Labs

The following is the complete blood count (CBC) on the day you see the patient and a baseline CBC from 2 months earlier at the time of her valve repair:

DayWBC (109/L)Hb (g/dL)MCV (fL)RDW-SD (fL)PLT (109/L)
Day of admission11.87.310167264
Preop (2 months ago)8.412.59241235

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW-SD, red cell distribution width-standard deviation; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45 fL, platelets (PLT) 150-450 x 109/L

Reticulocyte count on day of admission:

The reticulocyte count is reported as 5.6%… 5.6% of what?

a
Nucleated red cells
b
Total red cells
c
White cells + red cells
d
Platelets

The patient, then, appears to have acute or subacute anemia (developing sometime over the previous 2 months) which is hyperproliferative based on an appropriate reticulocyte count and macrocytic probably owing to the reticulocytosis. Let’s consider the hemolysis labs:

Classic for hemolysis, right? Elevated LDH, AST:ALT ratio and bilirubin; and low (in fact, undetectable) haptoglobin.

Hemolysis can be classified in several ways:

  • Congenital vs. acquired
  • Immune vs. non-immune
  • Extracorpuscular vs. intracorpuscular
  • Intravascular vs. extravascular

Intravascular hemolysis describes the lysis or rupture of red cells inside the circulation with leakage of intracellular contents into the bloodstream. Extravascular hemolysis refers to the ingestion of red cells by macrophages and the subsequent intracellular degradation of hemoglobin into its constituent parts.

LDH, AST, bilirubin and haptoglobin may be abnormal in both intravascular and extravascular hemolysis. However, the presence of free hemoglobin in plasma and hemoglobin in the urine is more common and more pronounced in intravascular hemolysis.

Let’s drill down a little.

Hemolytic anemia may be classified according to where the hemolysis is taking place: inside the circulation (intravascular) or in macrophages of the reticuloendothelial system, especially in spleen and liver (extravascular).

Intravascular hemolysis:

  • Causes:
    • Complement fixation, typically via IgM and direct lysis:
      • Cold autoimmune hemolytic anemia
      • ABO antibody-mediated alloimmune hemolytic anemia following transfusion
      • Paroxysmal nocturnal hemoglobinuria (PNH)
    • Mechanical, shear-force-associated red blood cell (RBC) lysis:
      • Thrombotic microangiopathy (TMA)
      • Valve hemolysis
    • RBC lysis secondary to membrane disorders, enzymopathies or hemoglobinopathies (i.e., non-immune-mediated intravascular hemolysis):
      • Hereditary spherocytosis
      • G6PD deficiency
      • PK deficiency
      • Sickle cell anemia
      • Thalassemia
  • Red cells are lysed in the blood, internal contents are leaked into the vascular space:
    • LDH is released directly into the circulation.
    • AST is released directly into the circulation.
    • Hemoglobin (Hb) is released directly into the circulation:
      • Hb binds haptoglobin.
      • Hb-haptoglobin complex taken up by macrophages.
      • Hb metabolized to bilirubin, with the latter being released into the circulation.
    • (Thousands of other proteins are also released into the plasma!)

Extravascular hemolysis

  • Causes – majority of cases of extravascular hemolysis are attributable to an immune-mediated cause.
  • Red cells are engulphed by macrophages in organs such as the spleen and liver:
    • Hemoglobin is degraded by macrophages, resulting in increased bilirubin production and release.
    • Presumably macrophages release some LDH, AST and free hemoglobin.
    • An alternative explanation for the changes in serum LDH, AST and haptoglobin is that most cases of extravascular hemolysis are also accompanied by some degree of intravascular lysis.

Here are the results of the uranalysis:

Visual inspection showed red colored urine, similar to that shown above.

The presence of blood on dipstick with virtually no red cells on microscopy is diagnostic of pigment in the urine, either hemoglobin or myoglobin. Distinguishing between these two pigments requires mass spec analysis which is rarely available or performed. Diagnosis is based on the clinical context (conditions associated with hemolysis vs. rhabdomyolysis).

Macrophages break down hemoglobin to unconjugated (indirect) bilirubin, which is released into the circulation. Indirect bilirubin is water insoluble. It binds to albumin and is transported to the liver. Indirect bilirubin is taken up by hepatocytes, where it is conjugated and released into the biliary system. From there, it is delivered to the small intestine. It is then metabolized by gut bacteria to urobilinogen. Urobilinogen, which is water soluble, is absorbed back into the blood. Some of the urobilinogen is excreted by the kidney into urine where it can be semi-quantitively assayed using dipstick.

Urine hemosiderin

Hemoglobin that is filtered by the kidneys is reabsorbed in the proximal convoluted tubule (PCT). PCT cells process the hemoglobin much the same way as macrophages do, only they hold onto the iron (instead of releasing it to feed erythroid precursors) and store is as ferritin or hemosiderin. The latter is a complex of iron with phosphate and hydroxide forms that is formed when the capacity for storage of iron in ferritin is exceeded. All PCT cells undergo turnover. When they die, they slough off with the hemosiderin and are excreted into the urine, where they can be detected using a Prussian blue stain. Urine hemosiderin is usually seen three to four days after the onset of hemolytic conditions.

Two examples of urine hemosiderin. Sources: Left and right images.

Urine hemosiderin was not performed in our patient.

I hope that you agree we have secured a diagnosis of hemolytic anemia.

The following is similar to the peripheral blood smear from the patient:

Differential diagnosis of hemolytic anemia

Recall our simple classification of causes of hemolytic anemia from the previous section:

TMA, thrombotic microangiopathy; MAHA, microangiopathic hemolytic anemia

Now let’s reorganize the causes of hemolytic anemia in table format and consider the peripheral smear findings. Clostridial sepsis and babesiosis are included as infectious causes. Can you fill in the table (in your head) before looking at the descriptions on the next slide (we can assume that polychromatophilia applies to all causes)?

Condition causing hemolytic anemiaSmear findings
Warm hemolytic anemia
Cold hemolytic anemia
Delayed transfusion reaction
TMA
MAHA
Cirrhosis
Infection – Clostridial sepsis
Infection – Babesiosis
Burns
Thalassemia
Sickle cell disease
PK deficiency
G6PD deficiency
Hereditary spherocytosis
Hereditary elliptocytosis
PNH
TMA, thrombotic microangiopathy; MAHA, microangiopathic hemolytic anemia; PK, pyruvate kinase; PNH , paroxysmal nocturnal hemoglobinuria.

Now let’s reorganize the causes of hemolytic anemia in table format and consider the peripheral smear findings. Clostridial sepsis and babesiosis are included as infectious causes. Can you fill in the table (in your head) before looking at the descriptions on the next slide (we can assume that polychromatophilia applies to all causes)?

Condition causing hemolytic anemiaSmear findings*
Warm hemolytic anemiaSpherocytes
Cold hemolytic anemiaRed cell agglutination
Delayed transfusion reactionSpherocytes
TMASchistocytes, decreased platelets
MAHASchistocytes
CirrhosisMacrocytes, target cells, acanthocytes (spur cells)
Infection – Clostridial sepsisMicrospherocytes
Infection – BabesiosisIntracellular parasites
BurnsElliptocytes, microspherocytes 
Thalassemia (minor)Microcytes
Sickle cell diseaseSickle cells, nRBC, HJ bodies
PK deficiencyAcanthocyte-like cells
G6PD deficiencyBite cells
Hereditary spherocytosisSpherocytes
Hereditary elliptocytosisElliptocytes
PNHMay appear normal
TMA, thrombotic microangiopathy; MAHA, microangiopathic hemolytic anemia; PK, pyruvate kinase; PNH , paroxysmal nocturnal hemoglobinuria.
* Most of these causes will be associated with polychromatophilia; nRBC, nucleated red blood cells; HJ bodies, Howell-Jolly bodies

Our patient has schistocytes on her peripheral smear. According to the table above, that helps to narrow the likely diagnosis to TMA or MAHA.

Concerning thrombotic microangiopathy (TMA) vs. microangiopathic hemolytic anemia (MAHA), which of the following statements is true?

a
TMA includes MAHA in its definition
TMA is clinically defined by microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and organ injury.
b
MAHA includes TMA in its definition
c
TMA, but not MAHA is associated with thrombocytopenia
d
MAHA, not TMA is associated with thromboctopenia

Thrombotic microangiopathy (TMA) is characterized by:

  • Microangiopathic hemolytic anemia (MAHA)
  • Thrombocytopenia
  • Organ injury

MAHA is characterized by red blood cell destruction within the microvasculature, and may occur in:

  • TMA
  • Extracorpuscular hemolysis due to mechanical trauma
    • Foot-strike anemia
    • Percussion hemolysis
  • Valve hemolysis (if one expands the definition of MAHA to include not just microvessels, but also a paravalvular leak.)

The last slide mentioned 2 interesting, but rare causes of microangiopathic hemolytic anemia (MAHA):

Foot-strike anemia (also called runners hemoglobinuria):

  • The first case was described in 1881 in a German soldier who demonstrated severe hemolytic anaemia reported the passage of dark urine after extended periods of marching on the field. Hence, it was initially called march hemoglobinuria.
This paper from 1944 refers to the original description in 1881. Br Med J. 1944;1(4355):844
  • Since then, foot-strike hemolysis has been demonstrated to be the greatest contributor to anemia in athletes. The risk of developing hemolysis in runners depends on individual running style, type of foot wear and the running surface. 
This is a paper from 2012 in which the focus is shifted to foot-strike hemolysis in elite athletes.

Percussion hemolysis

Hemolysis and hemoglobinuria have been reported in passionate hand percussion drum players, for example those playing following African drums. Percussion hemoglobinuria is very similar in pathogenesis to march hemoglobinuria, except that it occurs in drum players and the intravascular hemolysis occurs in the palms instead of soles.

This study reported extracorpuscular hemolysis in a cohort of 45 Candombe drummers in Uruguay. Clin J Am Soc Nephrol. 2008;3:1022-7.
Candombe, Afro-Uruguayan Culture in Montevideo - Upscape
Candombe drummers in Uruguay. Source.

In cases of acute hemolysis, ruling out thrombotic microangiopathy (TMA) is a priority because a diagnosis of thrombotic thrombocytopenia purpura (TTP) has a mortality rate of about 90% if left untreated. Even patients with a recent valve replacement may develop TTP.

These are the results of a transesophageal echocardiogram as they related to the mitral valve:

The diagnosis, then, is most certainly valve hemolysis.

Before moving on to treatment, we want to look at the renal function.

The patient’s renal function was normal.

Now let’s move on treatment considerations.