Serial Lab Testing: Worthwhile or Worthless?: Final Answer

In my last two posts, I detailed the serum sodium measurements in a hypothetical patient two ways. The first was a listing of daily values, and the second provided values obtained every six hours or so. It also showed the sodium supplementation that was ordered based on those values. (I’ve included the table at the bottom of this post)

What did you think? Did the extra determinations help you decide what, if any, treatment was needed? Did the therapies ordered help?

Here are my thoughts:

  • Overall, there was not a huge or rapid decline in sodium values. Given the initial values, I would not have started a saline infusion on day 1, just watched a few daily values and the patients physical exam. The infusion only provided 3gm of salt per day, and the serum Na remained fairly stable for the first 3 days.
  • There was a significant amount of intra-day variation seen on the six hour table. You need to know the normal “within-person ” variation for any lab test you order. If two assays on specimens drawn at the same time can vary by 5%, you must factor this in to your decision making. If the value is 3% lower than the previous draw, the difference could represent normal variation. Obtaining more frequent assays exacerbates the amount of variation you see and my be confusing.
  • From day 5 to 6, the sodium appeared to be rising without any salt supplementation! But then a higher dose was given, and one of the intra-day values dropped to 124. What’s up with that? More variation?!
  • Is the morbidity of frequent blood draws worth it if there is no clinical change in the patient’s exam? What morbidity, you ask? Sleep disturbances, with all the cascading problems like delirium, sundowning, administration of additional meds to compensate, and on and on. Unnecessary medication or interventions. Plus it does not promote patient or family satisfaction at all.

Bottom line: Unless your patient has a clinical problem that may deteriorate rapidly, serial lab determinations are probably not of much value. The example patient was many days out from a TBI with some extra-axial blood. So yes, he could develop hyponatremia, but it would have probably surfaced earlier. Know your within-person  variability, which for sodium is roughly +2 meq. Is your new value within that limit? Then it is statistically the same as the first value unless you see a trend over several measurements. And as always, if you note a marked change in just one value, repeat it immediately before beginning any more drastic interventions.

In general, think twice before ordering serial lab tests. Can the result actually make a significant move during that period of time? Is the margin of error for the test greater than that variability? Are you truly worried about a number that would have a real clinical impact? Always ask yourself these questions before ordering an expensive and uncomfortable series of tests!

Reference: Biological variation of laboratory analytes based on the 1999-2002 national health and nutrition examination survey. Natl Health Statistic Reports 21:March 1, 2010.

Day/Time Na Treatment NaCl per day
Day 1 18:30 131
Day 1 22:54 132 0.9% NS @ 125/hr 3G
Day 2 05:59 133 continues 3G
Day 2 12:19 129 continues
Day 2 17:50 129 continues
Day 3 07:18 127 continues
Day 3 12:09 127 continues
Day 3 17:58 126 continues
Day 3 23:53 126 continues
Day 4 07:45 125 continues
Day 4 11:38 122 2% NS @ 25/hr 6G
Day 4 15:25 125 continues
Day 4 19:31 125 continues
Day 5 00:06 122 continues 6G
Day 5 04:04 126 continues
Day 5 08:01 122 continues
Day 5 11:50 132 stop
Day 5 16:14 126
Day 5 19:26 127
Day 6 00:20 129 9.2G
Day 6 04:42 127 2% NS @ 40/hr
Day 6 08:30 124 continues
Day 6 12:29 127 stop
Day 6 16:16 127 Salt tabs 2G tid
Day 6 20:28 132 continues
Day 7 05:22 134 Salt tabs 2G qid 8G
Day 7 12:33 135 continues
Day 8 07:02 131 stop None
Day 8 13:33 136

Serial Lab Testing: Worthwhile or Worthless? Part 2

Yesterday, I posted a series of sodium levels that were drawn daily. There was no change in clinical status as the levels varied from 131 to 125 and back up.

Now let me give you a bit more information. The patient was actually getting serial checks every 6 hours (or more)! Here’s the updated chart:

Day/Time Na Treatment NaCl per day
Day 1 18:30 131
Day 1 22:54 132 0.9% NS @ 125/hr 3G
Day 2 05:59 133 continues 3G
Day 2 12:19 129 continues
Day 2 17:50 129 continues
Day 3 07:18 127 continues
Day 3 12:09 127 continues
Day 3 17:58 126 continues
Day 3 23:53 126 continues
Day 4 07:45 125 continues
Day 4 11:38 122 2% NS @ 25/hr 6G
Day 4 15:25 125 continues
Day 4 19:31 125 continues
Day 5 00:06 122 continues 6G
Day 5 04:04 126 continues
Day 5 08:01 122 continues
Day 5 11:50 132 stop
Day 5 16:14 126
Day 5 19:26 127
Day 6 00:20 129 9.2G
Day 6 04:42 127 2% NS @ 40/hr
Day 6 08:30 124 continues
Day 6 12:29 127 stop
Day 6 16:16 127 Salt tabs 2G tid
Day 6 20:28 132 continues
Day 7 05:22 134 Salt tabs 2G qid 8G
Day 7 12:33 135 continues
Day 8 07:02 131 stop None
Day 8 13:33 136

Confused? Me, too! This poor person had 30 blood draws in 8 days, with 6 per day for two of those days. Carefully look at the amount of salt given in each 24 hour period, and look at the sodium levels for that day.

See the variability, even when getting high doses of sodium chloride? What does this tell you? Was the salt administration helpful? Was seeing the lab value every 4-6 hours valuable?

Tell me what you think. Leave comments or tweet your opinions. Next, I’ll discuss the known variability of the serum sodium assay, and give you my opinion on the value of serial testing.

Serial Lab Testing: Worthwhile or Worthless?

We’ve all done it at some point. Serial hemoglobin. Serial sodium. Serial serum porcelain levels. What does serial mean to you? And what does it tell us about or patient?

Today and tomorrow, I’d like to present an example from real life. For today, have a look at the daily sodium tests done for a patient with a head injury. The concern was for cerebral salt wasting, which is probably grounds for its own blog post.

So have a look at this series of sodium determinations. It represents serial values based on daily testing.

Day/time Na
Day 1 18:30 131
Day 2 05:59 133
Day 3 07:18 127
Day 4 07:45 125
Day 5 04:04 126
Day 6 04:42 127
Day 7 05:22 134

At what point, if any, would you be concerned with significant hyponatremia, and begin some type of supplementation?

Tomorrow, I’ll provide a little more info on levels and treatment

2 Ways To Perform A Pericardial Window For Trauma: Part 2 With Video!

In my last post, I reviewed the classic, “old-timey” subxyphoid approach to the pericardial window procedure for trauma. Today, I’ll describe the operative approach if you are already in the abdomen managing injuries there.

The same considerations apply to these patients in deciding to perform the window. Either there is a suspicion of actual pericardial tamponade based on physiology or diagnostic imaging, or an injury has been noted in proximity to the heart that raises that suspicion.

If you are already exploring the abdomen, the procedure is much simpler. The instruments required are already in your laparotomy setup:

  • Two toothed forceps
  • Tissue (Metzenbaum) scissors

First, and most importantly, the upper abdomen must be evacuated of all blood. This is critically important since a positive window is solely determined by the presence of blood in the pericardial fluid. If it is contaminated with blood as it flows into the peritoneal cavity, a false positive may result leading to an unneeded thoracotomy or sternotomy.

The midline incision must extend to the xiphoid process in order to get adequate exposure of the diaphragm. The left lobe of the liver is retracted downwards by your assistant, and the two of you can then grasp an area of the pericardial portion of the diaphragm with the toothed forceps. As it is tented away from the heart, the scissors are used to dissect through both the diaphragm and pericardium. Although some use cautery for this, I’m a weenie using electricity near the heart.

The diaphragm is thick, so expect to cut through several mm of tissue before you see pericardial fluid. Watch the color of the fluid carefully. If it is the least bit blood tinged, the result is positive. And be sure to watch for 15-30 seconds. Sometimes the initial fluid is amber, but it becomes bloody as more is drained.

Bloody fluid equals positive result. This means that a thoracic procedure is indicated to evaluate the heart and repair the injury. The choice of sternotomy vs thoracotomy is determined by mechanism, foreign body trajectory, and suspected area of injury on the heart.

If the result is negative, you may close the hole with your suture of choice. If the abdomen is contaminated from a bowel injury, I recommend you use the traditional subxiphoid approach separate from the laparotomy incision to avoid contaminating the pericardial sac.

Here’s a YouTube video of a transdiaphragmatic window created laparoscopically. Since abdominal explorations for major trauma seldom lend themselves to laparoscopy, don’t get any ideas from watching this!

2 Ways To Perform A Pericardial Window For Trauma: Part 1 with Video!

In this two-part post, I’ll describe two ways to perform a pericardial window for trauma. The pericardial window should be considered in any trauma patient who has one of the following:

  • A suspected diagnosis of pericardial tamponade. These patients do not yet have classic signs and symptoms. If they did, a thoracotomy or sternotomy is in order, not a window.  But they do have a mechanism that could produce bleeding into the pericardial sac, and a positive imaging study. Typically, this study is a FAST exam of the heart. Occasionally, pericardial fluid may be seen on chest CT. This is uncommon but significant when detected.
  • An injury in proximity to the heart that is of concern for cardiac injury with a negative or indeterminant FAST. These are typically penetrating injuries so close to the heart that it’s hard to believe it wasn’t injured. If the FAST is not helpful, a window will make the definitive diagnosis.

Pericardial window is a very invasive procedure. For trauma, it is usually performed in the operating room and requires general anesthesia. It could be performed in the ED if the patient is already intubated and sedated.

There are two ways to perform this procedure. Today, I’ll discuss the old-timey subxiphoid approach.  The equipment required is minimal:

  • Scalpel
  • Tissue (Metzenbaum) scissors
  • Once or two toothed forceps
  • Your finger
  • Good lighting

A 4-8 cm incision is made extending from the top of the xiphoid, extending about 4 cm down onto the abdominal midline. Enter the retrosternal space with your finger, and head to the heart. Usually, some fatty tissue must be bluntly dissected out of the way. Note: the heart is frequently further away than you think!

Sweep the fat out of the way, exposing the pericardium. Grasp the pericardium with the toothed forceps and tent it away from the heart. Use the Metzenbaum scissors to incise the pericardium immediately adjacent to the forceps. If this is difficult, then have an assistant grasp the pericardium with another pair so a short line of pericardium is elevated. (Note: sometimes having a second set of forceps in the incision makes it too difficult to see, which is why I prefer the single forceps technique).

Make sure that the wound is bloodless when you incise the pericardium! There is always at least a small amount of pericardial fluid that will squirt out, and you are looking at its color. If it is anything but amber, you have a positive result. If you have a bloody field that contaminates the fluid, a false positive diagnosis could occur leading to an unnecessary thoracotomy.

If the window is positive, cover the wound and head immediately to the OR if your’re not already there. Your patient has a cardiac injury until proven otherwise. If negative, then close the skin wound with your sutures / staples of choice. Do not attempt to close the tiny pericardial hole!

Here’s a video that shows the basic technique. The procedure depicted is being performed for non-trauma, so the operator takes his time. He has the luxury of dissecting and exposing the field well. But in trauma, we don’t usually have time to resect the xiphoid or take 10 minutes to dissect out the field.

In my next post, I’ll discuss the technique that is used if you already find yourself in the abdomen when a cardiac injury is suspected.

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