but without any good support data. Until now…

A clever study looked at the presence of human DNA recovered from nasopharyngeal swabs as a marker for adequate specimen collection quality and found that human DNA levels were significantly lower in samples from patients with confirmed or suspected Covid-19 that yielded negative results compared to those of representative pool of samples submitted for Covid-19 testing.3

Interestingly, major commercial assays do not include any internal controls that ensure adequate sampling before testing for SARS-CoV2.

A typical microbiology lab can reject a sputum culture if gram-stain suggests poor quality specimen (eg, saliva only) but it looks like no similar rule exists for nasopharyngeal PCR tests for SARS-CoV-2 through commercial labs. Apparently, the US-CDC diagnostic panel does include a human RNAseP RNA-specific primer/probe set but the interpretation criteria for this control may also be too liberal.3

For these reasons, in patients highly suspected of having Covid-19 but with a negative initial PCR test, a repeat test on the same day or next 2 days is recommended.4

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References

  1. Prinzi A. False negatives and refinfections: the challenges of SARS-CoV-2 RT-PCR testing. Available at https://asm.org/Articles/2020/April/False-Negatives-and-Reinfections-the-Challenges-of     Accessed October 5, 2020.
  2. Kucirka LM, Lauer SA, Laeyendecker O, et al. Variation in false-negative rate of reverse transcriptase polymerase chain reaction-based SARS-CoV-2 tests by time since exposure. Ann Intern Med 2020 May 13:M20-1495. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7240870/
  3. Kinloch NN, Ritchie G, Brumme CJ, et al. Suboptimal biological sampling as a probable cause of false-negative COVID-19 diagnostic test results. J Infect Dis 2020;222:899-902. https://academic.oup.com/jid/article/222/6/899/5864227
  4. Green DA, Zucker J, Westbade LF, et al. Clinical performance of SARS-CoV-2 molecular testing. J Clin Microbiol 2020. DOI:10.1128/JCM.00995-20. https://jcm.asm.org/content/58/8/e00995-20

 

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!

Why would my patient with Covid-19 infection test negative by PCR?

Is the average body temperature in adults lower than 98.6 ᵒF (37 ᵒC)?

Despite the widely-held belief that the normal body temperature is 98.6 ᵒF (37.0 ᵒC), it is becoming increasingly clear that the average body temperature among adults (at least in the U.S.) is actually lower than 98.6 ᵒF (37 ᵒC).

The concept of a single normal body temperature dates way back to the 1800’s, based on measuring axillary temperatures by mercury thermometers. 1 However, a 2001 systematic literature review of 20 studies (1935-1998) of normal body temperature measured in adults found the following mean temperatures: oral 97.5 ᵒF (36.4 ᵒC), rectal 98.4 ᵒF (36.9 ᵒC), tympanic 97.7 ᵒF (36.5 ᵒC), and axillary 97.3 ᵒF (36.3 ᵒC ).  A British study involving >35,000 patients also found a lower mean oral temperature of 97.9 ᵒF (36.6 ᵒC). 2 A 2020 US study of a cohort of >150,000 adults (2007-20017) found a mean oral temperature of 98.1 ᵒF (36.7 ᵒC) in men and 98.2 ᵒF (36.8 ᵒC) in women; these values were lower than that of an earlier cohort (1971-1975). 3

So is the discrepancy between the body temperature in 1800’s and the more recent era due to the differences in measurement techniques or the population? In other words, are we cooling off?

The weight of the evidence suggests that our bodies are cooling!3  The study of an 1860-1940 cohort—presumably using similar thermometer techniques —found a gradual drop in the mean temperature during that period alone. Since axillary temperature (accounting for some of the values in the earlier cohort) is about 1 ᵒC lower than that of oral temperature, the magnitude of the drop in mean temperatures over the past 150 years is likely higher that those reported. 3

Potential explanations for our cooling bodies over the past 2 centuries include reduction in the population level inflammation due to improved standard of living, sanitation, lower incidence of chronic infections. improved dental hygiene, and cooler ambient temperatures. 3

 

Fun Fact: Did you know that in 1851 Carl Wunderlich, a German physician, obtained millions of axillary temperatures from 25,000 patients in Leipzig and thereby established the standard body temperature of 98.6 ᵒF (37 ᵒC)? ᵒ

 

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References

  1. Sund-Levander M, Forsberg C, Wahren LK. Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review. Scan J Caring Sci 2002;16:122-128. https://www.ncbi.nlm.nih.gov/pubmed/12000664
  2. Obermeyer Z, Samra JK, Mullainathan S. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ 2017;359:j5468. https://www.ncbi.nlm.nih.gov/pubmed/29237616
  3. Protsiv M, Ley C, Lankester J, et al. Decreasing human body temperature in the United States since the industrial revolution. Human Biology and Medicine, Jan 7, 2020. DOI: 10.7554/eLife.49555. https://www.researchgate.net/publication/338433061_Decreasing_human_body_temperature_in_the_United_States_since_the_industrial_revolution
Is the average body temperature in adults lower than 98.6 ᵒF (37 ᵒC)?