what labs to order...

[Bluesaurus]

I have trouble knowing which labs to order.

When to order a BMP...how often to order a HA1c...how often a thyroid...
etc.

Anyone have a decent reading source that just talks about stuff like this? I know some of it can be gotten just as you go, but it would helpful to have a reading source.

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[Droopy Snoopy]

I have trouble knowing which labs to order.

When to order a BMP...how often to order a HA1c...how often a thyroid...
etc.

Anyone have a decent reading source that just talks about stuff like this? I know some of it can be gotten just as you go, but it would helpful to have a reading source.

Order a renal panel for baseline assessment of a patient's metabolic status and prn when you have suspicion that these values could change and therefore alter your treatment plan. Start thinking about your patient in terms of problems and ways to evaluate those problems, the necessary lab tests should become more self-evident as you go along. To teach you to do this is to teach you medicine, so I'm not sure there is such a reference short of Harrison's.

For example, an A1c is good for 3 months, since that's about how long RBCs last before getting chewed up by the liver and spleen. As the PCM of a controlled diabetic you might check it once a year, but for patients who are newly diagnosed, uncontrolled, or in whom you are otherwise adjusting therapy you might check their 'sugar' more frequently. Thinking about all the organs affected by diabetes should give you a good idea of what other lab tests and exam findings you might want to check on a regular basis.

 

[Guile]

Allow your knowledge of pathology and pathophysiology to guide your decision-making regarding labs. What's the chief complaint? Is it concerning for a thyroid problem? Order TSH and FT4. Most medicine patients will get CBC and BMP on admission and often q day, whether they need it or not, unfortunately.

If you're asking about routine labs to order in an outpatient setting, I believe there the US Preventive Health Task Force (or some body with a name along those lines) publishes guidelines annually.

 

[jtlc2345]

In the acute setting, it is often helpful to think about it in terms of:
1. investigations that will give a diagnosis
2. investigations that will tell you the cause of the diagnosis
3. investigations that will tell you about complications of the diagnosis

Jonathan

 

[Super Mario]

Outpatients: As discussed earlier, outpatient laboratory monitoring is based on the specific disease.
E.g:
(a) Thyroid = recheck levels 4-6 weeks after a dose change as it takes that long to equilibrate. Monitoring TSH only is usually sufficient for primary hypothyroidism. Check T4 in secondary hypothyroidism as goal is to normalize TSH.
(b) Potassium and creatinine = I generally check these prior to initiating diuretics or ACE inhibitors/ARBs and then within 2-4 weeks after the patient has been on these medications. Patients on a stable dose of these medications with stable K and Cr should have these labs monitored at least every 6 months.


Inpatients: rather than ordering a slew of "routine" labs, try to think about the problems and differential to guide what tests to order. That being said, the majority of inpatients will get at least a CBC and chem 7 on admission (but this is because you can usually think of a reason these 2 sets of labs would be helpful in most medical inpatients).

 

[Bluesaurus]

Ok when do we order magnesium and phosphorous and why?

The interns order CBC, CMP, Mg and Phos often. I have some impression that Mg is ordered either for cardio issues because it's needed for good heart function, and also because it tends to become depleted when you have to rehydrate the patient. What about Phos?

Also when do I do a CMP vs. a BMP. Lots of times, i think a BMP is fine, and then I look and they want a CMP. Basically they're getting Calcium and liver enzymes and protein\albumin from what I cant tell.

Also I always thought a CMP was a chem 12...but it seems more like a chem 15 to me.

E.g:
(a) Thyroid = recheck levels 4-6 weeks after a dose change as it takes that long to equilibrate. Monitoring TSH only is usually sufficient for primary hypothyroidism. Check T4 in secondary hypothyroidism as goal is to normalize TSH.
(b) Potassium and creatinine = I generally check these prior to initiating diuretics or ACE inhibitors/ARBs and then within 2-4 weeks after the patient has been on these medications. Patients on a stable dose of these medications with stable K and Cr should have these labs monitored at least every 6 months.

useful tip

Most medicine patients will get CBC and BMP on admission and often q day, whether they need it or not, unfortunately.

I've noticed this too. It's always either those two things: CBC, BMP or four things: CBC, BMP, Mg, and Phos.

 

[Guile]

You should never order a magnesium level. If you're worried that your patient might have a low Mg then just give Mg. It's not worth the resources it takes to order the test when you can just give it to the patient. It's cheap and there's a very large therapeutic window, but the consequences of low Mg are great.

I think you're thinking about this backwards. You're just trying to memorize a list of scenarios of when you should order tests. Sit and think about the patient. Do you want to know his albumin? His liver enzymes? Do you NEED to know? How will the result change your management? Your school and attendings should be teaching you this.

 

[Bluesaurus]

You should never order a magnesium level. If you're worried that your patient might have a low Mg then just give Mg. It's not worth the resources it takes to order the test when you can just give it to the patient. It's cheap and there's a very large therapeutic window, but the consequences of low Mg are great.

I think you're thinking about this backwards. You're just trying to memorize a list of scenarios of when you should order tests. Sit and think about the patient. Do you want to know his albumin? His liver enzymes? Do you NEED to know? How will the result change your management? Your school and attendings should be teaching you this.

Sometimes they don't have time to explain. The doctor I was with told me that she pretty much orders Mg whenever they did fluid replacement because Mg tends to get lost when there is fluid replacement. The other scenario was during heart conditions she likes to know the value.

Phosphorous, I have no idea why we need that.

 

[Guile]

Sometimes they don't have time to explain. The doctor I was with told me that she pretty much orders Mg whenever they did fluid replacement because Mg tends to get lost when there is fluid replacement. The other scenario was during heart conditions she likes to know the value.

Phosphorous, I have no idea why we need that.

This is really poor form. I'm sorry you learning things this way. You don't order a test "just because you like to know." I see this all too commonly on medicine. Just brainless test ordering. So many people order daily CBC w/ diff and BMP. What are you actually doing with that information? Why don't you actually lay hands on your patient and assess them clinically? It drives me crazy.

Another good rule of thumb on Mg is that if you're replacing K then you probably need to replace Mg too. Just give it. You're not going to hurt the patient.

 

[Droopy Snoopy]

hypophosphatemia is mainly an issue in the ICU for ventilated patients and others with respiratory issues. Theoretically there's less ATP available for diaphragmatic function, =poorer respiratory function, less successful extubation, etc. I have no idea if this is true and have simply accepted the dogma and just repleted the phos if <2 or so.

Doesn't matter really since where I train at least you get the albumin and phos 'free' with the basic renal panel. Mg is ordered separate (again, really only order for admission lab and then for folks who have a chance of seeing a cardiologist during their stay), and a complete met panel includes LFTs.

 

[Guile]

Sometimes they don't have time to explain. The doctor I was with told me that she pretty much orders Mg whenever they did fluid replacement because Mg tends to get lost when there is fluid replacement. The other scenario was during heart conditions she likes to know the value.

Phosphorous, I have no idea why we need that.

Have you read about phosphorus? Why don't you go to the library and read about phosphorus in Harrison's? If you understood phosphorus' metabolism you would have a better handle on when to order a level. This is a higher level of functioning and one you need to strive for. If you don't understand something, READ ABOUT IT. Go look it up. Then ask questions about what you've read. Frankly your approach to these questions is all wrong. (Looking for an answer like "order phosphorus on intubated patients.") Get a grasp on electrolytes and then ask questions about what you don't understand. Move toward the next level and away from this "spoon-feed me" mentality.

 

[Droopy Snoopy]

Have you read about phosphorus? Why don't you go to the library and read about phosphorus in Harrison's? If you understood phosphorus' metabolism you would have a better handle on when to order a level. This is a higher level of functioning and one you need to strive for. If you don't understand something, READ ABOUT IT. Go look it up. Then ask questions about what you've read. Frankly your approach to these questions is all wrong. (Looking for an answer like "order phosphorus on intubated patients.") Get a grasp on electrolytes and then ask questions about what you don't understand. Move toward the next level and away from this "spoon-feed me" mentality.

This is great advice for the clinical years in general. I'm constantly having to remind the medstuds on my teams that they are there to learn. Not work, not get out by 3, not pal around for a better eval, but to learn. Everybody should pitch in and help with this, from the residents teaching you about COPD management and knot-tying to staff allowing you to skip rounds to see an EGD or do a joint aspiration.

By the same token OP you gotta do your part. Be curious. That can be anything from finding out why this lab needs a purple top and this one needs a red to reading about why a percutaneous drain might be more suitable for one patient but cholecystectomy is indicated for another. Asking your stoma nurse about decubitus staging at the bedside will fix that knowledge into your brain way better than that droning last-15-minutes-of-noon-lecture you had 4 months back will. Simple common sense stuff right? And in general it's what your evaluators mean you're doing or not doing when they say things like "student showed enthusiasm and initiative for learning".

Intern year is all about the details, and for every detail you nail down you'll provide better care and give yourself less work. That patient with the PICC you discharged to an LTAC, since you didn't take 2 minutes to verify the facility stocks IV zosyn they'll miss 3 days worth and bounce back septic. That diabetic patient on prednisone taper whom you forgot to adjust their insulin because you never bothered to look up some simple pathophys, yeah that patient's going to see you in follow up clinic with home glucose readings in the 300s and your staff will yell at you. That pneumonia patient you discharged without a follow-up chest xray, he's going to bounce to the ICU with adenoCA + a huge malignant effusion and freaking die man, because you weren't curious as to the reasons why f/u CXRs are necessary and appropriate.

I'm using IM examples here because we're talking about electrolytes, but no matter your field these are the kinds of details you need to start paying attention to and teaching yourself about. Your patients, your team, your step II score, and your future intern-self will all thank you for it I promise.

 

[Bacchus]

We havent done many systems yet, but have recently had a patient perspective on AKI and CKI. Isn't phosphorous something you want to check in dialysis patients? We learned of calcium/bone wasting due to secondary hyperthyroidism since phosphorous "irreversibly" binds to calcium rendering it useless. Don't you have fears of precipitates too with high serum phosphorous and thus calcium?

Edit: Only a 1st year so be nice 🙂

 

[spicedmanna]

OP, are you just starting your third year? It's normal to feel overwhelmed by what to order, how to manage a patient. This is something, as you move forward, will become more evident with practice and exposure. Don't freak out. Stick with it.

Guile and Tic are providing some great advice here. They are correct that you have things backwards. You are putting the cart before the horse. THINK about your patient and how they are presenting. Get a good H&P and come up with your differential. THINK, read about, and research the pathophysiology behind those conditions. THEN order studies that help you differentiate between, understand, and manage what you see. Don't just start randomly ordering labs before you get a basic picture of what you are dealing with.

Don't expect people to hand feed you information. You have to put in effort. Then, after you've reasonably researched and are read up on things, ask questions, or try your hand at coming up with the assessment/plan and get feedback about it. This will provide much better learning, as you will have a baseline understanding. At my hospital, third year students on IM are required to do all the H&P's for admission. We come up with an A&P and go over the whole thing with an intern or resident. Very valuable learning ensues. If your hospital doesn't do this, I recommend unofficially doing this.

When you are ready to order labs and other studies, THINK about what information will help you differentiate between possible conditions, provide critical information about your patient, given his presentation, that will need to be addressed, and/or will determine or change your management plan. If the test won't fulfill any of those criteria, you probably don't need it. Remember that medicine is part research science; you come up with hypotheses (your differential) based on your clinical suspicions and run studies (with an N of 1) so that you can diagnose your patient and come up with a treatment plan. You have to think critically.

Think economy of action. At beginning of third year, when I was pimped on how to manage a patient, I would often come up with the textbook tests to order, only to be told by my attending that while some of those results would indeed be abnormal, we wouldn't necessarily require them to nail the diagnosis and it wouldn't change the treatment plan. This is where experience comes into play. Now, I have a trigger in my head that asks, "is this test going to tell me more about my patient (that I don't already know) and will it change or help my treatment plan?"

You have to read, my friend. You have to do the leg work. There's no way around it. Try to follow what is happening and try your hand at it; get feedback. A lot of the time, residents and attendings will pimp you about it, but even if they don't, work as if they were. You have to be proactive.

Good luck and hang in there.

 

[spicedmanna]

We havent done many systems yet, but have recently had a patient perspective on AKI and CKI. Isn't phosphorous something you want to check in dialysis patients? We learned of calcium/bone wasting due to secondary hyperthyroidism since phosphorous "irreversibly" binds to calcium rendering it useless. Don't you have fears of precipitates too with high serum phosphorous and thus calcium?

Edit: Only a 1st year so be nice 🙂

Ah, you must mean secondary hyperPARAthyroidism. 😉

Read up on normal phosphate homeostasis/metabolism, PTH, vitamin D, Ca, and how the kidney is involved in the process. First understand the normal, then look at the pathophysiology in these systems (i.e., PTH, vitamin D, Ca, and kidneys), such as with secondary hyper-PTH in CRF and how it differs from primary hyper-PTH.

In essence, because your kidney is in chronic failure, you can't clear the phosphate due to insufficient active Vitamin D (read about the metabolism of Vitamin D and how the kidneys are involved), so you get hyperphosphatemia, which binds to calcium, forming calcium phosphate, which is insoluble. You have less available calcium in your blood, so you have hypocalcemia (what are the consequences of low serum calcium?), which your parathyroid responds to by increasing PTH to try to compensate, resulting in the hyper-PTH that you see.

In regard to your second question: there are several sequele of secondary hyper-PTH. Straight from little Robbins & Coltran (p. 582), you may see hyperplasia of your parathyroid glands, low serum calcium levels, osteitis fibrosa cystica (and bone pain or pathological fractures secondary to it), osteomalacia, and metastatic calcifications.

 

[Bacchus]

Ah, you must mean secondary hyperPARAthyroidism. 😉

Read up on normal phosphate homeostasis/metabolism, PTH, vitamin D, Ca, and how the kidney is involved in the process. First understand the normal, then look at the pathophysiology in these systems (i.e., PTH, vitamin D, Ca, and kidneys), such as with secondary hyper-PTH in CRF and how it differs from primary hyper-PTH.

In essence, because your kidney is in chronic failure, you can't clear the phosphate due to insufficient active Vitamin D (read about the metabolism of Vitamin D and how the kidneys are involved), so you get hyperphosphatemia, which binds to calcium, forming calcium phosphate, which is insoluble. You have less available calcium in your blood, so you have hypocalcemia (what are the consequences of low serum calcium?), which your parathyroid responds to by increasing PTH to try to compensate, resulting in the hyper-PTH that you see.

In regard to your second question: there are several sequele of secondary hyper-PTH. Straight from little Robbins & Coltran (p. 582), you may see hyperplasia of your parathyroid glands, low serum calcium levels, osteitis fibrosa cystica (and bone pain or pathological fractures secondary to it), osteomalacia, and metastatic calcifications.

Yeah, I meant PTH, haha. You say it so eloquently.

 

[spicedmanna]

Phosphorous, I have no idea why we need that.

What does phos have to do with ATP? What are the causes of hypophosphatemia? What conditions cause an intracellular shift of phosphate? What is the sequele of hypophosphatemia? What are the signs and symptoms? How do muscular weakness, problems with breathing and cardiac contractility, digestion, anemia, and rhabdomyolysis (what important lab value do you follow if you suspect rhabdo, or if your patient has rhabdo?) relate?

You are having problems knowing what labs to order because you don't understand your patient's condition(s). Do some research. A quick and convenient source could be UpToDate.

 

[Substance]

Here's what you need to do:

1. Read Robbin's and Cotran pathologic basis of disease. The big one. Not the little handbook. This is the bible of medicine, and explains how the body goes wrong. If your knowledge is a house, this is the foundation.

2. Get the Goljan USMLE audio files that are around the internet somewhere. He's one of the best lecturers I've ever heard speak. He'll put the info into a very excellent clinical perspective.

3. Get the Rapid Review path, also by Goljan.

Essentially, read and understand the science behind the conditions. Once you do this, knowing what labs to order is a cakewalk because you have a logical question to ask regarding your patient's presentation.