random thoughts on lab utilization

[EMEDPA]

wanted to take a minute to talk about a few labs, common misconceptions, helpful tips, etc

cbc: obviously an important test, but only helpful if abnormal. remember one can have an appy, chole, diverticulitis, etc with a nl wbc. remember to consider anemia in your ddx for any person with fatigue. it's surprising how many folks have a slow GI bleed and no idea their h+h is 6/18.

magnesium: great test and not included in cmp, probably should be. I order one anytime I order a cmp. anytime the K is low the mag is likely to be low as well. folks on diuretics, etc. Had a lady recently with fatigue and mag level less than .04(critically low below level of assay). a few grams of mag(as an inpt) and she perked right up. alcoholics run really low mags. make sure your facility adds mag to their banana bag, not all do. you may need to add it.

D-dimer: know when to order it, when not to, and how to interpret it. age adjusted d-dimer is 10x pts age (75 yr old is nl to 0.75, when nl cut off is 0.5). pregnancy doubles nl range.

sed rates and CRPs. great tests, know when to order and how to interpret. know the few things that will give you a sed rate over 100(board question).

lactates and procalcitonins, great tests. know when to use them and how to interpret them. remember to trend them. know things that will cause elevations other than sepsis, like seizures, etc remember lactate is a good serum marker for mesenteric ischemia.

troponin: the go to test now for cardiac dz. know nl ranges and non-cardiac causes of elevation.

urine pregs. remember you can use a few drops of blood too if they won't pee.

 cultures: really more helpful for the hospitalist, but they always appreciate the sputum culture on the pneumonia admit.

may add more later. others feel free to chime in.

[EMEDPA]

a few more things about K and Ca:

know the ekg changes of high and low levels of each. think of potassium as living in the T-wave and Ca as living in the QT segment. know how to correct for dangerously high and low levels of each. remember if giving Ca that chloride is for codes(pulseless) and gluconate is for folks with a pulse. really know the symptomatic hyper-K txs like the back of your hand, and there are several. many/most of the folks you see with ridiculously high K levels who are symptomatic will be folks with renal failure and/or missed dialysis.

[south]

this is GOLD

[EMEDPA]

here's another, prolactin to help differentiate pseudoseizure from real seizure:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1464160/

 

[UGoLong]

If you are going to order an ABG anyway, order an ABG with lactate. I highly second EMEDPA's recommendation. 

More on K+ (in chem 7 and CMP and renal panel) and Mg++ (not in any panel I know of):

- Keep K+ >= 4.0 and Mg++ >=2.0 to avoid paroxysmal AFib (K+)  or nonsustained vtach (Mg++). Loss of K+ is often from vomiting (or Lasix). Loss of Mg++ is often from diarrhea (or PPIs).

- Hard to hold the right K without the right Mg++. To replace K+: for every 0.1 below 4.0 for K+, give 10 meq K+ by any route (po or IV). I never give more than 40 meq po q4h x 2 without a recheck.

- For every 0.2 below 2.0 for Mg++, give 1 gram magnesium sulfate IV. I've had poor luck replacing it orally: too much and they get diarrhea and it is way less bioavailable than IV Mg++. I never give more than 4 grams IV without a recheck. In a jam, 400-800 mg MaxOx bid or 5 mL milk of magnesia po bid.

Remember that K+ and Mg++ are mostly intracellular lytes and you are testing the value extracellularly. It might take a while to reach equillibrium. (Like dripping water on a sponge until it is uniformly wet on the outside).

Hope this helps someone.

[DStillwagon]

PA student and former lab tech here: please don't use blood for an HCG unless it's a combo test approved for both samples! Also, whole blood is not ok to use, needs to be serum. Sure, your test may come up positive but that's not what it was made to test! 

[karebear12892]

A few things I was taught during my recent internal medicine rotation w/ hospitalists:

1. Always check SPEP/UPEP in patients w/ anemia + renal insufficiency. Multiple myeloma until proven otherwise. 

2. If your patient is seizing and you can't figure out why, check a phosphorus level. 

3. The degree of D-dimer elevation does not directly correlate with the likelihood of PE/DVT. 

4. Prothrombin time is the best measure of acute liver function. 

5. The presence of dysuria + urinary frequency (in the absence of fever, flank pain, pregnancy, and vaginal bleeding/discharge) is more sensitive & specific for uncomplicated UTI than a urinalysis due to high rates of contamination. 

[ohiovolffemtp]

CPK: very important in any patients who have been lying in a single position for a long while: "found down" - age, illness, drugs; any dehydrated and/or hyperthermic; pretty much any unknown drug OD, or known meth or anticholinergic.

[Parashooter79]

Sed-rates are as antiquated as bleeding times, they are non specific and vary greatly depending on methodology. Use CRP or more specific testing.

Edited December 16, 2019 by Parashooter79

[ohiovolffemtp]

Rapid strep test:

From a Cochrane review: 

On average, rapid tests for strep throat had a sensitivity (ability to correctly detect people with the disease) of 86% and a specificity (ability to correctly identify people who do not have the disease) of 95%.

However, a carrier (vs actual pharyngitis) will also show positive.  Look for clinical presentation.

[ohiovolffemtp]

Urinalysis:

• Look at specific gravity: good measure of hydration status
• Blood: menstrual, renal calculi, hemorrhagic cystitis, etc.
• Protein: examine if worried about rhabo
• Ketones: poor nutrition, DKA
• Leukocyte esterase: good indication of UTI
• Nitrites: before you consider it a sign of UTI, find out if patient has been on pyridium or AZO
• Epithelials: if high, sign of a dirty catch

[GetMeOuttaThisMess]

On 9/28/2017 at 12:26 AM, EMEDPA said:

cultures: really more helpful for the hospitalist, but they always appreciate the sputum culture on the pneumonia admit.

I was thinking that back in the stone-age when bronchoscopies became available that there was a study (studies) that showed a dissociation between cultures from expressed sputum and those collected during bronchoscopy, thus we don't tend to see people provide sputum specimens any longer, EXCEPT in what situation (think outside the box a bit)?

[ventana]

Honestly unsure but my guess (cause my local hospital still does). Acid fast for TB

[GetMeOuttaThisMess]

"Confirms" successful treatment if bacilli are absent.